16—
SUSTAINED YIELD PRODUCTION IN RIPARIAN SYSTEMS

MEETING THE CHALLENGES OF ECONOMIC AND ECOLOGICAL VIABILITY

Productivity in Native Stands of Prosopis Glandulosa

Mesquite, in the Sonoran Desert of Southern California and Some Management Implications^[1]

E.T. Nilsen, P.W. Rundel, and M.R. Sharifi^[2]

Abstract.—Species of Prosopis , in particular, Prosopisglandulosa , form the dominant woody elememt of wash woodlands over 30 million hectares of the desert and semiarid communities of southwestern United States. During 1980–81, productivity and biomass were measured in a Prosopisglandulosa stand near the Salton Sea in southern California. Total stand production was 3,650 kg. per ha. per year, and the aboveground standing crop was 14,000 kg. per ha. These values are similar to those found for a desert wash dominated by the same species in Baja California. Such production rates are remarkably high for desert ecosystems particularly in relation to the low annual rainfall of 70 mm. per year. These high levels of productivity for mesquite are possible because the mesophytic nature of Prosopis and its ability to fix nitrogen allow these plants to be decoupled from normal limiting water and nitrogen resources in desert ecosystems. Our productivity and related studies have considerable significance for managed stands of Prosopis and the influence of managed Prosopis stands on wash woodland ecosystems.

Introduction

In the hot deserts of southern California, wash woodlands are the major riparian systems. To date, only a few of the many ecosystem and community investigations carried out in the southern deserts have concerned themselves with wash woodland environments. The few available published data on productivity of plantations in Chile and Pakistan indicate that remarkably high productivity rates can be maintained for Prosopis species in arid lands. The paucity of information concerning native stands of Prosopis stimulated our research investigation on productivity of Prosopisglandulosa in the Sonoran desert of southern California.

Wash woodlands support many life forms but the most dominant are leguminous trees. Prosopis , mesquite, is probably the most abundant leguminous tree inhabiting wash woodlands in both southern California and northern Mexico. P . glandulosa accounts for the dominant woody woody element in wash woodlands in over 30 million ha. of the desert and semiarid plant communities of the southwestern United States. Even though mesquite has widespread ecological and economic importance, few data are available which characterize the stand biomass and productivity of mesquite woodlands (Klemmedson and Barth 1975; Sharifi etal . in press).

Quantification of productivity and biomass of Prosopis is very desirable for several reasons. First, as stated above, wash woodland ecosystems have not been previously studied extensively, despite their importance to desert fauna and desert water resources. Second, the few available published data on Prosopis net primary production from plantations in Chile and Pakistan suggest that remarkably high production rates can be maintained in desert communities (Salinas and Sanchez 1971; Ahmed 1961). Third, there has recently been increasing interest in the importance of energy and food production of arid zone plants such as Prosopis species, particularly in many third world countries.

In this investigation we quantified biomass and production in a P . glandulosa wash woodland riparian system in the Sonoran Desert of California. We also investigated the influence of normal desert environmental parameters, which limit the aboveground production of this Prosopis stand. These data were then used to

[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].

[2] E.T. Nilsen, P.W. Rundel, and M.R. Sharifi are associated with the Department of Ecology and Evolutionary Biology, University of California, Irvine, California.

project the possible ramifications of managing such a Prosopis stand for biomass and/or fodder production.

Site Description

The research site was Harper's Well, located 10 km. west of the southern tip of the Salton Sea near the base of the Fish Creek Mountains, Imperial County. Elevation of the site is –30 m. msl. Its age can be dated back 500 years when the site was covered by a larger Salton Sea (Lake Cahuilla). Climatic conditions at Harper's Well are extreme (fig. 1). There is a large yearround evaporative loss of water while precipitation is low (65–70 mm. annually). Temperatures reach a maximum in July, averaging 45C., while temperatures below 5C. are extremely rare. Soil texture is variable, ranging from a sandy loam beneath trees to clay between trees. Water is constantly available as groundwater is at a depth of about 5 m. There are occasional short periods of surface flooding of the wash area by runoff from the Fish Creek Mountains. Tropical storms occasionally create sheet flow over the impermeable clay soil.

Methods

Biomass and Productivity

Prosopis biomass and productivity were measured by the dimension analysis techniques (Whittaker and Marks 1975). Regressions were formulated between productive branch basal diameter and biomass components. Trunks were measured to determine volume, which was multiplied by wood density to determine trunk weight. Biomass and production were determined as dry weight. The biomass components investigated were trunk weight, productive branch wood, current twigs, leaves, inflorescences, and pods.

Figure l.
Climatic conditions at Harper's Well in the
Sonoran Desert of southern California.

Productivity was calculated as the sum of the clipping production and the woody increment. Clipping production was measured as the sum of maximum leaf, inflorescence, pod, and current twig biomass. Annual woody increment was determined by measuring the average of five years wood growth as per Whittaker and Marks (ibid .).

Ten trees were measured carefully to determine total biomass and production for all biomass components on a canopy area and volume basis. Canopy volume per ground area was then determined in 30 0.1-ha. quadrats on two transects which best represented the Prosopis stand. The canopy volume per ground area was multiplied by the biomass and production per canopy volume to yield biomass and productivity per ground area.

Water Use

Several measurements of water use were made during 1980–81. Diurnal cycles of transpiration per leaf area were determined monthly on four representative trees using a steady state porometer (Licor, model 1600). Seasonal variability in total leaf area was ascertained by labeling 20 branches on each tree, determining the number of leaves per branch, and multiplying by the mean leaf area per leaf (determined seasonally for each tree) to give mean leaf area per branch.

These data, when converted to a percent of seasonal maximum basis, were used to adjust maximum leaf area per tree, yielding seasonal leaf area per tree. The monthly diurnal cycles of transpiration per tree were multiplied by the seasonal leaf biomass to yield monthly water use. Soil moisture determinations were made to a depth of 5 m. by the use of neutron activation technique.^[3] Relative environmental plant water stress was determined by measuring pre-dawn xylem pressure potential seasonally by the pressure chamber technique.

Nitrogen

Nitrogen concentrations were measured in the soils and in plant tissues by co-workers.^[3] These nitrogen values were combined with the productivity values to produce a nitrogen budget for this stand of Prosopis .

Results and Discussion

Biomass and Production

Regressions which were formulated between basal diameter and biomass or production components were quite accurate, with correlation coefficients greater than 0.95 for all components except fruit and flower production (r² = 0.80). This resulted in an accurate determination of biomass per tree, which was found to be within 5%

[3] Data collected by University of California, Riverside co-workers Dr. Wes Jarrell and Dr. Ross Virginia.

of the harvested biomass of a sample tree. Average biomass and production values for aboveground components are shown in table 1 on a stand and canopy area basis.

Total stand biomass was close to 14,000 kg. per ha. on a stand basis and 42,000 kg. per ha. on a canopy area basis. A majority of biomass (85%) was retained in trunk and productive branch wood. Productivity was high in relation to biomass (biomass accumulation ratio = 3.84), indicating that this stand is still actively growing. Fruit production accounted for 21% of the total production and leaf production was 33% of the total.

There was a considerable variability in stand production and biomass (table 2). Near the wash watercourse itself biomass and production were twice as high as the average. On the outer edges of the stand biomass and production decreased to 25% of the average. The highest stand value for a 0.1-ha. plot was found in the proximity of the wash with 77% cover of P . glandulosa .

These biomass and production values are very high in relation to those of other desert communities. This Prosopis stand at Harper's Well has a biomass and production which far exceeds that measured for other Prosopis stands in New Mexico and Arizona (fig. 2), but is very similar to that measured in Baja California. The Baja California Prosopis stand is older than that of Harper's Well, therefore the biomass is larger and productivity is less. In comparison to other desert shrub communities, Prosopis biomass and production at Harper's Well is by far the largest (fig. 2 and 3).

Figure 2.
Biomass and production for several desert communities.
a = Whittaker and Neiring 1975; b = Turner and McBrayer
1974; c = Balph et  al . 1974; d = Chew and Chew 1975.

Based on these comparative data, the Harper's Well Prosopis wash woodland has a higher biomass and production than that measured for any other American desert community. While both biomass and productivity of this Prosopis stand are small in comparison to other California riparian plant communities, aboveground productivity of Prosopis is large in relation to rainfall (fig. 3). This is unusual when compared to other desert ecosystems, and is a clear reflec-

Figure 3.
The relationship of productivity to rainfall
in several desert communities.

tion of the riparian character of this wash woodland. It is particularly interesting because moisture is a limiting factor for most growth in desert environments.

Moisture Availability

Several measurements of moisture availability were made at this site during 1980–81 (fig. 4). Soil moisture at 4 m. depth stayed constant throughout the year while soil moisture at 0.3 m. depth increased during the period of rainfall from January through March. Pre-dawn xylem pressure potential gives an indication of the general environmental water availability to the plants. Water is slightly more available during January through March when surface soil moisture increases, although throughout the rest of the year there is no significant water deficit in the plants. This is unusual considering the extreme evaporative demand in July (fig. 1), and can only be the result of a deep tap root system which utilizes the available deep groundwater (Nilsen etal . 1981).

Figure 4.
Measurements of environmental and plant water
relationships for Prosopis   glandulosa  at Harper's
Well, Sonora Desert, southern California.

Water Use

Water use per individual plant was considerable, partly due to constant available water at depth, large leaf area, and minimal diurnal water stress. Figure 5 is a representative diurnal curve of transpiration rate for one tree in July, 1981. Most water use occurs in the morning and early afternoon when water stress is minimal and temperatures are moderate.

Such diurnal cycles of transpiration, when summed over each month, produce a measure of monthly water use (fig. 6). A majority of seasonal water use occurred during late spring and summer when leaf evaporative demand was the highest.

Figure 5.
Diurnal cycle of transpiration for  Prosopis
glandulosa  at Harper's Well, Sonoran
Desert, southern California.

Figure 6.
Average monthly water use per leaf area for a
population of Prosopis  glandulosa  at Harper's
Well, Sonoran Desert, southern California.

Seasonal leaf area, multiplied by monthly water use per leaf area, results in stand and plant water use characteristics. Maximum water use occurred in June and August when up to 700 kg. of water were transpired per tree per day. Total seasonal water use on a stand basis and a canopy area basis are shown in table 3.

It is clear that Prosopis uses tremendous quantities of water per year, far greater than the annual input by precipitation (70 kg. per ha. per year). Despite the large volume of water transpired, there was little change in the availability of groundwater (fig. 4) which served as the major water resource. On a yearly basis approximately 11,353 kg. of water were transpired per tree which results in a transpiration ratio (water transpired/net primary production) of 300. Such a transpiration ratio is similar to that of forested and riparian plant communities with ample water supplies. Therefore, these trees are not water conservative by any means and groundwater availability may be a critical limiting factor for management of a Prosopis wash woodland site in the Sonoran Desert.

Nitrogen

Nitrogen is another limiting factor for production in desert ecosystems. This may be especially true in wash woodlands, where the soils are often sandy and frequently flushed by flooding, commonly resulting in low inorganic and organic nitrogen availability. Table 4 represents the soil nitrogen characteristics in general for the upper 30 cm. at Harper's Well in comparison to other communities. It is clear that organic nitrogen is much higher under the trees than between the trees outside the stand. This is the result of the build-up of decomposed litter and humus under these deciduous trees. The nitrate values under these trees are extremely high for any ecosystem; particularly in relation to the organic nitrogen content. The very high nitrate content is the result of low denitrification rates (Virginia etal . in review), infrequent soil leaching by rainfall, and high deposition rates of nitrogen in abscissed leaves.

The flux rates of nitrogen through this Prosopis stand indicate considerable accumulation (52 gm. per m² per year). Translocation from storage tissues accounted for 8% of accumulation, and uptake from deep groundwater could account for only less than 1% of total accumulation. Thirty percent of accumulated nitrogen was deposited through litter fall. The accumulated nitrogen for the most part (>80%) must be derived from the upper soil layers or nitrogen fixation (Rundel etal . 1981). Clearly, the large amount of nitrogen in surface soils must have been derived from nitrogen fixation in the past since the surrounding areas are very low in nitrogen, as are the lower soil layers. The question still remains whether present nitrogen accumulation in biomass is derived from nitrogen fixation or soil uptake in a recycling manner (presently under investigation). Yet it is apparent that nitrogen fixation must be very important in maintaining such high productivity rates, whether or not the fixation is presently continuing.

Production Potentials

The potential biomass and production of a managed Prosopis stand can be estimated by extending the percentage cover of canopy to 90%. In this case, total production would be 9,900 kg. per ha. per year, and component production would be: leaves 3,300 kg. per ha. per year; fruits 2,080 kg. per ha. per year; wood 5,075 kg. per ha. per year. This is considerably lower than the leaf and pod production (15,000 kg. per ha. per year) found for Prosopistamarugo in Chile (Salinas and Sanchez 1971), but this would be an extremely high production for North American desert ecosystems. This high pod and leaf production is a potential resource for animal fodder, and the large wood production is a potential resource for biomass fuel, particularly because of the high wood density. However, it is also important to ask: what influence would such a managed stand have on the wash woodland ecosystem?

There is a considerable number of bird, reptile, insect, and mammal species which inhabit and utilize Prosopis for food and shelter (Mares etal . 1977). Increasing the density of the native Prosopis species to 90% should only increase the resources for the wash woodland fauna. The physical environment would also be influenced, particularly in relation to water and nitrogen availability. Clearly, an increased density of Prosopis would increase the upper soil nitrate and total nitrogen contents. Increasing nitrogen availability in upper soil layers could only benefit the nutritional stability of the community. Water resources used by a stand with density of 90% would be three times that presently transpired, or 3 × 10⁶ kg. per ha. per year, yet there was no significant change in groundwater availability. Even though there seems to be a considerable groundwater recharge potential, a non-irrigated, managed stand of 90% cover could conceivably reduce groundwater resources, particularly from June through August.

This problem of groundwater recharge ability could become critical in other desert washes which have ephemeral water tables.

Conclusion

This study indicates that the Prosopisglandulosa stand of Harper's Well has extremely high productivity rates in relation to other desert plant communities. These high productivity rates are possible because Prosopis fixes nitrogen and is mesophytic, which decouples this taxon from productivity limitation by rainfall and soil nitrogen availability. Even though productivity is decoupled from rainfall, large quantities of groundwater are required for growth. It seems that the major ecosystem disruption as a result of managing a Prosopis stand for biomass fuel production and fodder production may be a reduction in groundwater availability. This may be critical in wash woodlands with ephemeral water tables. Prosopis has considerable potential for biomass fuel production and fodder production in arid ecosystems, with limited disruption of ecosystem stability.

Literature Cited

Ahmed, G. 1961. Evaluation of dry zone afforestation plots. Pakistan J. of Forestry 168.

Balph, D.T., R.S. Shinn, R.D. Anderson, and C. Gist. 1974. Curlew Valley validation site. US/I.B.P. desert biome research memorandum 74–1. 61 p. Ecology Center, Utah State University, Logan.

Chew, R.M., and A.E. Chew. 1975. The primary productivity of a desert shrub (Larrea tridentata ) community. Ecol. Monog. 35:355–375.

Johnson, D.W. 1979. Some nitrogen fractions in two forest soils and their changes in response to urea fertilization. Northwest Science 53(1):22–32.

Klemmedson, J.O., and R.C. Barth. 1975. Distributional balance of biomass and nutrients in desert shrub ecosystems. US/I.B.P. desert biome research memorandum 75–5. 18 p. Ecology Center, Utah State University, Logan.

Mares, M.A., F.A. Enders, J.M. Kingsolver, J.L. Neff, and B.B. Simpson. 1977. Prosopis as a niche component. p. 123–149. In : B.B. Simpson (ed.). Mesquite, its biology in two desert ecosystems. Dowden, Hutchinson and Rass, Inc., Stroudsburg, Penn.

Nilsen, E.T., P.W. Rundel, and M.R. Sharifi. In press. Summer water relations of the desert phreatophyte Prosopisglandulosa in the Sonoran Desert of southern California. Oecologia.

Rundel, P.W., E.T. Nilsen, M.R. Sharifi, R.A. Virginia, W.M. Jarrell, D.H. Kohl, and G.B. Shearer. In press. Seasonal dynamics of nitrogen cycling for a Prosopis woodland in the Sonoran Desert. In : Nitrogen cycling in ecosystems in Latin America and the Caribbean, Cali, Columbia.

Salinas, H.E., and S.C. Sanchez. 1971. Estudio del tamarugal como productor de alimento del grande hanan en la Pampa del Tamarugo. Informe Tecnico Institutio Forestal Section Silvicultura (Santiago, Chile) 38:1–35.

Sharifi, M.R., E.T. Nilsen, and P.W. Rundel. In press. Biomass and net primary production of Prosopisglandulosa (Fabaceae) in the Sonoran Desert of California. Am. J. of Botany.

Turner, F.B., and J.F. McBrayer. 1974. Rock Valley validation site report. US/I.B.P. desert biome research memorandum 74-2. 64 p. Ecology Center, Utah State University, Logan.

Virginia, R.A., W.M. Jarrell, and E. FrancoVizcaino. In review. Direct measurement of denitrification in a Prosopis (mesquite) dominated Sonoran Desert ecosystem. Oecologia.

West, N.E., and J. Skojins. 1978. The nitrogen cycle in the North America cold-winter semidesert ecosystems. Oecologia Plantarum 12:45–53.

Whittaker, R.H., and P.L. Marks. 1975. Methods of assaying terrestrial productivity. p. 55–118. In : H. Leith and R.H. Whittaker (ed.). Primary production of the biosphere. Springer-Verlag, Heidelberg.

Whittaker, R.M., and W.A. Neiring. 1975. Vegetation of the Santa Catalina Mountains, Arizona. V. Biomass, production, and diversity along an elevational gradient. Ecology 56:771–790.

Woodmansee, R.G., I. Vallis, and J.J. Mott. 1981. Grassland nitrogen. p. 443–462. In : F.E. Clark and T. Roswell (ed.). Terrestrial nitrogen in cycles. Ecological Bulletin (Stockholm) 33.

Riparian Woodland Regulation

Pros and Cons^[1]

David E. Pesonen^[2]

Abstract.—The Forest Practice Act could be applied to riparian woodlands by action of the Board of Forestry. The act does not, however, apply to non-commercial timber cutting, and it provides little leverage to prevent conversion of such lands to other uses. The situation could change as stumpage values increase. Other laws should be explored for solutions.

Introduction

In recent years, it has been suggested that the Z'berg-Nejedly Forest Practice Act of 1973^[3] might be used to facilitate riparian woodland management. The issue has often been discussed within the California Department of Forestry (DF), and it has come before the Board of Forestry ("Board") on at least one occasion.

At first glance the Forest Practice Act would appear ideally suited to at least prevent abuse of riparian woodlands. The act has done a great deal of good in regulating and preventing the more destructive practices of timber operators in upland timber areas, and the process is continuing to be improved under the impetus of Section 208 of the Federal Water Pollution Control Act.^[4] Interestingly enough, the primary motivation for these efforts has been the need to prevent damage to water quality, an important factor in the discussions at this conference.

Nevertheless, the Forest Practice Act may not provide the protection to riparian woodland that it appears to offer. To be sure, nothing in the act prevents its application to any area ". . . which is available for, and capable of, growing a crop of trees of any commercial species used to produce lumber and other forest products . . ." (PRC 4526). This provision goes on to assign the determination of "commercial species" to the Board. The Board could determine that riparian woodland species are commercial species, thus placing riparian woodlands under the act. The absence of steady markets for such species might cause some hesitation in making this determination, but technically it could be done.

The question of "adequate protection," however, is quite another thing. It is actually very doubtful whether the act, as written, could provide any meaningful protection or facilitate the management of bottomland riparian vegetation.

The act has as its main thrust the "maximum sustained production of high quality timber" (PRC 4513). A clear underlying presumption is that the timber involved has a relatively high value and that the underlying land has value primarily for timber production. This assumption, which permeates the act, assumes an economic incentive to manage the timber for its own sake. Thus, the land must be protected from damage during the harvesting process, so as to maintain its capacity to produce timber.

A different situation exists in the riparian woodland. There, the land is typically valued primarily for agriculture, and the pressure to remove the trees comes from a desire to clear the land to make it available for agriculture. If the owner can sell the trees to help pay for the cost of clearing, well and good, but the existence of a market for these trees is not the only incentive for conversion. If he cannot sell the trees, the owner will often simply bulldoze and bunch them for burning.

The Forest Practice Act and the rules adopted under the act apply only to timber that goes into commerce (PRC 4527). If the owner chooses to bunch and burn his riparian trees, or to use them for his own non-commercial purposes, none of the rules will apply. The act deprives the Board of jurisdiction over such operations.

Stumpage values (the value of standing timber) of riparian woodlands are on the increase, however. Eventually, the value of retaining the land in timber may come to be seen by landowners. In such event, the Forest Practice Act may come to have more relevance in preventing improper har-

[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].

[2] David E. Pesonen is Director of Foresttry, State of California, Sacramento, Calif.

[3] Section 4511–4628, Public Resources Code (PRC) Section 4511–4628.

[4] PL 92-500, as amended by the Clean Water Act of 1977, PL 95-217.

vesting practices where sustained yield becomes a recognized goal. The act itself would still do little to prevent conversion of such lands to other uses, however.

Conversion of timberlands to non-timbergrowing uses is covered by the Forest Practice Act (PRC 4621 etseq .). These sections require a permit from DF, acting on behalf of the Board, before timberland can be converted to non-timber-growing uses. Currently, of course, such permits are not required on riparian woodlands because the Board has not included these lands in the timberland category.

The Board would have to change still another of its regulations to make this permitting process effective in the riparian areas. Through the definition of "Timberland Conversion" (14 CAC [California Administrative Code] 1102), conversion of non-Timberland Preserve Zone (TPZ) lands can be done without permit where commercial timber operations are not involved. None of the bottomland riparian woodlands are in TPZs. Thus, if the owner carries out conversion by the bunch-and-burn technique, he would not need a conversion permit. The Board would need to amend this regulation to require a permit for any timberland conversion, including many thousands of acres in upland timber not included in a TPZ.

Nevertheless, even if these two things were done, DF could rarely justify the denial of applications for permits to convert riparian woodland. The grounds for denial listed in PRC 4624 are quite limited for non-TPZ lands. Denials may be for only the following reasons:

a) the applicant is not the real person in interest;

b) there is material misrepresentation or false statement in the application;

c) the applicant does not have a bonafide intention to convert the land; and

d) there is a failure or refusal of the applicant to comply with the rules and regulations of the board and the provisions of the chapter.

California Environmental Quality Act^[5] requirements might allow DF to impose strict mitigation measures to discourage unwise conversions, but outright denial would be vulnerable to serious legal challenge.

To summarize, nothing in the act prevents the Board from including riparian woodlands under the Forest Practice Act. This could stem abusive harvesting practices. The act would, however, provide little or no relief from conversion of such areas to agriculture or other uses. Owners could evade the Forest Practice rules by non-commercial disposal of the trees. DF lacks authority to deny conversion permits on all non-TPZ lands for any except technical reasons. Therefore, meaningful protection for riparian woodlands under the Forest Practice Act, as it is currently written, is very unlikely.

Two changes in the act could make meaningful protection possible: 1) amend PRC Section 4527 to extend the act to all timber-cutting and removal operations, not just commercial operations; and 2) amend PRC Section 4621.2 to require the findings now necessary to allow conversion of TPZ lands to apply also to conversion of non-TPZ timberlands. Thus, application for conversions could be denied for more than merely technical reasons.

Such change could require considerable augmentation of DF staff. An estimate of the exact amount is impossible, but it could easily require double the present staff. DF would need additional inspectors not only to inspect known operations, but also to ferret out conversions where no commercial tree harvesting is involved. This would involve more than just riparian woodlands; non-timber harvesting conversion of non-TPZ lands everywhere would come under the regulations as postulated. The kinds of detective work needed to keep track of these conversions could become staggering, and provision for the resources to accomplish the task in an era of tight money—not to mention the spectre of massive regulation of agricultural land—could be politically unpalatable.

Other factors threaten the riparian woodlands beside timber harvesting or conversion to agriculture. Such developments as bank protection and flood control works, power developments, water storage, water diversions and drainage, and recreational developments are just a few that come to mind. Such developments have in the past had devastating effects on riparian woodlands, and none would come under the Forest Practice Act.

Two other areas of law might offer protection. The Wild and Scenic Rivers Act^[6] might be used in its present form to guarantee protection of riparian vegetation along portions of the Klamath, Trinity, Smith, Eel, and American rivers. Every effort should be made to assure that valuable stretches of river frontage on these rivers are included in management plans or are recognized for their unique values in Special Treatment Area rules by the Board of Forestry if AB 1600 were to pass in its present form.

Moreover, the act could be amended to include important stretches of river not now included. Riparian woodlands certainly add to the scenic value of the river, to say nothing of their other values.

The Native Species Conservation and Enhancement Act and the Native Plant Protection Act^[7]

[5] PRC Section 21000–21178.

[6] PRC 5093.50–5093.65.

[7] California Fish and Game Code Sections 1750–1913.

should also be explored for their applicability. These acts offer the possibility of habitat protection for rare and endangered species of plants and animals. Not every riparian woodland serves as habitat for a rare or endangered species, but those that do require the most urgent protection.

I am not unmindful of the political sensitivity of these suggestions. The American farmer has a traditional abhorrence of governmental regulation, and much of the land in question belongs to farmers. Adding this to the more recent generalized backlash against regulation, it seems to me that the times are not propitious for a move toward greater regulation. I cannot help but think that an all-out educational effort, coupled with the increasing value of riparian areas for wood products, could do a lot to reduce destructive conversions.

Perhaps, too, there is a place for a "Riparian Woodland Improvement Program" similar to our California Forest Improvement Program already in place. Such a program could provide incentive payments to maintain the woodlands. Less than 25 years ago, Douglas-fir timber on the North Coast was still being regarded as a weed. It was being slashed, girdled, and burned in wholesale quantities for conversion to grazing land. Very few such conversions are being made today, and much of the stipped land is being allowed to revert; some is being actively reforested. The turnaround occurred when landowners finally realized the true worth of their resources.

I don't say this to minimize damage done in the interim. Not at all. The damage in many instances is irreparable. I offer it only to point out that all is not lost. What was lacking on the North Coast 25 years ago was an effective educational effort or incentive program to take advantage of rapidly improving markets.

DF maintains a keen interest in the issue of riparian system protection and enhancement, even though the resource is not traditionally within our lead agency responsibility. We look forward to creative solutions that will surely emerge from this very timely conference, and we hope to have a significant part in carrying the theme of the conference forward into the next decade.

Economic Values of Three Furbearers Inhabiting California Riparian Systems^[1]

Lauren B. Scott^[2]

Abstract.—Information on the monetary value of sustained harvest of beaver, mink, and muskrat from California riparian systems is not readily available to planners and decisionmakers developing management plans for these systems. This paper summarizes an effort to establish some basic information on the economic value of sustained harvest to California. The effort was focused on answering three questions: 1) What is the relative economic value of each county's fur harvest? 2) What is the relative importance of each species to each county? 3) What is the direct economic value to California of harvesting the three species? Summaries of rankings of counties and species are presented.

Introduction

When man interacts with an ecological system, using it to satisfy his wants and needs, he tends to assign his values to it, comparing it to other systems and evaluating the components making up the system. These values can be both monetarily and non-monetarily based.

Knowledge of these values is important to those involved in making management decisions for an ecological system in today's world of conflicting wants and needs. Whether a management plan includes proposals to change or to not change a system, the evaluation used to determine the relative merit of each proposal is often based on trade-offs among components of a system, measured in terms of economic and non-economic values. The value of a particular component can often determine whether a management plan will favor, disfavor, or not consider that component.

This paper presents information on the economic values of three fur-bearing animals—furbearers—inhabiting California riparian systems. Although furbearers are a component of riparian systems, the author has found that their values often are not considered in riparian management plans.

Several reasons can be cited for omitting detailed consideration of these values. Three are listed below.

1. Current analyses of existing data on values of furbearers are not readily available to planners and decisionmakers.

2. The number of individuals knowledgeable about or participating in activities directly associated with furbearers is small.

3. Furbearers are characterized as "cautious animals that are mainly active at night" (Kellert 1979) and are thus difficult for most people to observe.

Kellert (ibid .) rated "attitudes people have toward wildlife as well as the knowledge they have in the area of animals: bird watchers are rated to be most knowledgeable, followed closely by trappers . . ." or fur harvesters. However, when he measured the "percentage of the general public" participating in "one of the animal activities groups," 78% consisted of people who watched "Wild Kingdom," 25.2% were bird watchers, and 1.7% of the population consisted of trappers. In California, less than 1% of the population buys a trapping license.

Species

The species considered in this study were chosen because they are, for the most part, exclusively riparian species and are known to have direct and indirect economic values. They are beaver (Castorcanadensis ), mink, (Mustela vison ), and muskrat (Ondatrazibethicus ). Other species considered were the raccoon (Procyonlotor ) and the gray fox (Canis (Urocyon ) cinereoargenteus ). They were not included because although they may spend some of their time feeding or resting in riparian

[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].

[2] Lauren B. Scott is Civil Engineer, USDI Bureau of Reclamation, Sacramento, Calif.

systems, they do not depend on riparian systems for most of their life requisites. The terms "species" and "animals" in this paper, therefore, refer to beaver, mink, and muskrat.

These species have several chracteristics in common (Nelson and Hooper 1976). Beavers need water several feet deep in which to live. In small rocky streams or shallow waters, they construct dams to create pools. The water backed up by the dam provides a safe travel way from lodge to food supply and a storage place for winter food (beavers do not hibernate). Along deep rivers and sloughs, beavers prefer to live in dens they dig in the banks. The beaver feeds at night on the bark and twigs of willows, cottonwoods, aspen, and other trees. It also eats roots, bulbs, leaves, and grasses found near water. Beavers can create problems by cutting down too many trees, blocking drainage ditches, and damming streams. These activities often result in flooded agricultural land and timberland, as well as damaged irrigation systems.

Mink are also found near water, preferring riverbanks and streams that have dense growths of vegetation. They live in old logjams, under the roots of trees near the water's edge, and in old muskrat burrows. Mink are nocturnal. They are good swimmers and often catch and kill their prey in the water. They eat mice, rabbits, muskrats, fish, crayfish, frogs, clams, birds, and other small animals and insects found near the water. They often kill more than they are able to eat. Mink may cause trouble because of their fondness for poultry.

Muskrats are always found in or near the water. They prefer the still or slow-moving waters of marshes, ponds, streams, and irrigation canals. They live in burrows dug in banks along the water's edge. In shallow, marshy areas, they build dome-shaped homes of tules, twigs, and mud. The muskrat is mainly nocturnal, although it is sometimes active during the day. It is primarily a vegetarian that feeds on aquatic plants, although it sometimes eats snails, mussels, insects, fish, and crayfish found near or in the water. Its burrowing habits make it a nuisance in areas where it may damage dams and ditches in irrigation systems by burrowing into the banks of these structures.

Values

This study examines the economic values of these three furbearers. The economic values of furbearers can be realized in many ways and are classified in this study as either direct or indirect. The economic value of a furbearer is realized in a direct manner when the animal is harvested and the pelt is sold. In California, this activity is regulated by the California Department of Fish and Game (DFG) and the California Fish and Game Commission. The term "harvest" or "take," as used in this paper, is defined under Title 14 of the Fish and Game Code (California Department of Fish and Game 1981–82). In addition, in this paper a sustained yield harvest is assumed. This assumption is based on an examination of nearly 60 years of DFG records.

The indirect economic value of furbearers is realized when:

1. these animals provide food, clothing, sport, education, or other needs and wants of man;

2. damage to irrigation and flood control systems and to timber- and cropland by these animals is reduced or prevented;

3. jobs and revenue are created by the fur industry's tanning, fur dressing, manufacturing, and retailing establishments; and

4. jobs and revenue are created by industries supporting and supplying fur-harvesting activities.

Of the two methods by which economic values can be realized, this paper addresses only the direct method.

Data Availability and Organization

The time period covered in this study is the 11 years from the 1969–70 to the 1979–80 fur harvesting seasons, designated the study period. Fur harvesting seasons generally run from November of one year through March of the following year. A season is designated by the year in which it begins, e.g., the 1969–70 season is designated as 1969.

The data in the annual licensed trappers' report (California Department of Fish and Game 1969–80), required by DFG, and in other sources (California Department of Fish and Game 1981) are compiled by county. This paper follows that same format for presenting and analyzing data.

Of the 58 counties in California, only Marin and Ventura counties are not included in this study. No take of beaver, mink, or muskrat was recorded for these counties during the study period, and thus they are considered to realize no direct economic value.

To organize and derive information on the direct economic value to California of the sustained yield harvest of these three furbearers, three questions were posed:

1. What is the relative economic value of each county's fur harvest?

2. What is the relative importance of each species to each county?

3. What is the direct economic value to California of harvesting the three species?

The information acquired by answering these questions is presented here.

Methods

County Values

The following methods were used to determine the relative economic value of each county's fur harvest. The value of each county's harvest is represented by the average annual value, measured in dollars, of the animals taken in each county during the study period.

The average annual values were calculated using equation 1:

where:

and:

y is a number from 1 to 11 where 1 represents data for 1969, 2 for 1970, etc.;

B is the statewide average value of a beaver pelt for a particular year;^[3]

M is the statewide average value of a mink pelt for a particular year;^[3]

R is the statewide average value of a muskrat pelt for a particular year;^[3]

N is the total number of pelts reported taken of a particular species for a county and year;^[3]

V is the average value of all species for a particular county;

b is the countywide average value for a beaver pelt for a particular year;

m is the countywide average value for a mink pelt for a particular year; and

r is the countywide average value for a muskrat pelt for a particular year.

After the average value (V) of each county's fur harvest was calculated, the counties were ranked by these values, revealing the relative importance and the degree to which one county is more or less important than another in terms of their fur harvests. Degree is measured by the absolute difference and percent difference between the value of any two counties' fur harvests.

The 10 counties with the highest value of fur harvest are listed in table 1. The total average annual value for all counties is $208,335. However, because of large fluctuations in the raw fur market, this value may not reflect future values. It reflects only the average for the 11-year study period.

Species' Value to Counties

The following method was used to determine the relative importance of each species to each county. For each county, the average annual value of the fur harvest calculated for the species in equation 1 was used to determine their relative importance. This determination was made by calculating each species' percentage contribution to the total average annual value (V) of a county's fur harvest. The species were then ranked by their percentages. The most important species, that species with the highest percentage, was assigned a rank of 1, the second most important a rank of 2, and the third, or least important, a rank of 3. In the case where two species' values contributed the same percent and were less than the third species, they were both assigned a rank of 3. The results of the ranking are shown in table 2.

Economic Value to California

To determine the direct economic value to California of harvesting the three species, the value of this harvest to California was examined in three ways: 1) the value contributed by each species; 2) the value realized by individuals engaged in fur harvest; and 3) any relationship between fur harvest to the extent of riparian systems.

Value Contributed by Each Species

Absolute values and percent contributions of the species' values to each county are summarized in table 3.

Value Realized by Individuals

To determine the value of the fur harvest to individuals, the average annual income of a California trapper resulting from harvest of these species and the percent of the trapper's mean annual income it represented were calculated using information from Boddicker (1980).

[3] Calculated by DFG each year from the number of a species sold divided into the total revenue from the species.

The mean 1980 income (State of California 1980), the number of licensed trappers during 1980 (California Department of Fish and Game 1981), and the average annual value of the fur harvest for each county was used to calculate the importance of this harvest to the income of individual trappers. The ten counties where trapping of the species made the greatest contribution to a trappers annual income are listed in table 4. The value of the annual harvest of these species appears by inspection to be low for all counties and for California as a whole.

Economic Value and Extent of Riparian Systems

After the counties were ranked according to the value of the harvested species to individual incomes, correlations between the extent of riparian systems in each county and the county's

rank in terms of average annual value of the fur harvest were examined. Those counties with similar habitats produce fur harvests of similar economic value. These high-ranking counties contain large areas of riparian systems as would be expected.

Information on the extent of riparian systems was available from Katibah etal . (1980) for 41% of the counties considered in this study. The areal extent of all vegetation/coverage classifications except urban and agricultural were summed. These values do not necessarily include all of the riparian vegetation occurring in the county. Nonetheless, an attempt was made to rank the counties, based on this information, according to the number of "riparian" acres found in the county. These ranks were compared with those for economic value (from equation 1), adjusted to the same scale. The average difference in ranks between these two values for all counties was 4.3 positions, with the maximum of 14 for Tehama County and a minimum of 0.

Discussion

The Value of Fur Harvest to Counties

Equation 1 was developed to account for those factors in the fur harvest for which data are available. These factors are the number of animals taken and the quality of fur (reflected in the value). Although this equation accounts for the number of animals taken and the average value of the animals, other variables not accounted for in the equation are known to affect fur harvest. They include:

1. the number and skill of people trapping in a county;

2. the availability of animals;

3. the accessibility and weather conditions of a region;

4. the types and cycles of animal diseases, including rabies, bubonic plague, leptospirosis, tularemia, and distemper; and

5. the seasons set for taking animals.

Data on the number of each species taken in each county, and the statewide average value for each species are available for the 11-year study period. However, if only these data were entered into equation 1, (and variables b, m, and r were set to 1) the equation would account for and the counties would be ranked according to only the differences in the number of animals taken, because the average value data would be the same for each county. The quality of fur would not be taken into account.

The quality of fur is related to the animals' habitat, including such factors as temperature (duration of low temperature), elevation, rainfall, type of food available, and density of animals in an area.^[4] Because of the wide range of climates and terrains in which California riparian systems are found, from low desert washes to high mountain meadows, the quality of fur may vary widely enough throughout the state to affect the importance or rank of a county. Thus, equation 1 takes into account differences in the quality of fur between counties. This quality variable is assumed to be independent of the number of animals taken; thus, a county's rank is based on two independent variables: number of pelts taken and the quality of fur.

The quality of a fur is measured in dollars. Higher quality fur is worth more, with the fur market generally controlling demand for the particular species. The standards of quality are defined by the fur market in general and the fur buyer, to whom the pelts are sold, specifically. Standards are defined for each species; species are not judged against each other. The quality of fur produced by a county is measured by an annual county-wide average value.

Data from the DFG on these county-wide average values were available for only 4 years, the 1969 to 1972 seasons. Thus, county-wide average values for 1973 to 1979 seasons had to be derived. First, a ratio was calculated of the average value of the harvest in a county to the statewide average value. Ratios were calculated for each county, species, and year that data were available. The average of these ratios was then calculated. Table 5 shows the ratios for Siskiyou and Plumas counties.

It was expected that the annual ratios would stabilize around a particular point, but this did not necessarily happen. Siskiyou County average values were consistently above the statewide average; the ratios are greater than 1.00. However, Plumas County ratios are consistently above and below the statewide average for beaver and muskrat respectively, and inconsistent for mink.

Assuming that with additional data, the annual ratios would stabilize around a number close to the average of values for 1969 to 1972, these averages were used to derive the annual average values for 1973 to 1979. The average values, as shown in table 5 for Siskiyou and Plumas counties, are represented by variables b, m, and r in equation 1 and are referred to as average value factors.

Several counties reported no harvest of one or more of the species during the 11-year study period. The average value factor for these counties was assigned a value of 0. Other counties

[4] Rether, Roger. Certified New York trapper training instructor. Personal conversation.

reported no harvest during the 1969 to 1972 period but did report during later years. They were assigned the average value factor of a county most alike in terrain and vegetative cover, determined by examining the Aero relief map of California.^[5]

The product of an average value factor and a statewide average value for a particular county, species, and year yields the county-wide averge value used in equation 1.

If the fur quality was not accounted for, the values and ranks of some of the counties would not be the same as those obtained in equation 1. Shasta County would, for example, trade places with Butte County. This is not considered significant, as they changed only one position in rank and the degree that Shasta is more important than Butte is only $1,129.83 or 5.6%. However, other counties shifted rank more dramatically. The shift in rank occurred for more than 30% of the counties. Accounting for fur quality in the value and rank of a county is thought to more closely represent the true relative value of a county than if the number of animals taken was the only variable.

Economic Importance of Species to Counties

The information in table 3 suggests that statewide, muskrat is the most important, beaver is the second most important, and mink is the least important of the three species, although they are not necessarily ranked in that order for each county. Although the average annual value of the species was used to determine their relative importance, it was found that the relative importance of the species is the same whether determined by average value (including fur quality) or by number of animials taken, for all but 14 counties.

A variable not taken into account in determining the relative importance of a species was legal restrictions on fur harvest in a given county. For example, the Fish and Game Code (California Department of Fish and Game 1980–81) lists 14 counties where beaver cannot be taken; however, this restriction was not necessarily in force in these counties during the entire study period. The effects this and other possible legal restrictions have on county and species rank is not known. However, of the 14 counties with restrictions in 1981, beaver ranked third for only nine counties, second for four counties, and first for one county, Santa Barbara.

The information in table 3 also suggests that determining the rank of a county is not necessarily dependent on knowing the value of all animals harvested, but perhaps only the value of the most important animal for that county. To determine if a county's rank is dependent only on the most important species, the counties were ranked according to average annual value, species by species. These rankings were then compared with the ranking shown (for the 10 highest-ranking counties only) in table 1 to identify similarities.

The ranks determined by the average annual value of each species can vary considerably, as shown in table 6 (San Mateo County, which had a rank of 27). It was found that when a county's rank was determined by the species of highest relative importance and when the relative importance was the same regardless of whether or not it was measured by value or number of animals taken, the rank of a county according to that species corresonded within one or two positions of the rank from equation 1 for: Shasta, Butte, Glenn, San Joaquin, Solano, Sutter, Modoc, Yuba, Lassen, Contra Costa, Fresno, Other, Placer, Tehama, Nevada, and Santa Barbara.

[5] Aero Service Corporation. 210 E. Courtland Street, Philadelphia, Pennsylvania. Aero relief map of California.

It should be noted that these ranks are determined by an average for an 11-year period. Graphs were prepared for each county which had a continuous harvest for nine or more years, showing number of animals taken and the average annual value for the study period. These graphs were analyzed to better understand the importance of each species to each county, in terms of number of animals, frequency of take, county-wide average value, and number of successful trappers (statewide), to answer the question of what determines its relative importance. An example is presented in figure 1.

Figure l.
Plot of fur harvest trends for Shasta County muskrat, 1969–79.

Importance of Harvest to Individuals

The characteristics of an average trapper compiled from a nationwide survey of trappers conducted by Boddicker (1980) at Colorado State University during 1978 and 1979 were examined. Although this was a nationwide survey, I assumed, after consultation with Boddicker, that the results would apply to California. This assumption was verified to the extent the existing data allowed. The results of this survey did not distinguish between those people taking riparian species and those taking other species; however, data could be adjusted to cover only riarian species. This information was used to obtain the average annual value to California trappers of the fur harvest, by county. Some of these values are shown in table 4.

Findings and Conclusions

1. Harvest of beaver, mink, and muskrat has been and can continue to be an economically and biologically viable sustained-yield harvest. This harvest has been largely overlooked by conservationists and planners because of: a) lack of information; b) lack of involvement in the harvest by a large segment of the population; and c) lack of interest in conserving these species by a large segment of the population, in part because of the damage they can cause and their nocturnal habits.

Protection of riparian systems can be based in part on economic values of the fur resource component of the system. When planning for conservation of riparian systems, sustained harvest of furbearers should be considered.

2. The rank of a county based on the economic value of fur harvested can, for some counties, be determined by examining only the value of the three species.

3. The economic importance of trapping and fur resources has wide-ranging implications. On a local level, trapping provides residents with additional income, often making a substantial contribution to an individual's livelihood in both an economic and non-economic sense. Fur harvesting of these species has provided up to 7% of an individual's income.

4. Statewide, harvesting beaver, mink, and muskrat is not economically important. It is, however, important to individuals in counties with a lower mean annual income.

5. Statewide, muskrat is the most important, beaver the second, and mink the least important species of the three, in terms of monetary values.

6. Counties with riparian systems of similar extent produce fur harvests of similar economic value. The greater the extent of riparian system, the higher the economic value of the fur harvest.

7. The quality of fur and thus the quality of furbearer habitat does influence the economic value of the fur harvest produced by a county.

Literature Cited

Boddicker, M.L. 1980. Profiles of American trappers and trapping. Proceedings of the first worldwide fur bearers conference. [Frostburg, Maryland, August 1980]. In press.

California Department of Fish and Game. 1969–80. Licensed trappers' report. California Department of Fish and Game, Sacramento, Calif.

California Department of Fish and Game. 1981–82. Fish and Game Code, Section 462.463, 2251.5, 465 Title 14. California Department of Fish and Game, Sacramento.

California Department of Fish and Game. 1981. List of 1981 licensed trappers, Department of Fish and Game, Sacramento.

Katibah, E.F., N.E. Nedeff, and K.J. Dummer. 1980. Areal and linear extent of riparian vegetation in the Central Valley of California. Final report to the California Department of Fish and Game, Sacramento. Remote Sensing Research Program, University of California, Berkeley.

Kellert, S.R. 1979. Public attitudes toward critical wildlife and natural habitat issues. Phase I (final report). 148 p. Yale University, School of Forestry and Environmental Studies, New Haven, Conn.

Nelson, Lewis, Jr., and Jon K. Hooper. 1976. California fur bearers and their management. Leaflet 2721, Cooperative Extension, University of California, Berkeley.

State of California. 1980. California Statistical Abstract.

New York Department of Environmental Conservation. About fur bearers and trapping in New York. Albany, N.Y.

US Department of Commerce. 1981. Facts about the United States fur industry. Voice of the Trapper July 1981.

USDI Fish and Wildlife Service. 1971. Fur catch in the United States, 1970 wildlife. Leaflet 497, USDI Fish and Wildlife Service, Washington, D.C.

Compatibility of Biofuel Production with Wildlife Habitat Enhancement^[1]

John Disano, Bertin W. Anderson, Julie K. Meents, and Robert D. Ohmart^[2]

Abstract.—A stand of native cottonwood trees (Populusfremontii ) with hedges of quail bush (Atriplexlentiformis ) would attract high avian densities and diversities. Densities and diversities of birds and rodents reached above-average levels for riparian vegetation in the lower Colorado River valley within two years from planting on two experimental plots and within one year on a third plot. The rapid growth rate of native trees and the acceptance of revegetated areas by wildlife, in conjunction with the current demand for wood as fuel, suggests that the two objectives are compatible and the latter can be economically productive.

Introduction

Since 1977 we have been studying the feasibility of reintroducing native riparian vegetation along the lower Colorado River. Previously, there was virtually no information about the environmental conditions necessary for growth or survival rates of native plant species used in revegetation. We have also studied the densities and diversities of wildlife species associated with the reintroduced vegetation.

The objectives of this report are to discuss: 1) environmental conditions which lead to greatest productivity of trees; 2) the economic potentials of tree farming; and 3) a planting and harvest rotation design which would be compatible with wildlife enhancement.

Factors Affecting Growth and Survival of Trees

We planted about 2,000 trees of four species in February 1979 on a riparian zone dredge-spoil site along the Colorado River about 16 km. south of Blythe (Riverside County), California. The ensuing discussion is based on generalizations drawn from data collected from growth of those trees. Details of this study have been presented elsewhere (Anderson and Ohmart 1981a). Anderson and others will present details concerning the growth of cottonwood (Populusfremontii ) elsewhere in these proceedings (Anderson etal . 1983).

Importance of Tillage

Tillage is defined as breaking up and mixing the soil. In this case tillage was provided with a power auger or backhoe at each tree planting site. When planted in sandy soil, cottonwood and willow (Salixgooddingii ) trees grew to an average height of 5 m. within 680 days of planting if the saplings were provided with tillage to a depth of 3 m. (fig. 1). With no tillage, growth averaged less than 2 m. The advantage of deep tillage is that it permits rapid root penetration to the water table. The effect of deep tillage was not pronounced during the first six months, but thereafter saplings with deep tillage rapidly outgrew those without tillage (fig. 1). Tillage also affected survival of the trees. Among 112 trees planted with no tillage, 43 (38%) died by the end of the second growing season; among 772 trees provided with tillage to 3 m., 20 (<3%) died. Tillage to 3 m. was the single most important factor affecting growth and survival in our study.

Other Factors Affecting Growth

Competition

Competition from other vegetation, which invaded tree planting sites as a result of irriga-

[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].

[2] John Disano is Research Biologist, Bertin W. Anderson is Faculty Research Associate, Julie K. Meents is Research Biologist, and Robert D. Ohmart is Associate Director; all are at the Center for Environmental Studies, Arizona State University, Tempe, Arizona.

Figure l.
Effect of tillage on growth of cottonwood
trees at three time intervals after planting in
sandy soil along the lower Colorado River.

tion, seriously affected growth and survival of trees. Among 77 willow and honey mesquite (Prosopisglandulosa ) trees planted with tillage to 3 m. which received moderate to severe competiton from Bermuda grass (Cynodon dactylon ), Russian thistle (Salsolaiberica ), and smotherweed (Bassiahyssopifolia ), 35 (45%) had died after two growing seasons, and growth of the survivors was significantly less than in areas where trees had little competitive interference.

Control of weeds potentially involves one of the greatest expenses associated with tree farming. This can be minimized if trees are planted in areas where the surface soil is very sandy, at least to a depth of 1 m. Competing species have difficulty becoming established because of the high temperatures and extremely dry conditions typical of surface sand.

Soil Density

Tree growth decreased as soil density increased below a depth of 1 m., even when tillage was to 3 m. and competitive vegetation was absent (Anderson and Ohmart 1981a). As the soil included more clay, tree mortality also increased. We recommend that tree farming not be attempted in dense soils, especially those containing significant amounts of clay.

Length of Irrigation Period

Growth rates were maximum and survival approached 100% when planting was in sandy soil, tillage was to 3 m., and irrigation was continued for 150 days at 30–40 1. of water per day. As soil density and competition from other vegetation increased, longer periods of irrigation were necessary (ibid .).

Avian Use of Riparian Vegetation

Value of Trees

In our studies of avian use of riparian vegetation along the lower Colorado River, we found that high avian density and species richness were consistently associated with stands of cottonwood and willow trees (Anderson and Ohmart 1981a, b; 1983). This was especially true for insectivorous bird species (fig. 2). Doves were attracted more to stands of mesquite trees (fig. 2). Frugivorous birds were virtually absent in stands of cottonwood and willow, but were primarily associated with honey mesquite. This, however, was not because of any intrinsic value of mesquite iself. Honey mesquite is parasitized by mistletoe (Phoradendron californicum ) to a greater extent than other species of trees along the lower Colorado River. Frugivorous birds eat mainly mistletoe fruit (Anderson and Ohmart 1978).

Value of Shrub-Like Vegetation

The term shrub-like refers to herbaceous vegetation such as Russian thistle, smotherweed, inkweed (Suaeda torreyana ), and quail bush (Atriplexlentiformis ). To examine the value of shrubs and shrub-like vegetation to birds, we compared three revegetation sites with varying shrub densities and composition. On one site shrub density was about one shrub per ha. Russian thistle and smotherweed reached densities of about 1,500 mature plants per ha. on a second site; quail bush and inkweed reached a combined density of 1,500 plants per ha. on a third site. All three areas had cottonwood and willow trees 3–7 m. tall, planted at a density of about 20 trees per ha. The combined density of all other tree species was less than one per ha.

Densities (number per 40 ha.) of various groups of birds on the revegetation sites were compared with the average bird densities in natural riparian vegetation along the lower Colorado River. These densities are expressed in standard units; thus, the average density for riparian vegetation is zero; positive and negative numbers indicate values above and below average, respectively.

When herbaceous vegetation was sparse, all avian groups except passerine granivores had below-average densities (fig. 3A). When herba-

Figure 2.
Number of times various vegetation variables were included
as a step in a significant multiple linear regression equation
(Y-axis). Maximum value possible for Y-axis is 25 (5 seasons
for 5 years). DVI—density of visiting insectivores; DPRI—
density of permanent resident insectivores; CW—number
of cottonwood/willow trees; HM—honey mesquite; SB—
screwbean mesquite; and SC—salt cedar per 0.4 ha;
FDD—foliage density and diversity at 0-0.6 m.

Figure 3.
Populations of various avian groups associated with various kinds and
densities of shrubs, other variables being equal. A. Very low density.
B. Shrubs abundant, primarily Russian thistle and smotherweed. C.
Shrubs abundant, primarily quail bush and inkweed. Avian densities
are expressed in standard deviation units where the mean was for all
riparian vegetation-types found in the lower Colorado River valley.
TOTAL SP—total species; VI—density of visiting insectivores;
PRI—density of permanent resident insectivores; GR—density of
passerine granivores; and GQ—density of Gambel Quail.

ceous vegetation consisted of Russian thistle and smotherweed, avian species richness and densities of passerine granivores and Gambel Quail (Lophortyxgambelii ) were above average, but densities of insectivores were about average (fig. 3B). When herbaceous vegetation consisted of quail bush and inkweed, all bird groups were above average, although visiting insectivores were near average (fig. 3C).

We conclude from these data that an area can be maximally enhanced for birds if cottonwood and willow trees, and shrubs such as quail bush and inkweed are planted. Dove densities would probably be somewhat below average, and frugivores would be absent unless honey mesquite with mistletoe was also present. Cavity-nesting bird spe-

cies, including woodpeckers, Lucy's Warbler (Vermivoraluciae ), Ash-throated Flycatcher (Myiarchus cinerascens ), and Wied Crested Flycatcher (Myiarchustyrannulus ) would be present in very low densities.

Profits from Farming

It has been shown that the trees grow rapidly and that tree farms could be attractive to wildlife, especially birds. By the end of the fourth growing season, trees grown in relatively sandy soil with tillage to 3 m. and competitive vegetation controlled will yield about one cord of wood per tree. In the Colorado River area one cord of wood sells for $80–$100 per cord (Fairbank 1980). We estimate a total production cost of $60 per tree, not including land rental or purchase. This estimate includes expenditures for management, labor and secretarial needs, moderate clearing and leveling requirements, irrigation system purchase, installation, and maintenance for 150 days, as well as harvest and transportation costs. An area planted with trees centered at 6 m., a 10% mortality rate, no harvest until the fourth year, which is then harvested at a rate of 40–60 ha. annually would yield a profit approaching $1 million in 12 years of operation if the wood was sold at $80 per cord (fig. 4). We believe our cost and mortality estimates are somewht high and the value per cord conservative. Our cost-income estimates are based on the design in figure 5.

Figure 4.
Profit estimates for a tree farm encompassing 160 ha. over
a 12-year period. See text for assumptions concerning
expenses and income. Mil = millions of dollars.

Honey mesquite wood is valued at twice as much per cord as cottonwood and willow (ibid .), but honey mesquite trees grow much more slowly. Nonetheless, because of their economic value, honey mesquite trees would be an important species in a tree-farming operation.

After first cutting, trees will sucker at the root crown without additional irrigation if the water table is 4.6 m. or less from the surface. We assume that biomass production in the four years after the first cutting will be decreased by 50%. This may be a conservative estimate because we have obtained greater growth rates after trees have been harvested than from initial plantings. However, our data are for only one year. If this high growth rate continued for four years, the profits indicated here might also prove conservative. The main disadvantages of tree farming are the high initial costs and the relatively long period until first returns on the investment.

Rotation of Harvest

A large variety of planting and harvest schemes are possible. We present only one for the purposes of discussion.

It is desirable to develop a tree farm so that the entire area is not harvested at one time. This would increase the diversity of vegetation height and foliage density on the area. If habitat values for wildlife are to be enhanced, it is also desirable to plant shrubs. Planting shrubs introduces extra costs, but will greatly enhance the area for wildlife.

Shrubs also have practical value. For example, it is desirable to have fire lanes;

Figure 5.
A plan for planting and harvesting trees on a 160-ha.
plot. The plot is divided into eight subplots of 20 ha.
each. Arabic numerals represent year of planting; Roman
numerals represent the year of harvest; e.g., 1, IV, VII,
 and X in one block mean that the first block would be
planted the first year and harvested the fourth, seventh,
and tenth years after planting.

these can include roads bounded by hedges of quail bush. Quail bush is an evergreen species and is fire resistant. Once established, quail bush is a vigorous competitor and will reduce costs associated with controlling invasion by salt cedar (Tamarixchinensis ) and annual weeds (fig. 5). Dried annuals and the large volume of litter produced by the deciduous salt cedar greatly increase fire hazards.

The cottonwood community development envisioned includes planting trees on 38% of the area each of the first two years and 24% the third year. In the fourth year the first trees would be harvested (fig. 5). With this rotation there would be four-year-old trees 10–15 m. tall with relatively open areas, attractive to quail and doves. On our revegetation sites, trees of this height attracted, among others, Northern Orioles (Icterus galbula ) and breeding Yellow-billed Cuckoos (Coccyzusamericanus ), a species listed as endangered in California. The combination of trees and quail bush would attract above-average densities and diversities of birds in all seasons. If some trees four or five years of age were not harvested but were killed by girdling, the area would also become attractive to at least a few cavity-nesting species.

Acknowledgments

We wish to thank Jeannie Anderson, Susan M. Cook, Jane R. Durham, Dr. Julie K. Meents, and Cindy D. Zisner for editorial assistance. Marcelett Ector and Cindy D. Zisner typed the numerous drafts of the manuscript. Elaine Hassinger and Julie Huff prepared the illustrations. We are grateful to Dr. F. Aljibury, Les Ede, and Jule Meyer, University of California Agricultural Extension Service, Riverside California, for their advice and cooperation. Ronald Gass, Mountain States Wholesales Nursery, Phoenix, Arizona, kindly provided many trees. The work was jointly funded by the USDI Bureau of Reclamation and the USDI Fish and Wildlife Service Contract No. 7-07-30-V0009.

Literature Cited

Anderson, B.W., J. Disano, D.L. Brooks, and R.D. Ohmart. 1983. Mortality and growth of cottonwood on dredge-spoil. In : R.E. Warner and K.M. Hendrix (ed.). California Riparian Systems. [University of California, Davis, September 17–19, 1981.] University of California Press, Berkeley.

Anderson, B.W., and R.D. Ohmart. 1978. Phainopepla utilization of honey mesquite forests in the Colorado River valley. Condor 80:334–338.

Anderson, B.W., and R.D. Ohmart. 1981a. Vegetation management final report. In prepara- tion. USDI Bureau of Reclamation, Boulder City, Nevada.

Anderson, B.W., and R.D. Ohmart. 1981b. Agricultural final report. In preparation. USDI Bureau of Reclamation, Boulder City, Nevada.

Anderson, B.W., and R.D. Ohmart. 1983. Avian use of revegetated riparian zones. In : R.E. Warner and K.M. Hendrix (ed.). California Riparian Systems. [University of California, Davis, September 17–19, 1981.] University of California Press, Berkeley.

Fairbank, W.C. 1980. On biofuel for air conditioning: a preliminary evaluation of using mesquite for powering residential and small commercial air conditioning systems in the Colorado River Desert. 9 p. Cooperative Extension, University of California, Riverside, Calif.

Considerations of Riparian Biomass for Management As an Energy Source^[1]

Gary Brittner^[2]

Abstract.—The biomass resource of riparian zones has potential for management for production of many products including energy. Riparian zones have been too long ignored for this potential. More intensive management of this important resource is needed to realize these benefits. A balanced approach is needed; environmental concerns need to be addressed.

Introduction

As described in the Conference announcement, the State's riparian systems are productive in many ways, but there is much we still need to find out about them. Our knowledge of the systems could tolerate improvement. Our current management, or as some may insist, mismanagement, of riparian systems could be better organized. Better management would bring about greater benefits from all resource areas.

Riparian systems are particularly important from the standpoint of renewable resources. Throughout much of California, the limiting factor for vegetation growth is water. Water is one common denominator among riparian systems and is the primary element that makes these systems so productive. From the forest production standpoint, riparian environments are especially important. These sites are usually most productive since the moisture requirements of fastgrowing mesic plants are satisfied.

Riparian Wood Products

Fiber

The fiber resource of riparian areas outside of commercial timberland is not well documented. These sites have great potential for wood production, but little effort has been focused in this direction. Wood demand is constantly rising in California; demand for wood products will triple in 50 years. The land base for commercial timber is constantly shrinking due to conversion to other uses: residential expansion, road construction, parks, and power line rights-of-way. To meet the increasing demand we will have to grow more fiber on fewer acres, look for alternative products, and recycle. Another solution to ease the problem is to manage our productive riparian zones more intensively. Getting more wood from these underutilized sites would ease some of the pressures on commercial timberlands.

The fiber resource of riparian zones is not only important for traditional products like lumber and chips for pulp, but also has great value for energy production. Plants are solar energy collectors and this energy can be harnessed through combustion.

Fuel

In California, there are over 124 megawatts of wood-fired power generated yearly, or enough power to meet the needs of a city of 75,000. By 1985, the figure is expected to jump to 625 megawatts. By the year 2000 we expect over 1140 megawatts to be produced in this way.^[3] According to the American Pulpwood Association, on a national scale wood supplies more energy now than does nuclear power.^[4]

Use of wood has many beneficial aspects. It is clean-burning and, unlike petro-fuels, does not produce noxious compounds; is a renewable resource; decreases our demand for foreign oil imports; stimulates local economies; and helps us to conserve our own finite resources.

Management Potentials

To develop the wood resource, there are a number of management schemes that can be

[1] Paper presented at the California Riparian Systems Conference. [University of California, Davis, September 17–19, 1981].

[2] Gary Brittner is Forrester II, California Department of Forestry, Sacramento, Calif.

[3] California Department of Forestry. 1981. Wood energy in California. 150p. Sacramento, Calif.

[4] American Pulpwood Association. August, 1981. Pulpwood highlights.

employed, depending upon the needs and desires of the owner or manager. One new approach is termed short-rotation intensive silviculture. Trees are essentially farmed for fiber in a short period of time, 2–8 years. This is a marked contrast to the longer rotations commonly associated with saw timber with a minimum rotation of about 40 years. Short-rotation intensive silviculture planting stocks are usually exotics, hybrids, or superior phenotypes that have been selected for breeding. Fertilizers, herbicides, and cultivation are used to accelerate crop growth. This management technique is one extreme of the spectrum and it is doubtful it should become a common practice in riparian zones. The other extreme is to totally ignore the resource and let the trees grow in a totally unmanaged state. This would be wasteful, and in a world of increasing demand on dwindling resources, is unacceptable. A balanced system between these two approaches is needed.

Manipulation of the fiber resource of riparian zones would provide many benefits. Increased planting on stream banks and floodplains can have positive effects on erosion control and stream bank protection, and make a contribution to the wood supply. Hardwoods are the main component in most riparian vegetation outside of commercial timberland zones. In California, the hardwood manufacturing industry is largely undeveloped and is likely to grow in coming years. Hardwood products include lumber for cabinets, furniture, pallets, ties, dunnage, chips for pulp, hog fuel for energy production, and firewood.

Clearing dense riparian growth can improve access to rivers and lakes for the benefit of recreationists for fishing, camping, swimming, and other activities. Planting and harvesting operations can be coordinated with wildlife managers to alter habitats for the improvement of animal populations. Also, more intensive management of the fiber should include fuel management activities to decrease potential for the spread of wildfire.

A well-orchestrated management plan for the riparian fiber resource would have many positive impacts on a variety of resources as I have briefly outlined. Implementation of this type of management does not require any exotic techniques or untried schemes. Application of existing methods would result in better utilization of the fiber resource.

The Wood Energy Program

The California Department of Forestry's Wood Energy Program is actively seeking ways to improve utilization of wood for the production of energy. Some of the pilot programs offer various means to improve the quality of the riparian fiber resource, and ways to utilize it.

Biomass Demonstration Projects

We are in the process of selecting sites for a number of biomass tree farming demonstration projects. This program was established by the Legislature to locate lands suitable for biomass farming, and determine which trees and plants are most suitable for high net energy yields. Care must be taken not to interfere with efficient use of forest and food crop lands. We will be researching the silvicultural techniques necessary to produce tree crops and identify and resolve problem areas in order to facilitate implementation of biomass farming by the private sector. Other products besides fiber and fuel will be identified. The program will be administered statewide, so we should be able to gather management information that will have applicability to many different covertypes. This information will be extremely valuable for management on all California lands.

Mobile Wood Densifier

In response to needs for better fuel management and utilization of wood waste, such as logging slash, precommercial thinnings, and cull trees, the Department has begun construction of a mobil wood densifier. The densifier will produce densified wood cubes at a rate of 1.5 tons per hour. The wood cubes can be substituted for charcoal briquets in barbeques and campfires, and for logs in fireplaces and wood heaters, and provide an excellent fuel for biomass burners and gasification.

Any clean woody substance is acceptable raw material for densification. The material is processed first by reduction through a tubgrinder. The shredded material is then passed through a modified hay cuber to produce the briquets. The Department of Forestry has contracted with the Papakube Corporation of San Diego to build the unit. Field testing should begin in the first quarter of 1982. This machine will be able to utilize woody material that has in the past been wasted, and turn it into a useable product. The machine will be valuable because it can consume as raw material, lower quality trees that are frequently associated with riparian zones. This project should help demonstrate the importance of looking at all biomass forms as potential resources.

Biomass Processing Technology

Technology to process biomass into more useful energy forms such as alcohol, fuel, and chemicals is proceeding at a rapid pace, yet the vast biomass resource remains largely untapped. Development of the resource is hindered by the need for economical harvesting methods. Currently there is no economical way to cut, collect, and transport biomass resources, such as logging slash, chaparral, and small hardwood species. To expedite a solution to the problem, the Department of Forestry proposes to work with university engineers and private industry, to fund research to develop machinery for the job.

The Department will take a leadership role in this field to make sure development takes into consideration the environmental impacts which can result from harvesting. Mechanical equipment will be designed to have minimum impact on the land. This is a very worthwhile project. It will shorten the length of time needed for technology and the resource to mesh and will lessen the demand for the other energy sources upon which we now rely. These biomass harvesting systems to be developed could be useful for harvesting in sensitive riparian zones.

Environmental Concerns

Before we plunge headlong into widespread harvesting of biomass, many environmental concerns must be addressed. Thorough studies of the environmental consequences of our proposed activities should be undertaken on a variety of natural system types. Such studies will help us avoid deleterious impacts. This is especially true for the fragile and fertile riparian zones. It is important to analyze what effects harvesting will have on nutrient cycling, plant succession, bank stability, wildlife populations, recreation, and other resources. These considerations are important aspects of a coherent management system. The Department will consider these items as we work together to provide management for this important productive resource.