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Gradient Analysis of a Wash 1 2 Peter L. Warren and L. Susan Anderson

Abstract.--Vegetation was sampled along two parallel environmental gradients in the Sonoran Desert, one in upland bajada sites and one in xeroriparian wash sites. The wash gradient was found to be more complex than the upland gradient, with three areas of species turnover compared to no turnover along the upland gradient. The more complex pattern of the wash gradient is likely due to the interactions of three major limiting environmental factors related to watershed area acting in different portions of the wash gradient, whereas the upland gradient is controlled by one overriding environmental factor.

INTRODUCTION in turn support animals that are rare or absent in the upland habitats (Lowe, 1964). For these reasons xeroriparian habitats have long been Vegetation in the deserts of southwestern recognized as contributing to the biotic diversity has been studied systematically of desert environments in disproportion to their since the opening of the Carnegie Desert Botanical area (Shreve, 1951). Laboratory in 1902. Plant ecology of this desert region is probably better known than any similar arid region in the world. Much of this research Despite their importance in desert has concentrated on patterns of vegetation ecosystems, xeroriparian systems have been the distribution along desert bajadas, the gently subject of remarkably little research. There are sloping plains of coalesced alluvial fans several reasons for this. First, riparian extending from the mountain foothills to the flat biologists have focused most of their atten~ion on valley floors. systems with permanent water to the excluslon of desert washes (Campbell and Green, 1968; Brown et al., 1981; Minckley and Brown, 1984). Second, Desert washes dissecting the bajada plain are studies of desert vegetation distribution patterns easily recognizable by the corridor of vegetation have focused on the upland bajada gradient and along their channels that contrasts strongly with expressly avoided xeroriparian corridors, treating the sparse vegetation of adjacent uplands. A them only as a factor that might throw confusion transect across a typical Sonoran Desert bajada into the general pattern of bajada vegetation. intercepts an average of approximately 14 desert Thus, plant distribution patterns along desert washes per mile. At an average riparian corridor washes have been largely ignored by riparian width of 25 feet, between six and seven percent of ecologists on one side and desert plant ecologists total bajada surface area is covered by desert on the other. wash vegetation. The intermittant nature of these watercourses results in less luxuriant vegetation In this paper we attempt to answer three than is found along streams with permanent flow, questions: 1) What are the patterns of vegeta­ and they have been termed xeroriparian systems tion distribution in a xeroriparian drainage (Johnson et aI, 1981). Runoff from surrounding system? 2) How does the pattern of riparian slopes increases the available water in and near vegetation compare to the surrounding bajada the wash permitting growth of plant species not vegetation gradient? 3) What environmental found in the surrounding desertscrub. The controlling factors might account for xeroriparian 1 vegetation distribution? Paper presented at the First North American Riparian Conference [University of , Tucson, April 16-18,1985]. METHODS 2 Peter L. Warren is a Research Assistant at We examined a simple, dendritic, xeroriparian the Arizona Remote Sensing Center, Office of Arid drainage system flowing from the wes~ slopes of Lands Studies, University of Arizona, Tucson, the Ajo Mountains west across the AJo Va~ley in Arizona. L. Susan Anderson is a Research Organ Pipe Cactus National Monument. ThlS area Ecologist with the Cooperative Park Studies Unit, receives an average of 6-8 inches of rain per University of Arizona, Tucson, Arizona.

150 year. At this level of precipitation washes carry water only a few days per year, and in some years Physical features recorded for each site may not flow at all. included elevation, watershed area, stream order, channel width, and corridor width. These were A series of paired upland and riparian sites determined from aerial photographs and topographic were. sampled along wash and adjacent bajada maps, depending upon the scale appropriate to the gradlents. Elevation of sample sites ranged from site. 2300 feet at the foot of the Ajo Mountains at Alamo Canyon to 1400 feet in Growler Valley. A Cluster analysis was used to compare total of 33 riparian and 22 upland sites were floristic similarity between sites and determine sampled. Fewer upland samples were taken because degree of association between groups of sites. some upland samples were associated with more than The similarity value is based on the euclidian one wash sample if they occupied an interfluvial distance, the square root of the sums-of-squares of the differences between species prominence s~te between two wash tributary samples of dlfferent size. values for each pair of sites. A larger value indicates a greater degree of floristic differ­ Sites were selected with the aid of aerial ence. Sites with identical species composition and photographs approximately every 1 to 2 miles along prominence values would show zero difference. A the length of the major tributary and represented combination of statistical and graphical analysis a range of stream orders and watershed areas for was used to examine patterns of diversity and the drainage system (Johnson et al., 1984). Sites species turnover along both the riparian and were selected to avoid problems such anastomozing upland gradients. cha~nels. Every site sampled was characterized by a slngle channel that carried all of the flow from the watershed area upstream. RESULTS AND DISCUSSION The upland gradient follows a pattern similar Vegetation sampling involved compiling a to that documented by other studies of desert complete plant species list for each site and bajadas: species diversity declines continuously assigning each species a prominence value from 1 from higher to lower sites with very little to 5 based on its abundance at the site. Wash species turnover (Fig. 1A). Species present on samp~es included the entire width of the riparian corrldor and extended approximately 100 meters higher sites decline in abundance at decreasing along the stream channel. This resulted in a elevation and disappear, but are not replaced by greater sampling area for larger washes because new species at lower elevations. Only two they have a wider channel and riparian corridor. species, creosotebush and white-bursage, show an Adjacent upland bajada samples covered an area of increase in abundance at the lowest sites, while approximately one-half hectare. the other nine common upland species all decline (Table 1). Few species have higher prominence at

Table 1.--Average prominence and species frequency (percent of sites encountered) in three upland bajada elevation zones along the Alamo­ Cherioni-Growler wash system at Organ Pipe Cactus National Monument. These elevation zones correspond to vegetation assemblages of the species rich upper bajada, transitional middle bajada, and the depauperate lower bajada.

Species 2300-1900' 1900-1600' 1600-1300' n=10 n=7 n=5

prom. freq. prom. freq. prom. freq.

Ambrosia deltoidea 4.0 100 1.6 57 1.2 40 Larrea tridentata 3.7 100 4.6 100 5.0 100 FOUqUieria splendens 2.8 100 1.3 57 0.4 20 Carnegiea gigantea 3.1 100 1.7 71 1.4 60 Cercidium microphyllum 2.5 100 1.4 57 0.4 20 Opuntia fulgida 2.9 100 0.9 43 0.2 20 Krameria ~ 1.7 80 1.6 71 0.6 60 Opuntia acanthocarpa 1.9 90 0.3 14 0.6 40 Olneya tesota 1.1 50 0.9 43 0.4 40 Ambrosia dumosa 0.6 30 2.0 71 2.2 80 Lycium an~ii 0.8 50 0.3 29 PrQSOPis glandulosa 0.2 10

151 BAJADA 5

~ 4 ...... :::::;> ......

.~ 3 E o a..L- .•..... 2 .... .••...... " ...... ,,'- .~--- ...... 1,1,,;1 ...... ::'::.,.::...... \.... :~ .. ::::::::~~::.:.:.:.::.::...... I ", . ". . . "-...... -+---""""""'----.------,----.-----'"-,-'--.------.---\ ~~~ ""- : ...... "'\" i··········· 2300 2200 2100 2000 1900 1800 1700 1600 1500 1400 1300 Elevation (feet)

RIPARIAN 5

..... ---- ..... 4 - ..~~ ...... ------:: ..... - - ~~..:- - -- - '::'.~. ..,.-- ."."...... Q) " ...... ,.::.. _- ". u / . t· ...... c / ..' .,' ...... ". - ~ 3 'E o a...L-

. '. e~~~L:::;;.~~~~~~~~~~~······- ...... -2.5 -2.0 -1.0 o 1.0 2.0 2.5 Log watershed area (sq. miles)

Figure 1.--Comparison of plant species distributions along upland and xeroriparian gradients in the Sonoran Desert. The upland gradient (A) exhibits a continuous decline in diversity with decreasing elevation. The riparian gradient (B) is more complex with three major areas of species turnover.

152 intermediate sites. Growth form diversity available soil moisture than the heavier silty­ parallels general species diversity. The numerous loam of the valley bottom by virtue of its higher growth forms present at the top of the gradient, infiltration rate and lower soil water potential. including trees, shrubs, and stem succulents, decline to one dominant form at the bottom. Water is likely to also be the primary limit­ ing factor along the xeroriparian gradient, This pattern of decreasing vegetction diver­ however it probably does not control species dis­ sity along bajada elevational gradients has been tribution simply as a function of plant-available documented at numerous sites throughout the soil moisture throughout the gradient. The Sonoran Desert. Near the southern edge of the frequency, volume, and duration of flow along desert Felger (1966) studied bajadas along the washes of different size is a function of both coast of . At the northeast margin of the watershed area and the regional rainfall regime. desert, bajada gradients have been documented in The most important and easily measured variable at least three locations (Yang and Lowe, 1956; affecting frequency and amount of runoff is water­ Whittaker and Niering, 1965; Lowe et al., 1973). shed area. Therefore, we used watershed area as Phillips and McMahon (1978) examined a bajada in our primary axis for ordination of species the north central Sonoran Desert, and Yeaton and distribution. Cody (1979) studied a northwestern location at the transition between the Sonoran and Mojave deserts. The riparian vegetation gradient shows In all of these studies the same basic pattern of several important differences from the upland decreasing diversity with decreasing elevation was gradient, suggesting that the controlling environ­ observed. mental factors for riparian vegetation operate differently from those in the upland community. The primary environmental factor controlling The riparian gradient differs from the bajada this pattern of decreasing diversity on desert gradient in two major ways. First, there is bajadas is available soil moisture (Klikoff, higher species turnover and therefore higher 1967). Under the same rainfall regime, the between site diversity along the riparian coarse, rocky soil of the upper bajadas has more corridor. Second, there is greater floristic

Table 2.--Average prominence and species frequency (percent of sites encountered) in four watershed area classes encountered along the Alamo-Cherioni-Growler wash system at Organ Pipe Cactus National Monument. The four zones are defined by watershed area and species distribution patterns: Class 1 is 0.002-0.02 sq. mi., Class 2 is 0.02-0.8 sq. mi., Class 3 is 0.9-50 sq. mi., and Class 4 is 60-400 sq. mi. For a more complete species list see Johnson et al. (1984).

Species Class 1 Class 2 Class 3 Class 4 n=9 n=9 n=11 n=4

prom. freq. prom. freq. prom. freq. prom. freq.

Krameria ~ 1.7 56 0.8 44 0.1 9 Cercidium microphyllum 1.3 67 2.8 89 0.7 27 Ambrosia deltoidea 4.0 100 3.2 89 0.9 54 0.5 25 Condalia spathulata 0.8 44 1.0 56 0.9 45 Acacia constricta 0.9 44 2.9 89 0.7 36 0.2 25 Sarcostemma cynanchoides 0.1 11 0.1 11 0.4 36 1.2 75 Sphaeralcea sp. 0.1 11 0.3 22 1.4 73 1.8 75 Prosopis glandulosa 0.2 11 1.2 44 3.2 100 3.3 100 Acacia greggii 0.3 22 1.8 89 3.4 100 3.5 100 Plumbago scandens 0.4 44 Ephedra nevadensis 0.8 33 Brickellia californica 1.6 67 0.4 27 Celtis pallida 1.2 67 0.4 27 Aloysia wrightii 1.0 56 0.3 27 Penstemon parryi 0.4 33 0.9 67 Zizyphus obtusifolia 0.3 33 0.9 67 0.5 50 Anisacanthus thurberi 0.3 11 1.1 54 0.5 50 Ambrosia ambrosioides 0.3 22 0.9 36 2.2 75 Cercidium floridum 0.3 11 2.6 82 1.2 50 sarothroides 1.5 82 1.5 75 Nicotiana trigonophylla 1.0 54 1.2 75 Clematis drummondii 1.1 46 2.0 75 Hymenoclea salsola 0.3 27 3.5 100

153 differentiation between different portions of the the loss of many of the preferential and obligate riparian gradient. riparian shrubs that dominate the second class. These shrubs are replaced by a group of shade­ Plant species distribution along the con­ tolerant shrubs which grow under or in the tinuum of watershed area shows three relatively canopies of the overstory trees. The primary distinct areas of species turnover (Fig. 1B). limiting factor controlling the second area of These turnover points, characterized by coincident species turnover appears to be shading by trees decline and loss of some species and addition and and consequent loss of the shade-intolerant shrubs increase of others, occur at approximately 0.02 that dominate the second floristic class. square miles, just under 1.0 square miles, and 40-50 square miles. The presence of multiple The fourth floristic class is characterized turnover points along the xeroriparian gradient by two distinct vegetated areas, the tree domin­ suggests that several environmental variables are ated bank~ and floodplain, and a shrub dominated operating to limit species distribution and that channel. The trees that became dominant in the they operate in a reciprocal fashion with differ­ third zone remain so here. The channel itself ent factors operating to a greater or lesser supports stands of scour resistant species includ­ degree in different portions of the continuum of ing Hymenoclea salsola, Ambrosia ambrosioides, and watershed area (Whittaker, 1967). Baccharis sarothroides. The factors limiting species distribution in washes with the largest A somewhat unexpected observation is that a watershed areas appears to be a combination of number of species share similar distribution pat­ scouring in the open channels and shading along terns along the gradient, increasing, decreasing, the heavily vegetated banks and floodplains. and reaching maximum prominence over the same portion of the watershed area continuum. The The xeroriparian gradient is much more result is the formation of four species assem­ complex than the upland gradient. The average blages, demarcated by the areas of turnover floristic similarity index among sites in the discussed above, along washes with different riparian gradient is almost twice the average for watershed area. The environmental factors con­ the upland gradient, indicating a higher degree of trolling the species composition of each of the florisitic differentiation between wash sites. In floristic classes can be inferred from the addition, species distributions along the xerori­ ecological distribution of the dominant species in parian gradient are controlled by at least three each assemblage (Table 2). environmental factors in contrast to the single overriding factor along the upland gradient. The first floristic class is associated with These results suggest that ecological response small washes with watershed area under 0.02 square and/or recovery of xeroriparian vegetation follow­ miles. In these small washes the dominant species ing disturbance may be different from upland are those with predominately non-riparian distri­ vegetation, and generalizations drawn from studies butions such as Cercidium microphyllum, Ambrosia of upland vegetation should not be extended to deltoidea, and Krameria~. Although a few xeroriparian habitats. preferential riparian species are found in these washes, the watersheds are too small to substan­ tially increase available moisture above that supplied by ambient precipitation. These drainages CONCLUSIONS probably do not flow every year and cannot support many preferential or obligate riparian species. The gradient of xeroriparian vegetaiion along a Sonoran Desert wash is more complex than the The second floristic class is dominated by adjacent upland gradient because it is controlled preferential and obligate riparian shrubs such as by several interacting environmental factors Acacia constricta, Celtis pallida, Brickellia related to watershed area, while the upland californica, and Aloysia wrightii. This first gradient is controlled by one overriding factor. major species turnover is probably controlled by The riparian corridor shows three areas of species the watershed area threshold at which increased turnover and four distinct vegetation classes moisture is reliably greater than that supplied by controlled by frequency and amount of runoff, ambient precipitation. These larger washes, with shading, and channel scouring mitigated by watershed areas greater than 0.02 square miles, watershed area. probably flow almost every year. The increased runoff Rllows faster-growing riparian shrubs to For these reasons, riparian vegetation will replace the non-riparian species which dominated likely respond differently to disturbance than the smallest washes. surrounding uplands. Riparian vegetation is The third floristic class includes moderate subject to high levels of natural disturbance and to large watersheds with areas from just under 1 may recover more rapidly than upland vegetation. square mile up to approximately 50 square miles. However, it may be more difficult to evaluate the The riparian corridor of these washes is dominated extent of recovery of riparian vegetation because by trees, mostly Prosopis glandulosa, Cercidium the expected natural diversity is the result of a complex interaction of watershed area above a floridum, and Acacia greggii. Two of these specific site, and the regional precipitation species, Prosopis and Acacia, are present in shrub regime. Recovery of vegetation structure in form in smaller washes, ~ut only achieve tree size xeroriparian habitats may be rapid compared to in washes with watershed areas greater than 1 recovery of species diversity. square mile. Species turnover associated with the shift to dominance by trees is characterized by

154 Lowe, C.H. 1964. Arizona landscapes and habitats. ACKNOWLEDGEMENTS In: C.H. Lowe, ed. The vertebrates of Arizona. Univ. Ariz. Press, Tucson. Several of the ideas presented here were developed during discussions with R.R. Johnson and Lowe, C.H., J.H. Morello, G. Goldstein, J. Cross C.H. Lowe. R.R. Johnson participated in the field and R. Newman. 1973. Analis comparativo de work and the Organ Pipe staff, especially Dick la, vegetacion de los desiertos subtropicales Anderson and Bill Mikus, provided logistical de Norte y Sud America (Monte-Sonoran). assistance. This research was supported by the Asociacion Argentina de Ecologia, Ecologia. Cooperative National Park Resources Studies Unit 1:35-43. at University of Arizona. Minckley, W.L. and D.E. Brown. 1984. Wetlands. In: D.E. Brown. ed. Biotic communities of the American Southwest- and . LITERATURE CITED Desert Plants. 4:223-287.

Brown, D.E., N.B. Carmony and R.M. Turner. 1981. Phillips, D. and J.A. McMahon. 1978. Gradient Drainage map of Arizona showing perennial analysis of a Sonoran Desert bajada. South- streams and some important wetlands. Ariz. west Naturalist. 23:669-680. Game and Fish Dept., map scale 1:1,000,000.

Campbell, C.J. and W. Green. 1968. Perpetual Shreve, F. 1951. Vegetation of the Sonoran succession of stream-channel vegetation in a Desert. Carnegie Inst. Washington, Pub. 591, semiarid region. Jour. Ariz. Acad. Sci. 192 p. 5:86-98. Whittaker, R.H. 1967. Gradient analysis of Felger, R.S. 1966. Ecology of the gulf coast and vegetation. BioI. Rev. 42:207-264. islands of Sonora. Ph.D. Dissertation. Univ. Ariz., 460 pp. Whittaker, R.H. and W.A. Niering. 1965. Vegeta­ tion of the Santa Catalina mountains, Arizona Johnson, R.R., S.W. Carothers and J.M. Simpson. (II). A gradient analysis of the south 1981. A riparian classification system. In: slope. Ecology. 46:429-452. Warner and Hendrix. eds. riparian systems: A conference on their ecology, Yang, T.W. and C.H. Lowe. 1956. Correlation of conservation and productive management. major vegetation climaxes with soil charac­ Univ. Calif., Davis. Sept. 17-19, 1981. teristics in the Sonoran Desert. Science 6:41-45 Johnson, R.R., P.L. Warren, L.S. Anderson and C.H. Lowe. 1984. Stream order in ephemeral Yeaton, R.I. and M.L. Cody. 1979. The distribu­ watercourses: a preliminary analysis from the tion of cacti along environmental gradients Sonoran Desert. Hydrology and water resources in the Sonoran and Mojave deserts. J. in Arizona and the Southwest--Proceedings of Ecology. 65:529-541. the American Water Resources Association, Arizona section. 14:89-100.

Klikoff, L.G. 1967. Moisture stress in a vegeta­ tional continuum in the Sonoran Desert. Amer. Midland. Natur. 77:129-137.

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