Results and Discussion
Table 1 lists perennial species recorded in the riparian inventory described in this paper. Plant common names are from the USDA Soil Conservation Service National Plant List (May 1980).
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Community Classification of Woody Species
Occurrence on each system studied and the assignment of woody species to one of four plant communities (broadleaf riparian, riparian scrub, adjacent desert, higher elevation) is also shown in table 1.
Figures 1, 2, 3, 4, and 5 depict the relative abundance of the woody species communities in each
Figure l.
Community classification of woody species and photograph of Mescal Creek.
Figure 2.
Community classification of woody species and photograph of the Gila River.
Figure 3.
Community classification of woody species and photograph of the San Francisco River.
Figure 4.
Community classification of woody species and photograph of Bonita Creek.
Figure 5.
Community classification of woody species and photograph of Aravaipa Creek.
riparian system, expressed as a running mean of the woody species per 20 sample points. Also included are photographs of the watercourses. A good distribution of mature broadleaf riparian trees without successful reproduction in the open areas made it more likely for riparian scrub, adjacent desert, or higher elevation species to be the nearest woody species to the sample point. Mescal Creek was found to be an example of fluctuation in community type caused by seedling absence. The Gila River and San Francisco River showed a very clear dominance of the riparian scrub community type. Bonita Creek also showed a riparian scrub community type with a slightly greater broadleaf riparian component.
Aravaipa and Mescal Creeks were the only systems with a dominant broadleaf riparian component. No apparent correlation appeared to exist between broadleaf riparian dominance and channel or terrace width. Broad areas on the watercourse might be expected to correspond to broadleaf riparian dominance, but this was not always the case.
Since no broadleaf riparian species was recorded if one was not present before the next sample point, the percent of sample points on each watercourse where broadleaf riparian species were recorded illustrates the ubiquity of these species. Broadleaf riparian species were recorded at the following percentages of sample points on the watercourses: Aravaipa Creek 99.5%; Mescal Creek 84.9%; Bonita Creek 73.1%; Gila River 22.5%; San Francisco River 21.1%.
Relative Abundance of Woody Species
Table 2 shows the relative abundance of woody species occurring at a frequency over 1.0%. Aravaipa Creek had a high percentage of broadleaf riparian species as would be expected from the community classification in figure 1. The San Francisco and Gila Rivers had a very high percentage of riparian scrub species. Salt cedar, burro brush, mesquite and seep willow account for 90% of the woody species recorded on the San Francisco River.
Relative Abundance of Herbaceous Perennials
Table 3 shows the relative abundance of the principal species of herbaceous perennials recorded. Bermuda grass is clearly the dominant herbaceous perennial on each of the watercourses. All the herbaceous perennials in table 3 are grazing-resistant with the exception of bent grass, a minor component of Bonita Creek.
After seven years of cattle exclusion on Aravaipa Creek, palatable grasses other than bermuda grass still occurred at negligible frequencies. Trend plot photos for Aravaipa Creek show bermuda grass in sparse clumps extending to form a solid mat after the cattle were removed. This extensive sod formation by bermuda grass has apparently limited establishment of other grasses.
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Population Data
Figures 6–9 depict the size-class data for broadleaf riparian trees and mesquite on the five watercourses.
Aravaipa Creek
Aravaipa Creek (fig. 6) has been excluded from cattle since 1973 and shows a high percentage of seedlings in every species population. The survival of seedlings to the 1- to 3-inch size-class is good for the broadleaf trees with the exception of sycamore and walnut. Sycamore seed production and establishment is sporadic and sycamores often rely on sucker sprouts for reproduction. It would require a follow-up study to determine the number of sycamore seedlings surviving to the 1- to 3-inch size-class. The absence of walnuts in the 1- to 3-inch size-class may be due more to the small walnut sample (31 trees) than to ecological factors.
Figure 6.
Size-class data for Aravaipa Creek (DBH size-classes in inches).
Gila and San Francisco Rivers
The Gila River and San Francisco River (fig. 7 and 8) show virtually no successful broadleaf riparian reproduction. The only trees in the 1-to 3-inch size-class are cottonwoods on the Gila River, and they comprise only 2% of the cottonwood sample. The low percentage or absence of 1-to 3-inch size-class trees indicates low seedling survival. On the San Francisco River, 82% of the cottonwood seedlings were browsed by cattle. On the Gila River, 62% of the cottonwood and willow seedlings were browsed, resulting in seedlings 6–10 in. tall with up to 0.6 in. diameters indicating the seedlings had been browsed for more than one growing season. Mesquite, however, is reproducing successfully on both of these rivers.
Figure 7.
Size-class data for the San Francisco
River (DBH size-classes in inches).
Figure 8.
Size-class data for the Gila River (DBH size-classes in inches).
Very poor broadleaf riparian tree establishment and successful mesquite establishment will eventually lead to the replacement of the broadleaf riparian community by mesquite and other riparian scrub species. This trend is already well established on the Gila and San Francisco Rivers (figs. 2, 3, 7, and 8).
Mescal Creek
All the broadleaf species on Mescal Creek (fig. 9) showed good representation in the seedling size-class. However, willows, sycamore, and walnut showed a total absence of 1- to 3-inch size-class trees. Cottonwood had 1.5% in this size-class and ash 5.5%. This strongly indicates seedlings were not surviving. Cottonwood seedlings were 70% browsed by cattle, willows 9%, sycamores 62%, ash 1%, and no walnut seedlings were recorded as browsed. Mesquite is reproducing successfully on Mescal Creek.
Figure 9.
Size-class data for Mescal Creek
(DBH size-classes in inches).
Poor broadleaf riparian establishment and successful mesquite establishment will eventualy lead to the replacement of the broadleaf riparian community on Mescal Creek. A good distribution of mature broadleaf riparian trees on Mescal Creek results in a dominant broadleaf riparian community (fig. 1). However, poor reproduction will not maintain this broadleaf riparian community over time.
Bonita Creek
Data from Bonita Creek (fig. 9) indicated poor reproductive success of all broadleaf riparian species:
1) only 4% of the cottonwoods were in the seedling size-class and all of these seedlings were browsed by cattle. The 1- to 3-inch size-class represented only 2% of the sample;
2) only 1.5% of the willows were in the seedling size-class and 67% of these were browsed. No 1- to 3-inch size-class willows were recorded;
3) only 1.5% of the sycamores and no velvet ash were in the seedling or 1- to 3-inch sizeclass.
Grazing has occurred on Bonita Creek for over 100 years. The palatability of broadleaf riparian seedlings in descending order of
Figure 10.
Size-class data for Bonita Creek
(DBH size-classes in inches).
preference is: cottonwood, willow, sycamore, ash, and walnut.[4] Prior to 1972, grazing pressure on Bonita Creek was perhaps severe enough to affect the most palatable species, but not severe enough to affect ash and walnut. Since 1972, grazing pressure has apparently increased sufficiently to affect the less palatable ash and walnut seedlings.
Sycamore, willow, and cottonwood populations indicate a size-class distribution the reverse of one needed to maintain these species in the community.
Arizona walnut and velvet ash are slow growing species. On Bonita Creek, size-classes for these trees show a normal size-class distribution from 4- to 16-in. and larger. This indicates successful seedling reproduction prior to about 8–10 years ago to establish the 4- to 16-in. trees. The grazing allotment history on Bonita Creek indicates a change in grazing allottees in 1972. The change in grazing allottees corresponds to the downward trend in successful reproduction of walnut and ash.
The trend on Bonita Creek is toward replacement of the broadleaf riparian community by mesquite and other riparian scrub species. This trend is already well established as illustrated in fig. 4 (community classification).
Summary
Aravaipa Creek is the only system where the trend is not toward replacement of the broadleaf riparian community by riparian scrub. Aravaipa is also the only system where cattle have been excluded.
Flooding
Flooding is another factor that influences reproductive success. In the winter of 1978–79, all five watercourses experienced severe floods. Mesquite is not damaged by cattle browsing, but is as susceptible to flooding as the broadleaf species. In each of the systems, mesquite indicated successful reproduction. Mesquite trees were recorded in all parts of the riparian systems from high on the upper banks to within feet of the water. Mesquite seedlings and 1- to 3-inch trees were found under mature broadleaf riparian species and in habitats suitable for broadleaf riparian reproduction.
Aravaipa Creek experienced severe flooding, yet did not indicate poor broadleaf riparian reproduction. Flooding does have adverse effects on tree seedlings, but is not, apparently, as detrimental to the broadleaf riparian communities as cattle browsing upon seedlings.
Importance Values
Table 4 shows the importance values for the major woody species on the five systems. An analysis of these data confirm the inferences drawn from the population analysis.
Aravaipa Creek
High coverage, density, and frequency of the cottonwoods indicate a mature population evenly distributed along the river with very successful reproduction. High coverage indicates large mature trees. High frequency indicates uniform distribution and high density indicates numerous seedlings and small trees. Walnut is the only broadleaf riparian tree with a low importance value, but Aravaipa Creek is the only system in the study where walnut is recorded in the 0.01-acre samples. High density relative to frequency of seep willow indicates clump-like concentrations. Nearly equal density and frequency of burro brush indicates a fairly uniform linear distribution.
[4] Steve Bingham. Personal communication.
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San Francisco River
The two broadleaf riparian species recorded on the San Francisco River have very low importance values compared to the riparian scrub species. High coverage and low but fairly equal density and frequency of the cottonwoods indicate large mature trees growing singly and sparsely along the river with very few seedlings. Low coverage of willows with none recorded in the
0.01-acre samples indicate very sparse distribution of mature trees with no successful reproduction. Very high coverage of mesquite and nearly equal, but high frequency and density indicate mature mesquite and successful reproduction. Extremely high density of salt cedar relative to its coverage indicates an abundance of young plants. The frequency is also high indicating uniform distribution and strongly suggesting salt cedar will become dominant.
Gila River
High coverage of cottonwood relative to its frequency indicates large mature trees sparsely distributed along the river. Low density of cottonwoods indicates few seedlings. Willow was recorded in only one of the 57 0.01-acre samples and this was a browsed seedling. Very high coverage of mesquite and nearly equal, but high frequency and density indicate mature mesquite and successful reproduction.
Mescal Creek
Data from Mescal Creek show a predominantly broadleaf riparian community (fig. 1). The very high coverage of ash relative to its density indicates large mature trees without successful reproduction. The low frequency value indicates these large trees are sparsely spread along the creek. The density of cottonwoods relative to frequency indicates good tree establishment. The coverage value for cottonwood is not disproportionately high indicating seedlings are fairly numerous. However, coverage, density, and frequency of cottonwoods on Mescal Creek are much lower than cottonwoods on Aravaipa Creek. Successful reproduction and establishment of the broadleaf riparian community on Mescal Creek is apparently less than on Aravaipa Creek.
Bonita Creek
Very high coverage of sycamore relative to its density and frequency indicates large mature trees distributed sparsely along the creek with very few seedlings. Sycamore was third in importance only because of its relatively high coverage. Riparian scrub species are clearly dominant. Willows, cottonwood, walnut, and ash all have low density compared to their coverage indicating a mature broadleaf riparian vegetation which is not successfully maintaining itself.
General Conclusions
Aravaipa Creek is the only system of the five studied with a dominant broadleaf riparian community and successful reproduction. Mescal Creek is also dominated by a broadleaf riparian community, but has poor reproductive success. The Gila River, San Francisco River, and Bonita Creek all show a well established trend toward replacement of the broadleaf riparian community by riparian scrub.
Cattle browsing appears to be the major contributing factor to the downward trend of broadleaf riparian communities. The only system in this study with an upward trend in the broadleaf riparian community is Aravaipa Creek, where cattle have been excluded since 1973.