Regeneration of Riparian Forests of the Central Valley^[1]

Jan Strahan^[2]

Abstract.—Riparian forests of the Sacramento River have an overstory and a regeneration pattern corresponding to the successional stage and fluvial landform associated with the forest stands. Cottonwood/willow forests form initially on gravelbars. With development of the floodplain and maturation of the forest, other species enter. Floodplain forest regeneration is primarily box elder, black walnut, and valley oak with few sycamore and ash. Riverside floodplain forests differ from oxbow lake forests in species diversity, density, and reproduction. Land use and water development projects alter fluvial landforms and fluvial events to create changes in forest composition and regeneration.

Introduction

Riparian systems provide an excellent opportunity to study the effects of landform and fluvial processes on vegetation distribution and forest regeneration. Erosion, deposition, and lateral channel migration regulate both the distribution and development of vegetation in the riparian zone. With continual changes in landforms as a result of seasonal and catastrophic fluvial events, vegetation dynamics remain in a state of "perpetual succession" (Campbell and Green 1968).

The generalized patterns of vegetation zonation resulting from fluvial processes have been described and illustrated by Conard etal . (1977) for the Sacramento Valley region. The Sacramento River Atlas (Upper Sacramento River Task Force 1978) illustrates the pattern of zonation as well as the successional stages found in riparian forests. McGill (1975, 1979) has also correlated the existing riparian vegetation with fluvial landforms. Gaines (1974) has noted that the more extensive remaining riparian forests occur on islands, along bends in the river, and adjacent to oxbow lakes and other areas subject to flooding. As such, the remaining forests are a result of the most dynamic interplay between the fluvial system and riparian vegetation.

Objectives

The primary objective of this study was to develop regeneration data for the dominant tree species in the riparian forests of the Sacramento River. The information compiled can be used to assess present conditions and future trends of the forests. Work by Conard etal . (1977) and Michny etal . (1975) illustrates the variety of plant community structure and composition encountered in the riparian zone. Recognizing this, information was gathered at two levels to gain a more comprehensive picture of the structure and composition than previously developed. A detailed quantitative study was undertaken at three sites near Princeton, Glenn County, where disturbance to the fluvial system and vegetation is relatively minimal. At these sites, regeneration was examined relative to landform in different successional stages: a young forest (less than 30 years) associated with a gravelbar; an established forest (less than 70 years) located on the floodplain along the current river channel; and a mature forest (greater than 85 years) adjacent to an oxbow lake. To understand the larger patterns occurring along the length of the river, a broad survey of the river as a fluvial system was undertaken. This survey relates the effects of land use and water resource development projects to regeneration potential and stand development. The survey included a review of the geomorphic and ecological literature as well as air and ground reconnaissance.

Methods

Floodplain vegetation was sampled using the point-centered quarter method of Cottam and Curtis (1956). Transects were located perpendicular to the water course at 50-m. intervals.

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

[2] Jan Strahan is a Graduate Student, Wildland Resource Science, Department of Forestry and Resource Management, University of California, Berkeley.

Points were centered at 10-m. intervals. At each point-center a 1-m. circular plot was used to tally the number of tree seedlings by species and a 10-m² circular plot was used for saplings. Seedlings were defined as having become established this season, and saplings were classed as other size-classes less than 10-cm. diameter-at-breast-height (DBH). Saplings were further classified into seven size-classes: (1) less than 0.3-cm.; (2) 0.3- to 1-cm.; (3) 1- to 1.5-cm.; (4) 1.6- to 2.4-cm.; (5) 2.5- to 5-cm.; (6) 5- to 7.5-cm.; and (7) 7.5- to 10-cm. Vegetative reproduction was not distinguished from seed reproduction as part of this tabulation, but was recorded wherever observed. Composition and cover of the shrubs and groundcover were also recorded. On gravelbars, seedling establishment was sampled through the use of 1-m² plots. Five-m. by 20-m. belt transects were used in the young forests on gravelbars and for levee sampling.

Physiography

Fluvial processes result in a number of characteristic landforms. Floods contribute to overbank deposition and aid in the building of floodplains. Lateral channel migration results in progressively building point bars which account for much of the existing natural topography of the Sacramento River riparian zone (Leopold 1973; Brice 1977). A cross-section through the riparian zone may have the following landforms: cut bank, point bar, natural levee, floodplain, oxbow lakes, meander scars, and islands. Variable surface features occur on these landforms, depending on the type of aggradation and frequency of flooding. The microtopography of the floodplain, consisting of ridges and swales, was formed by flows of old channels and is periodically altered by flood channel flows (Nanson and Beach 1977). These slight variations in elevation lead to considerable differences in soils and drainage conditions which provide the opportunity for tree species with different flood tolerances to occupy different elevations of the floodplains (Hosner and Minckler 1960). Vegetation, once established, also plays an active role in the depositional environment by acting as a sediment collector. Erosional bowls frequently form around trees and shrubs in the active channel.

Distinct landform changes occur in the downvalley progression of the Sacramento River. Brice (1977) describes the following features which change in the reach between Chico Landing and Colusa. As with most rivers, there is a downward progression in gravel size as one moves downvalley. The Sacramento River is classified as a gravel-bed stream from Red Bluff to Glenn. Below Glenn, it is a sand-bed stream. (Note: This shift was noted by Bryan in 1923, prior to the construction of Shasta Dam.)

Natural levees are composed of coarser materials deposited as floods flowed over the top of channel banks. Beginning at Hamilton City, the levees form a strip 4.8 to 8 km. (3 to 5 mi.) wide between Hamilton City and Colusa. Levees are discontinuous for several miles south of Stony Creek and continuous from near Butte City southward.

Brice (1977) also reviews the changes in the river which have occurred since white settlement, using the "natural" river of 1870 as a baseline. According to Brice, channel sinuosity has decreased while channel width has increased. Morphologic changes have been attributed to both clearing of riparian vegetation and the effect of levees in reducing overflow areas. These changes have caused the main river channel to be scoured deeper and wider and water velocities to increase. Meander loops from Butte City to Colusa are confined by artificial levees and tend to be distorted and unstable. Flow regulation by Shasta Dam has resulted in an increase in mean monthly flows at Red Bluff for June, July, and August from 6,190 ft³ /sec. (1889 to 1944) to 10,520 ft³ /sec. (1945 to 1970). Maximum observed flood peaks at Red Bluff before regulation attained about 250,000 ft³ /sec. with subsequent peaks of 140,000 ft³ /sec.

The California State Department of Water Resources (McGill 1979) identified 29,352 ha. of riparian zone from Butte Creek to Keswick Dam in 1977. This includes 3,828 ha. high terrace riparian vegetation (rarely flooded), 3,395 ha. low terrace (frequently flooded), 2,096 ha. gravelbars, 162 ha. oxbow lakes, and 3,942 ha. water surface, for a total of 13,423 ha. undeveloped lands. Agricultural lands comprise 14,852 ha. of the zone and 1,097 ha. are in other developed uses. Of particular significance in this study is the reduction of high terrace lands by 15% in the five years between 1972 and 1977, mostly through agricultural conversion. Erosional losses from bank undercutting are not concurrently offset by building processes.

These variations in physiography have major ecological significance in the riparian zone. Lindsey etal . (1961) attributed the different plant communities to the differences in soil-water relationships resulting from physiographic variation. The amount of floodplain activity and influence of the river on landforms results in different degrees of community stability. Wilson (1970) found stabilized forest communities developed along the Missouri River floodplain after the river had been stabilized by a series of dams and reservoirs. Campbell and Green (1968) link "perpetual succession" to rivers which actively meander over their floodplains. They found the frequent shifting of landforms and channels resulted in early successional stages occupying the majority of the floodplain. Everitt (1968) and Fonda (1974) attributed spatial distribution of the riparian plant communities primarily to the meandering pattern of the river.

Along the Sacramento River, physiographic variation was sampled through the use of transects perpendicular to the river. The three main

landform categories sampled were: gravelbar, floodplain adjacent to the riverside, and floodplain adjacent to oxbow lakes. These three categories are representative of a sequence of landform and soil development which led to progressively older forests with distance away from the channel.

Forest Establishment and Composition

Establishment and distribution of species in riparian forests is controlled by the interaction between fluvial events and ecological requirements of the species.

Fluvial Processes

The water regime of the river influences distribution through both seasonal fluctuations and catastrophic occurrences (Sigafoos 1964; Bell and Johnson 1974). Both the low-flow regime and high flows or floods causing inundation influence distribution. The low-flow regime, which provides freshly exposed surfaces, is the most important factor for successful seedling establishment and is critical for survival of young trees.

Whether the result of flooding is an adverse or beneficial effect on the plant is dependent on the frequency, duration, and depth of inundation (Teskey and Hinckley 1978). Susceptibility to flooding affects species location on the floodplain relative to the height of the water table. Tolerance to flooding may also vary between young and old trees of the same species (Lindsey etal . 1961). Inundation may result in the death of young or established plants through mechanical abrasion or through lack of sufficient soil oxygen. For established dormant plants, floods deposit soil nutrients necessary to maintain high productivity rates (Johnson etal . 1976). Time between major disturbances determines the amount of forest stands that will be in early, middle, or late successional stages throughout the floodplain. Both scour and fill processes, resulting from high flows, determine vegetation patterns: a flood may eliminate a portion of a mature forest through bank undercutting with the undercut material forming new depositional surfaces for seedling establishment further downstream. Aside from being the agent of plant mortality, flooding can also cause topping or "flood-training" (Sigafoos 1964) of both young and mature trees, resulting in the formation of sprout groups.

Ecological Characteristics

Ecological characteristics of the dominant tree species are important determinants of successional events in the riparian zone. Of particular importance are the light-weight seeds of the pioneer species dispersed by wind or water. Seed disperal at the time of a falling water level is essential for successful establishment of the pioneer species. These characteristics result in the initial colonization of a site by the pioneer species cottonwood (Populusfremontii ) and willow (Salix spp .). Shade intolerance of cottonwood and willow has been noted to be the limiting factor in preventing their establishment in mature forests as well as the need for a mineral seedbed for germination (Sigafoos 1964; Johnson etal . 1976; Lindsey etal . 1961). Mid-successional stages have species with both light-weight seeds (box elder, ash) and heavy seeds (black walnut, oak). However, they all are able to germinate through litter and under the canopy of a cottonwood/willow forest.

Results

The interaction of fluvial events, landforms, and autecological requirements has led to the development of heterogeneous forest stands along the Sacramento River. The following tree species were encountered frequently in the floodplain forests: box elder (Acer negundo ssp. californicum ), Fremont cottonwood (Populusfremontii ), willow (Salix spp., including S . hindsiana , S . laevigata , S . gooddingii , S . lasiandra , and S . lasiolepis ), and black walnut (Juglanshindsii ). Sycamore (Platanusracemosa ), valley oak (Quercuslobata ) and ash (Fraxinuslatifolia ) occurred less frequently.

Overstory composition associated with each landform in the intensive survey is shown in table 1. This mixed riparian forest (of the species listed above) is found in different successional stages along the majority of the floodplain. As shown by the table, early stages are usually pure cottonwood/willow. Mid- and late-successional stages frequently have a cottonwood/willow overstory and oak and sycamore ocasionally. Box elder, black walnut, and ash comprise the second canopy layer in these later stands. The broad survey also revealed different types of forest stands than encountered in the intensive survey of the river. There are several older groves of pure oak or oak/sycamore on high terraces. Within the later stages of the mixed forest, small stands of pure box elder or box elder/black walnut, approximately 200/m² or less, were encountered in several locations. In the pure box elder stands average densities were 100 stems per 100 m² . In these stands, saplings from 2.5 to 7.5 cm. were most frequent with no stems greater than 15 cm. found. Several standing dead stems and many small stems on the ground were evidence of an even higher density at one time. Shade was sufficient to prevent groundcover but a few Prunus sp . and valley oak seedlings were in the stand.

Regeneration

Establishment and survival of riparian species are related to landforms and a sequence of fluvial events (table 2). Most seedling establishment occurs along the newly exposed surfaces of gravelbars and is significantly

different in species composition than regeneration in the established forests.

Gravelbar Regeneration

Seasonal variation in flow regimes greatly influences establishment and survival of the pioneer species on gravelbars. During the winter, streamflows must remove humus and freshly fallen leaf litter from the surface so that the seeds land on mineral soil. A receding water level in late spring and early summer must coincide with cottonwood and willow seed dispersal. As establishment is directly related to the low-flow line (McBride and Strahan 1983), a 1-m. wide band of seedlings and saplings is often found along the river's edge. Prior to further flooding, seedlings must achieve sufficient size to withstand mechanical injury. The subsurface of bars must remain moist throughout the summer in order for the seedlings to withstand late summer drought. Late summer desiccation results in the death of many seedlings (McBride and Strahan ibid .). Winter floods often wash away or bury many seedlings. While density in the initial stages of establishment on bars is extremely high (table 3), the latter two factors account for significant mortality.

Floodplain Regeneration

Within the mature riparian forests of the floodplain, the link between regeneration and the flow regime of the river is not as direct. The most influential flows here are the floods which may remove seedlings established for a season or longer and at the same time prepare seedbeds. While low flows have less direct influence on these species than on those of pioneer species, McGill (1979) attributed some losses on high terraces of riparian vegetation to the lack of occasional flooding during the drought of 1976 and 1977.

In the mature forest, young cottonwoods and willows are rare while box elder and black walnut are common (table 2). The latter two species enter at a later successional stage, establishing through litter and under the shade of a cottonwood/willow canopy. While regeneration in the floodplain is currently occurring primarily in swales or on the banks of swales, young trees are much more scattered throughout the forest and much less dense (table 2), than on gravelbars. Some riverside forests 30–40 years old have little reproduction. Thus, distinct compositional differences exist between reproduction in the riverside floodplain forests and the oxbow lake forests with an increase in seedling density occurring in the oxbow lake forests. Factors limiting successful seedling establishment in the floodplain forests appear to be associated primarily with extremely dense groundcover. Grape vines were noted entwined around many dead saplings.

Succession

The successional progression of forest stands in the riparian zone begins with seedling establishment on gravelbars. The amount of available soil moisture may be an important factor governing these zonal sequences, with the younger land surfaces significantly drier than the older ones. Vegetation establishes on fresh surfaces of the point bar when sufficient sediment accumulates above summer low-water levels. Young cottonwood and willow stands do not form a continuous protective cover on the gravelbar because of the river cutting across point bars during floods. Providing floods do not alter the bar significantly, plant colonization creates additional deposits. Several inches of soil may be deposited by a single flood. As the bar builds higher, it is less frequently flooded. This deposition, in combination with channel migration, results in a stabilized floodplain developing from a shifting gravelbar.

If bars remain relatively undisturbed for a number of years, deposition gradually occurs until the floodplain supports mature cottonwood/willow forests. Eventually an understory of shade tolerant species enters the forests. Should the forests be missed by flood scouring

for many years, the cottonwood/willow may be replaced by these understory species. In places where the river has moved progressively across the floodplain in a uniform direction, a sequence of stand ages is produced, chronologically arranged in the direction of bend migration with the youngest stands nearest the river.

A broad perspective of Sacramento River successional stages is available through aerial reconnaissance. Bands of vegetation of successive ages can be found to occupy the floodplain (Murray etal . 1978). Channel lateral migration studies (Brice 1977) show the maximum ages of the forests in the intensive survey to be 32 years for the developing forest, 73 years for the riverside forest, and >85 years for the oxbow lake forest. Everitt (1968) noted similar findings for the Little Missouri River with germination and growth of cottonwood intricately related to the discharge of the river, movement of the channel, and development of the floodplain. Tree age increases both upvalley and away from the channel according to Everitt (ibid .) and is the result of the rise of sapling thickets along gravelbars.

Physiognomy

The forest structure and physiognomy differ considerably according to the age of forests and landforms on which they develop. Young cottonwood/willow forests are dense with many small trees, but have few other woody species. These gravelbar forests develop in progressive bands, each associated with a rise in elevation of the ridge-swale topography. In the older cottonwood/willow forests, the trees are tall and widely spaced, allowing sufficient light for shrub and herb development. Lianas are prominent in some stands and non-existent in others. Older forests have a two-layer tree canopy and are denser than

the mid-successional stage forests. Forests adjacent to cut banks are more frequently composed of alder, oak, or sycamore than are the forest edges that develop behind bars. Young oaks and sycamores were only found in mixed species stands, while old oaks and sycamores are found in groves without the associates. Diameter-classes (table 4) of the oxbow lake and riverside forests show the difference in species composition and structure of the two forests.

Reproductive Strategies

The most common method of reproduction is by seed. However, throughout the floodplain vegetative reproduction is also common. Sandbar willow (Salixhindsiana ) was frequently observed sprouting on higher portions of the gravelbars. This was explained by Wilson (1970) who noted an adaptive value of vegetative reproduction on sandy soils where seedling establishment is limited by surface soil moisture availability. Sprouting was also recorded on the floodplain in areas infrequently flooded: older sycamore trees frequently had basal sprouts. In areas which undergo severe mechanical abrasion from flows (banks downstream from reservoir flow releases or banks receiving a high degree of wave action from boats), vegetative reproduction was as common as seedling establishment.

Survival

In the developing cottonwood/willow forests, survival is reduced by both drought and winter flooding as well as shade and competition from groundcover. Significant attrition occurs for different stages of cottonwood development (compare tables 2 and 3). Floodplain forests had many dead trees, probably a result of the 1976–1977 drought (McGill 1979).

Discussion

Initial Establishment

The study indicates that the initial establishment of riparian forests is along point bars. Cottonwood and willow can be regarded as classic pioneer species; within this region, their seeds germinate almost exclusively on fully exposed alluvium recently deposited by the river. Not a single seedling of these species was found in any of the floodplain samples (table 3). This indicates that neither functions as a gap-phase species (Watt 1947) by establishing seedlings in forest openings following disturbance. Smaller stems of cottonwood/willow in the floodplain forests (table 4) appear to result from suppression or sprouting (Note: the two size-classes in table 3, plot 4 had all established at the same time). In areas with sufficient light, flood deposits of fresh alluvium may provide areas for a younger age class to develop. Other dominant tree species, such as box elder and black walnut, all have the ability to germinate and grow under the cottonwood/willow overstory. Without disturbance, they in time could replace the cottonwood/willow overstory.

Structure and Composition

The data also show that the structure and composition of the overstory are strongly related to stand age and horizontal and vertical position of the floodplain. For example, cottonwood and willow predominate in young stands on low terraces near the river. Ash, box elder, and black walnut enter cottonwood/willow stands over time and predominate in stands away from the river. Oak and sycamore are found in old stands on high terraces with the other dominants and along banks high above the river. Reproduction in these stands is very limited (table 5). Thus, species diversity initially increases as stands age, reaches a maximum in stands with mixtures of both pioneer and later successional species, and may decline slightly in oldest stands.

The high frequency of sapling box elder and black walnut in cottonwood/willow forests suggsts that the next successional stage will consist predominantly of these two species. However, although box elder was found in small pure patches, there is no evidence available at this time that large scale replacement of the cottonwood/willow type along the Sacramento River by these two species is occurring. Despite the establishment of cottonwood only on point bars, mature cottonwoods remain throughout the floodplain. Lateral channel migration occurs frequently enough to retain cottonwoods and willows in most stands except the few high terraces where only oak and sycamore remain.

Cultural Impacts on Regeneration

The Sacramento River riparian system is much altered both in its natural flow regime and floodplain characteristics. Land use and water resource development projects may have a significant effect on the current regeneration situation and on the future regeneration potential. While further research into these areas is necessary to provide quantitative data for the Sacramento River, correlation with other major rivers provides us with clues to changes caused by alteration of the riparian zone. Historical research, although qualitative, provides a picture of the riparian forests of the past upon which we may also draw.

Land Clearing

Removal of all but the frequently flooded areas of the riparian forests has had obvious impacts on the reduction of certain species such as oak and sycamore in the Sacramento Valley. Thompson (1961) cites several descriptions of the riparian forests prior to extensive clearing which speak of forests of oak, cottonwood, and sycamore. While the oak is found in large groves in several areas along the upper river, individual sycamores are scattered very infrequently throughout the forests. Ongoing reduction of the high terrace lands (McGill 1979) will contribute to a further reduction of these two magnificent species.

Introduced Species

The introduction of exotic species in the area has also changed species composition. For example, figs (Ficuscarica ) in patches in the forest create such a dense shade that reproduction under them is limited to sprouting figs. While these patches are fairly small in extent (100 m² ), they have created a major change in the localities in which they are found by their high reproduction (table 2). Prune seedlings (Prunus sp.), and tree of heaven (Ailanthusaltissima ) are also found in many areas along the river.

The native black walnut (Juglanshindsii ), now so common in the riparian forests, appears to have become widespread in the forest through the use of its rootstock for commercial propagation of the English walnut (Juglansregia ). The only population noted along the Sacramento River prior to the arrival of European man was between Freeport and Rio Vista (Fuller 1978). This was discovered by Richard Brindsley Hinds of the Sulphur Expedition in 1837 (Thomsen 1963).

Grazing

Grazing of the forest may lower reproduction densities in floodplain areas. When grazed, forests are kept clear of groundcover and young trees. When grazing is excluded, the regrowth of a thick understory which may prevent seedlings from establishing has occurred in the riparian forests. Thus, grazing could be responsible for the lack of establishment of certain age-classes in the flood-induced age structure through seedling elimination. Further work is necessary to substantiate the degree to which this has affected the Central Valley riparian forests. Carothers (1977) had shown it to be a major cause of reduction in reproduction in the Southwest riparian forests.

Water Resource Development

Levees : Aerial photography of the river reveals a large-scale change resulting from the artificial levees. Above Colusa, artificial levees are either non-existent or are far away from the channel. This allows lateral migration to form point bars at most bends and provides new surfaces for cottonwood and willow establishment. Below Colusa, the levees are adjacent to the river channel preventing point bar formation. Aerial photography (Murray etal . 1978) depicts 18 bars forming in a 20-river-mile (RM) reach above Colusa and only four bars forming in a 20-RM reach below Colusa. Bars below Colusa are much smaller in size than those above Colusa. Without the initial landform on which to colonize, riparian forest formation and regeneration will not continue in the same pattern.

NewLandforms : Development of man-made levees has caused a disruption of gravelbar formation thereby limiting reproduction. However, the levees themselves could provide new habitats for the development of new forests, providing current management practices were discontinued. The following species were common on levees: alder, ash, fig, cottonwood, valley

oak, sandbar willow, and tree willow. Densities for saplings ranged from 3/100 m² for most species up to 85/100m² for willows and cottonwood <2.5 cm. stem diameter. Regeneration density was partially dependent on levee management. Survival in burned areas was mainly in swales with sandbar willow and a few sapling oaks near the top of the levee. Species zonation is very noticeable with oaks often lining the tops of the levees; alder, ash, and cottonwood near the water level; and willows in swales. Weirs also provide a place where seasonal water flows and abundant light have created an oak phase of riparian forests along their levees.

FlowRegulation : The impacts of controlled flows on seedling establishment and survival have two effects. On certain rivers, willow encroachment on the streambanks has occurred as a result of controlled flows (Pelzman 1973). Pelzman (ibid .) attributed this to a prolonged soil moisture which allowed greater establishment and survival. McGill (1979) and Brice (1977) have also noted an increase in vegetated bar surfaces for the Sacramento River. They both attributed this to the moderating effect of Shasta Dam which has resulted in the lack of scour. The data for seedling establishment for the Sacramento River as a controlled stream reveals a lower density of seedlings (table 3) than similar data collected for a non-controlled stream (McBride and Strahan 1983). This suggests that the annual falling of the water level that coincided with seed dispersal and allowed abundant germination on the non-controlled stream did not occur on the Sacramento River. Daily flow data (USDI Geological Survey 1978, 1979; 1980 and 1981 data not available) for the Butte City gauging station reveal a wide fluctuation of streamflow with high flows following low flows frequently during the months of May-September. Thus, the absence of a continual lowering of the water level could have resulted in a limited amount of seedling establishment this year. However, the controlled flows may result in a higher survival percentage through lack of scouring. Also, a continual provision of moisture throughout the summer would reduce losses from desiccation for those seedlings which do become established.

LandandWaterEffects : Forest composition for the entire Sacramento River riparian zone must differ from the earlier forests because only frequently flooded areas remain to be sampled. Thus our results probably show a more flood-tolerant community dominating the area than we once had. With the decrease in bank stability of the river (Brice 1877), bank erosion has caused the loss of high terrace lands resulting in further decrease of sycamore and oak forests. Infrequent flooding and higher stands due to controlled flows and levees has probably resulted in the development of a greater proportion of older trees, since flooding of the areas does not clear out the undergrowth and provide bare areas for establishment to occur. As the rate of meandering is a major factor in determining the proportion of the floodplain in pioneer, transitional, and later successional stages, changes in meandering noted by Brice (1977) would suggest different proportions of forest stands in these stages may occur in the future than we had in the past.

Summary

Existing riparian forests have been shown to have an overstory and regeneration that corresponds to landforms and fluvial processes as well as successional stages. Establishment, growth, maturation, and death of floodplain trees are merged with the complete flow regime of the river and the erosion and deposition of sediment. The heterogeneity of forests is an indicator of a dynamic fluvial system. Establishment of the forests begins on gravelbars with the development of a cottonwood/willow type, making bars a critical landform in forest development. With deposition and time, the forests develop and mature, with understory species of box elder and black walnut becoming frequent. While regeneration on the bars is almost totally cottonwood and willow, regeneration on the floodplain is predominantly box elder and black walnut, especially on the low terraces. High terraces have minor amounts of oak, sycamore, and ash establishing. Forests surrounding oxbow lakes are older and have higher densities of reproduction than riverside floodplain forests. Water resource development projects and land uses have significant impacts on regeneration potential of riparian forests.

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