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Riparian vegetation of ephemeral streams
Journal of Arid Environments 138 (2017) 27e37
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Riparian vegetation of ephemeral streams Juliet C. Stromberg a, *, Danika L. Setaro a, Erika L. Gallo b, Kathleen A. Lohse c, Thomas Meixner b a
Arizona State University, Tempe, AZ 85287-4501, USA University of Arizona, Tucson, USA c Idaho State University, Pocatello, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2016 Received in revised form 17 November 2016 Accepted 2 December 2016
Ephemeral streams are abundant in drylands, yet we know little about how their vegetation differs from surrounding terrestrial zones and about their projected response to regional warming and drying. We assessed plant communities at seven ephemeral streams (and terrestrial zones) distributed among three climatic settings in Arizona. Compared to terrestrial zones, riparian zones had similar herbaceous cover but greater woody vegetation volume. They supported more plant species, with several woody taxa restricted to the ephemeral zone (consistent with the idea that herbaceous plants are rain-dependent while riparian trees rely on runoff stored in stream sediments). Their herbaceous communities had high compositional overlap with terrestrial zones and may sustain regional diversity as droughts intensify. Presumably owing to periodic flood disturbance, riparian plant communities had greater evenness than terrestrial zones, many of which were dominated by Eragrostis lehmanniana. Patterns along the climatic gradient suggest that increasing aridity will reduce the number of herbaceous (and total) plant species within riparian zones (110 species per stream in semihumid settings, 88 in semiarid, 48 in arid) and drive compositional shifts from perennials grasses and forbs to annuals. Hotter and drier conditions will drive sharp declines in herbaceous cover, converting riparian savanna to xeroriparian scrubland. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Aridity Climate change Eragrostis lehmanniana Savanna Species richness Riparian vegetation
1. Introduction In hot dryland regions, the groundwater-dependent riparian ecosystems that border perennial to intermittent rivers and streams have substantially more biomass and greater productivity than the surrounding terrestrial vegetation (Scott et al., 2014). They are vegetated by distinct suites of fast-growing and flood-adapted wetland plant species (obligate riparian taxa), while also providing habitat for many plant species that are typical of more xeric habitats (facultative riparian taxa). They maintain levels of plant species diversity that are greater or lesser than in adjacent desert habitat, depending on context, and increase regional diversity (Sabo et al., 2005). Ephemeral streams are the predominant stream type in desert regions but are understudied relative to their wetter counterparts. By definition, ephemeral streams are decoupled from regional
* Corresponding author. E-mail address: [email protected] (J.C. Stromberg). http://dx.doi.org/10.1016/j.jaridenv.2016.12.004 0140-1963/© 2016 Elsevier Ltd. All rights reserved.
groundwater and flow only in response to major storm runoff events (Meinzer, 1923). Their plant communities-drought-adapted taxa including small-leaved shrubs and short-canopies trees-have been referred to as xeroriparian (Warren and Anderson, 1985; Johnson et al., 1984). The trees grow in narrow linear bands, sustained by water supplied by periodic floods (run-on events) that recharge the often-sandy stream sediments and/or create shallow perched water tables (Atchley et al., 1999; de Soyza et al., 2004; Rassam et al., 2006). Soils often are deeper than on alluvial fans. The extent to which the plant communities of these ephemeral streams differ in biomass, species composition, and species diversity from the terrestrial vegetation remains little studied. While many parts of the world are becoming warmer and wetter, many arid and semiarid regions are becoming hotter and drier (Dominguez et al., 2010; Vicente-Serrano et al., 2012). Analyses of long-term data sets in the American Southwest have documented recent drought-related declines of desert shrubs and upward elevational range shifts of various taxa (Bowers, 2005; McAuliffe and Hamerlynck, 2010; Brusca et al., 2013). Vegetation
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patterns along spatial aridity gradients reveal yet other changes that may occur through time as climate changes, including decreases in above and below ground biomass, shifts in plant growth form, and declines in species richness (Schulze et al., 1996; Munson et al., 2013; Ulrich et al., 2014). Plant communities in riparian ecosystems are changing in response to direct effects of temperature and precipitation and to indirect effects of climate change on watershed processes that regulate flows of surface water and groundwater (Perry et al., 2012; Davis et al., 2013; Garssen et al., 2014; Kløve et al., 2014). For groundwater-dependent-ecosystems (GDEs) in dryland regions, declining groundwater inflows will induce shifts from perennial flow to intermittent flow in susceptible river reaches (Seager et al., 2013). This will cause riparian vegetation to transition from tall, broad-leaf riparian forests to more deeply-rooted and smallerleaved shrublands (Stromberg et al., 2010), and drive initial increases in species richness followed by declines to lowest levels as flow becomes highly infrequent (Katz et al., 2012). The effect of increasing aridity on ephemeral stream vegetation remains little studied. Particularly for streams draining small watersheds, effects may mimic those in the adjacent terrestrial vegetation. There has been a recent surge of interest in understanding and ~ a et al., conserving ephemeral streams in desert landscapes (Acun 2014; Datry et al., 2014). Our goals were to increase understanding of the ways in which vegetation along ephemeral streams differs from the desert matrix and how vegetation will change under a climate scenario of increasing aridity. Focusing on the American Southwest, we asked, (1) to what extent do biomass, diversity, and composition differ between ephemeral streams and adjacent terrestrial zones, and (2) how do plant community attributes vary along an aridity gradient? 2. Materials and methods We selected seven ephemeral streams (colloquially, washes or arroyos) in central and southern Arizona, spanning three aridity zones (Table 1; Appendix 1, electronic version only). The aridity gradient is created by elevational change, and encompasses changes in mean annual precipitation and mean annual temperature. All sites have a summer wet season (JulyeSeptember) and winter wet season (December through March), although the westerly sites receive a higher percentage of rainfall in winter (56%) relative to summer (44%) compared to other sites (42% and 48% respectively). Study streams were functionally Type I (Shaw and Cooper, 2008) in that they drain small watersheds and are regulated more by the local climate than by that of distant mountains. We determined the percentage of time that surface water was
present in study streams by instrumenting one 200-m stream length per site with three electrical resistance sensors with exposed leads (ER, Tidbit v2 UTBI-001 data logger, Onset Corporation, Bourne, MA) (Blasch et al., 2002). (The 200-m stream length also served as the base area for vegetation sampling). The sensors logged a resistance signal every 10 min over a two-year period. For one stream, Sauceda Wash, surface flow data were obtained from USGS stream gaging station 09519760. Collectively, these data indicated that study streams had surface flow for only a few days per year (Table 1; Fig. 1). We calculated aridity using the de Martonne Aridity Index (mean annual precipitation in mm divided by the sum of mean annual temperature in C plus a constant of 10) (Quan et al., 2013). A value of <5 is arid, 5e10 is semiarid, 10e20 is semihumid, 20e30 is humid, and >30 is perhumid. Black Gap Wash and Sauceda Wash, within the Barry M. Goldwater Air Force Range near Gila Bend, Arizona, were arid, with Aridity Index of 3. Both are in the Lower Gila River Basin, on semi-consolidated alluvial basin fans. Their matrix vegetation is Sonoran Desertscrub (Lower Colorado Valley subdivision) (Brown, 1994). Two unnamed streams in the Santa Rita Experimental Range (currently administered by the University of Arizona College of Agriculture), Santa Cruz River Basin, had a semiarid climate with Aridity Index of 8. Both are center-of-basin braided unconsolidated sandy channels. Their matrix vegetation is degraded semidesert grassland (a mix of cacti, grasses, and woody plants). The three ephemeral streams with the greatest precipitation and lowest temperature (semihumid climate, Aridity Index from 11 to 15) drained the foothills of the Huachuca Mountains near Sierra Vista, Arizona, within the San Pedro River Basin. These piedmont streams have semi-consolidated alluvial channels. Two of the three were on land managed by the Department of Defense (Fort Huachuca) and the third was on the Coronado National Forest. For convenience, we named these three streams based on the name of the closest canyon. Their matrix vegetation is semidesert grassland. We delineated the riparian zones from the terrestrial zones based on vegetation cues including changes in size of facultative riparian shrubs (i.e., those that also grow in the dryland). We determined the boundary between the channel and riparian zone using hydrological cues (recent evidence of flow) and vegetation cues (differences in cover). The combined riparian/channel zone width ranged among sites from 15 to 58 m. We established a terrestrial zone of comparable length and width at each site, with a buffer of at least 15 m between the riparian and terrestrial zones. Given the importance of edaphic factors to vegetation, we collected soils or sediments in the channel, riparian, and dryland zones from the 0e5 cm depth using a core sampler (n ¼ 5 replicates
Table 1 Attributes of ephemeral streams. Precipitation and temperature values are 30-year means (US Climate Data; http://www.ncdc.noaa.gov/). A ¼ arid, SA ¼ semiarid, SH ¼ semihumid. DoD ¼ Dept. of Defense; USFS ¼ U.S. Forest Service. Site name
Goldwater sites Black Gap Wash Sauceda Wash Santa Rita sites Small Santa Rita Large Santa Rita Huachuca sites Nr. Huachuca (DoD Nr. Garden (DoD) Nr. Ramsey (USFS)
Lat. and long. in decimal degrees
Mean annual temp.( C)
Mean annual precip.(mm)
Winter precip. as percent of total
Coefficient of variation in precip.
Aridity Index
Aridity zone
Elev-ation(m)
Surface flow (% of time)
Catchmentarea (km2)
32.711123, 112.831066 32.878405, 112.752874
22.3 21.6
97 97
56% 56%
0.48 0.48
3.0 3.1
A A
324 258
0.6 1.1
10 326
31.885414, 110.88042 31.880545, 110.883672
19.0 18.1
227 227
42% 42%
0.23 0.23
7.8 8.1
SA SA
947 952
1.1 2.0
2 18
31.540278, 110.334113 31.506705, 110.316744 31.468538, 110.294548
16.7 16.6 16.3
293 335 397
42% 42% 42%
0.34 0.34 0.34
11.0 12.6 15.1
SH SH SH
1453 1494 1533
1.9 2.0 1.3
1.3 0.5 0.3
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Fig. 1. Streamflow presence through time for ephemeral streams in three climatic settings. Data points are the means for two to three sites per climatic setting.
per stream and zone). In the lab, soils were passed through a 2 mm sieve before determining particle size (percent sand, silt, and clay). We calculated gravimetric soil moisture seasonally, after drying the soils at 105 C. We sampled vegetation during 2010, 2011, and 2012. As an index of above ground biomass, we measured vegetation volume using the vertical line intercept method: We determined the number of vegetation intercepts at decimeter intervals along a telescoping pole, at ten random points in each riparian and terrestrial zone. For plant species richness, we collected incidence data in ten, 2 5 m (10 m2) plots in each riparian zone (70 total) and each terrestrial zone (70 total). We estimated herbaceous ground cover of vascular plants, by species, in ten randomly placed 1 m2 quadrats in each riparian zone (70 total) and terrestrial zone (70 total). We sampled for species richness and herbaceous cover four to five times per site to capture temporal variation (March/April and September/October at the Goldwater sites, May/June and September/October for other sites). Precipitation was above average during 2010, for both winter and summer, and below average during 2011 and 2012 (https:// www.ncdc.noaa.gov/sotc/and http://www.prism.oregonstate.edu/ explorer/). We identified vascular plants to species using Kearney and Peebles (1960), the Vascular Plants of Arizona Project (http://nhc. asu.edu/vpherbarium/vpap.html), and other regional references, and deposited voucher specimens in the Arizona State University herbarium. We grouped species as woody (trees, shrubs, woody vines, and stem succulents in the Cactaceae) or herbaceous. We further classified herbaceous species as predominantly perennial or predominantly annual, using data from the USDA Plants Database (http://plants.usda.gov/) in conjunction with field knowledge. The few biennials were classified as perennials. We used general linear models to determine whether vegetation volume, species richness (totaled across four sampling times), and herbaceous cover and Simpson's E (averaged across sampling times) differed between riparian and terrestrial zones (categorical variable) and with Aridity Index (continuous variable). To determine if the numbers of species exclusive to the riparian zone differed as a function of aridity, we pooled the incidence data from two sites and four collection times per climate setting (using the two Fort Huachuca sites for the semihumid setting) and then classified species as riparian-exclusive (at least five occurrences
with 100% of these in riparian zone), terrestrial-exclusive (at least five occurrences with 100% of these in terrestrial zone), or infrequent (less than 5 occurrences out of 160 possible). We calculated community similarity between zones using Sørensen similarity coefficients, for data pooled across two sites and four seasons. We consider values to be of probable biological significance if P 0.10. 3. Results 3.1. Soils The riparian soils had greater water content than terrestrial soils only during periods with above-average flood runoff (e.g., 2010) (Appendix 2). They had a slightly higher percentage of sand compared to the terrestrial zone (Appendix 3). The percentages of silt, sand, and clay were relatively constant across the aridity gradient, although semihumid sites had slightly more clay (across topographic zones) than arid sites. 3.2. Vegetation volume and cover Woody (and total) vegetation volume were significantly greater in riparian zones than terrestrial zones, but values for herbaceous volume and cover did not differ (Fig. 2., Fig. 3, left panel; Table 2). Species contributing substantially to woody riparian vegetation volume were the legume trees Olneya tesota, Parkinsonia florida and Prosopis velutina (arid setting), P. florida and P. velutina (semiarid), and P. velutina (semihumid). As sites became hotter and drier, woody vegetation volume changed little in riparian and terrestrial zones, whereas herbaceous vegetation volume and cover decreased sharply. Total vegetation volume thus also decreased. At arid sites, herbaceous cover was essentially zero during dry seasons. There were no trees in the terrestrial zone of the arid site. 3.3. Species richness Aggregate numbers of herbaceous, woody, and total species sampled per site were greater in riparian zones (vs. terrestrial), with the between-zone differences considerably greater for
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Fig. 2. Vegetation volume by stratum, parsed by woody and herbaceous, for riparian zones of seven ephemeral streams and adjacent terrestrial zones.
herbaceous species than for woody (Fig. 3, right panel; Table 2). Plot-level differences emerged during wet seaons. At the semihumid site during a summer in which over 30 cm of rain fell during the five months prior to sampling, the riparian zones had up to seven more species per m2 than terrestrial zones. Plot values were uniformly low in riparian and terrestrial zones during dry as well as very hot sampling times (Fig. 4). Species richness patterns across the aridity gradient also differed between herbaceous and woody vegetation. As sites became hotter and drier, numbers of herbaceous species site decreased, while woody species number remained similar for riparian and terrestrial zones alike. Total vascular plant species richness decreased two to three-fold along the aridity gradient (Fig. 3, right panel). Evenness of herbaceous species in the community was higher in riparian zones (vs. terrestrial) (Table 2), with differences most evident in the semihumid-climate streams during the summer wet season. In the terrestrial zone of the near-Huachuca stream, for example, the perennial grass Eragrostis lehmanniana was a clear dominant (Appendix 4). In the riparian zone, in contrast, E. lehmanniana shared dominance with four annual graminoids (Cyperus odoratus, Eriochloa acuminata, Panicum hirticaule, Urochloa arizonica). 3.4. Species composition Compositional similarity between riparian and terrestrial zones
was high in all climatic settings (Sorenson coefficient ¼ 0.69 for arid; 0.79 for semiarid; 0.71 for semihumid). Similarity did differ by plant type, however. The Sorenson community coefficient averaged 0.75 for the herbaceous subset of species and 0.51 for the woody subset. Although the percentage of species that were annuals (vs. perennials) did not differ greatly between topographic zones, it doubled as sites became more arid (Fig. 5). Eight-seven percent of species in the riparian zones of the arid setting were annuals, with the remainder being small trees, shrubs, or succulents. Of these annuals, most were cool-season species (e.g., Amsinckia tesselata) that germinated during the winter wet season; only a few, including Pectis papposa, were present during the hot summer wet season. At the semiarid sites, annuals comprised 53% of the species, and were a mix of cool-season (e.g., Pectocarya recurvata) and warm-season (e.g., Amaranthus palmeri) species. At semihumid Huachuca, annuals comprised only 35% of species (with most, such as Bidens leptocephala, growing during the summer wet season); herbaceous perennials were the prevalent form. The percentages of species in the grass family (Poaceae) differed sharply between the arid setting (3%) and the other two climate settings (22% and 19%). The percentage of species exclusive to the riparian zone was greatest in the arid setting, with many of these being shrubs or trees. Sixteen percent of species sampled at the two Goldwater sites combined, including Ambrosia salsola, Lycium andersonii, Olneya tesota, Parkinsonia florida, and Prosopis velutina, were sampled exclusively in a riparian zone (Fig. 6; Appendix 5). Only one species,
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Fig. 3. Vegetation volume and plant species richness in relation to Aridity Index, for riparian zones of seven ephemeral streams and adjacent terrestrial zones. Value for richness are totals of four sampling seasons per site. Also shown are best-fit regression lines and adjusted r2 values.
an annual, was terrestrial-exclusive. For the two sites at semiarid Santa Rita, four percent of species including Anisacanthus thurberi were riparian exclusive and none were terrestrial exclusive. At semihumid Fort Huachuca, six percent of species, including
Amaranthus palmeri and Epilobium canum, were exclusive to the riparian zone and two percent were terrestrial-exclusive (Fig. 7; Appendix 6).
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Table 2 Effects of aridity and topographic zone (riparian vs. terrestrial) on vegetation volume and plant species richness (total across four seasons, as sampled in 10 m2 plots) based on general linear models (df ¼ 1,12). Aridity Index
Vegetation volume-total Vegetation volume-herb. Herbaceous cover Vegetation volume-woody Species richness- total Species richness- herb. Species richness- woody Herbaceous evenness
Topographic zone
F-ratio P value F-ratio
P value
10.4 20.7 58.9 <0.01 24.4 31.4 0.9 <0.01
0.02 0.94 0.79 0.01 0.05 0.06 0.09 0.10
0.01 <0.01 <0.01 0.97 <0.01 <0.01 0.35 0.91
7.0 <0.01 0.1 9.3 4.7 4.3 3.5 3.2
Model adj. r2
0.61 0.65 0.84 0.46 0.72 0.76 0.29 0.22
infiltrates deeply into the porous sediments of the stream bed and banks. This allows for survivorship of the small legume trees (Olneya, Prosopis, Parkinsonia) that provide the major biomass structure of many ephemeral desert streams in southwestern USA (Smith et al., 1995). Indeed, one study noted that woody plants along an arroyo showed drought-alleviation in response to a channel-filling flow, but did not show ecophysiological response simply to rainfall (de Soyza et al., 2004). In sharp contrast to the woody vegetation, herbaceous plants did not differ in mean cover or volume between topographic zones. Shallow-rooted herbaceous plants respond to events that wet surface soils (Weltzin and McPherson, 2000; Ogle and Reynolds, 2004; Throop et al., 2012), and seasonally-pulsed response to precipitation was evident for herbaceous plants in the washes and drylands alike.
Fig. 4. Mean herbaceous species richness in relation to prior precipitation and temperature (five-month blocks) for riparian and terrestrial zones. Data are shown for all three climatic settings.
4. Discussion 4.1. Conservation value of ephemeral streams in drylands: productivity Individually and collectively, the many small, unnamed ephemeral streams (aka washes) in dryland regions have high conservation value (Ludwig, 1987). Despite their infrequent stream flow, the riparian zones of ephemeral streams are ‘hot spots’ of productivity. The riparian zones we studied supported greater vegetation volume than the surrounding deserts or semidesert grasslands, consistent with observations of greater above-ground biomass, cover, or net primary productivity along ephemeral streams versus adjacent plains or hillsides (Ehleringer and Cooper, 1988; Burquez et al., 2010; Free et al., 2013). The seasonal differences in surface soil water content between ephemeral stream margins and the matrix vegetation are small except during periods of above average runoff, at which time water
4.2. Conservation value of ephemeral streams in drylands: species diversity Ephemeral stream riparian zones sustain many plant species. Our multi-season and multi-year sampling yielded higher species numbers (values of 39e130 among individual streams) than typically reported for ephemeral streams. Most studies to date have captured only one growing season or year or have excluded plant types such as forbs (e.g., Leitner, 1987; Shaw and Cooper, 2008). In addition to supporting more plant species, the ephemeral streams supported more unique plant species than the adjacent piedmont or alluvial plain. Potential explanations for the higher richness include differences in resources (e.g., water) and in dispersal (e.g., more seed flow in the stream network). Differences in diversity between riparian and terrestrial zones are context-dependent and not always apparent. Leitner (1987), for example, found similar numbers of plant species in canyon slopes as in riparian zones of ephemeral streams in Punto Cirio, Mexico, presumably owing to
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Fig. 5. Percent distribution of plant species by lifespan and growth form. Data are based on four sampling times at two streams per climatic setting (the two semihumid streams are on Fort Huachuca, DoD). T ¼ Terrestrial, R ¼ Riparian. Values on the right are total numbers of plant species.
the abundant water stored in the fractured bedrock of the rocky slopes. Ephemeral streams increase regional diversity by supporting a small number of riparian-affiliated species (many of these being woody taxa). And, given the high compositional overlap in herbaceous taxa between ephemeral streams and terrestrial zones, they also may influence regional diversity by providing seed reservoirs for plants that are declining in the dry terrestrial zones. As droughts become more frequent or severe, ephemeral streams may become increasingly important as seed reservoirs for drought-sensitive regional species. Composition of the matrix plant community influences that of desert washes (Levi and Fehmi, 2014) and the converse is presumably true, as well. For example, herbaceous species including Sphaeralcea ambigua and Eriogonum fasciculatum may become locally dependent on washes for persistence (and subsequent spread) in areas of the Colorado Desert that are experiencing extreme drought (Miriti et al., 2007) as may species of Bouteloua in the Chihuahuan Desert (Snyder and Tartowski, 2006). Source-sink dynamics of these ecosystems warrant further study.
4.3. Increasing aridity Aridity is a product of precipitation and temperature. Vegetation of ephemeral streams and terrestrial zones in the American Southwest will be influenced by the interactive effects of declining precipitation and rising temperatures. Per unit of precipitation decline, there is less biologically available soil water under hotter conditions owing to higher rates of evapotranspiration (Fu and Burgher, 2015). High temperatures severely restrict vegetation abundance when water is limiting, as seen in the limited response to summer storm events by the ephemeral stream herbaceous plant community in the hot, arid setting. Extreme resource limitation reduces productivity and the potential for coexistence of multiple plant species (Huston, 2014; intermediate productivity hypothesis). Indeed, we observed a sharp decline in plant species richness with increasing aridity. Most
species in the arid setting were annuals, a life history strategy that is favored by unpredictable environments such as typifies hot deserts (Free et al., 2013; Friedman and Rubin, 2015). Ground cover is sparse for much of the year in arid settings, along ephemeral streams and adjacent deserts alike (Reynolds et al., 2004). Following episodic storm events, short-lived annual plants emerge from storage in the soil as cued by appropriate temperature and moisture conditions (Facelli et al., 2005; Gremer and Venable, 2014). As aridity (and climatic variability) increases in the American Southwest, the riparian zones of ephemeral streams and the adjacent deserts the herbaceous vegetation will have less biomass and fewer plant species and will support a single plant functional type (annual species). Vegetation changes often are non-linear. Mean annual precipitation of 180 mm (a value in between our arid and semiarid sites) has been identified globally as a threshold at which vegetation composition differs sharply (Ulrich et al., 2014). Indeed, we observed stark difference in growth form between riparian ecosystems of the arid climate zone (xeroriparian scrub with seasonally abundant annuals) and the semiarid and semihumid climatic zones (xeroriparian savannahs with abundant perennial and annual grasses). In sharp contrast to herbaceous species, the woody plant community of the ephemeral streams maintained constant abundance and richness (although changing in dominant species) along the spatial aridity gradient. As aridity increases in the American Southwest, the grassland component of ephemeral streams will diminish, and xeroriparian savannahs will be replaced by xeroriparian scrublands. We refrain from making specific projections as to how individual sites will be effected by climate change. Space-for-time substitutions are useful models, but do have limitations. Our different geographic sites are not a perfect analogy for temporal changes because of the existence of some inter-site differences in geomorphology and in seasonal weather patterns (winter vs. summer rain percentages; which may itself change in the future).
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Fig. 6. Number of occurrences of plant species in riparian and terrestrial zones of arid Goldwater. Maximum possible occurrence per species is 80 (two sites four times 10 plots). Plants with less than five occurrences are omitted. Species are listed from least to most abundant within the riparian zone. Woody species are shown separately in inset figure.
4.4. Flood disturbance Water serves as a resource and an agent of disturbance, and climatic models predict an increase in intensity of rain events (Dominguez et al., 2010; Garssen et al., 2015). Well known to influence diversity patterns in large river floodplains, periodic flood scour and sediment transport also create patches available for colonization in ephemeral streams. In our study, the riparian zone had greater evenness of herbaceous species compared to the terrestrial zone, presumably owing in to reduction in competitive
exclusion processes following periodic flood scour. Patterns of Eragrostis lehmanniana exemplify these differences. This Africanorigin grass, viewed by some as a nuisance species, was widely seeded onto overgrazed rangelands in the mid 1900s. Its populations have fluctuated over recent decades but it continues to be a dominant species in many desert grasslands (Bagchi et al., 2012; Morris et al., 2013). The many small ephemeral streams that fragment the semidesert grasslands allow a variety of species to coexist in high numbers with this and other perennial grasses.
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Fig. 7. Number of occurrences of plant species in riparian and terrestrial zones of semihumid Huachuca Mountain foothills. Maximum possible occurrence per species is 80 (two sites four times 10 plots). Plants with less than 10 occurrences are omitted. Species are listed from least to most abundant within the riparian zone. Woody species are shown separately in inset figure.
4.5. Broader context Drylands and their embedded ephemeral streams are being influenced by many local to global changes, in addition to climate. For example, CO2 fertilization may offset projected declines in grass cover resulting from aridity increase (Notaro et al., 2012). In contrast, grazing, a common land use in drylands, has disproportionally negative effects on grass abundance. Grazing can convert
mesic riparian vegetation to a more xeric condition, necessitating careful management of rangelands (and ephemeral streams) to prevent degradation from increasing aridity (Allsopp et al., 2007; Beschta et al., 2013; Golodets et al., 2015). On a more local scale, urban encroachment can exacerbate effects of regional aridity on ephemeral streams by increasing temperatures or can mitigate them by increasing urban runoff (Hutmacher et al., 2014).
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Acknowledgments This study was funded by the Strategic Environmental Research and Development Program (SERDP), grant number RC-1726. We thank the Barry M. Goldwater Air Force Range, Fort Huachuca, Santa Rita Experimental Range, and the Coronado National Forest for allowing site access. We also thank the graduate and undergraduate students who assisted with field studies and laboratory work. Particular thanks to Dr. John Hall for his support of this project. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jaridenv.2016.12.004. References ~ a, V., Datry, T., Marshall, J., Barcelo , D., Dahm, C.N., et al., 2014. Why should we Acun care about temporary waterways? Science 343, 1080e1081. Allsopp, N.L., Gaika, R., Knight, C., Monakisi, M.T., Hoffman, 2007. The impact of heavy grazing on an ephemeral river system in the succulent karoo, South Africa. J. Arid Environ. 71, 82e96. Atchley, M.C., de Soyza, A.G., Whitford, W.G., 1999. Arroyo water storage and soil nutrients and their effects on gas-exchange of shrub species in the northern Chihuahuan Desert. J. Arid Environ. 43, 21e33. ndezBagchi, S., Briske, D.D., Wu, X.B., McClaran, M.P., Bestelmeyer, B.T., Ferna nez, M.E., 2012. Empirical assessment of state-and-transition models with Gime a long-term vegetation record from the Sonoran Desert. Ecol. Appl. 22, 400e411. Beschta, R.L., Donahue, D.L., DellaSala, D.A., Rhodes, J.J., Karr, J.R., et al., 2013. Adapting to climate change on western public lands: addressing the ecological effects of domestic, wild, and feral ungulates. Environ. Manag. 51, 474e491. Blasch, K.W., Ferre, T.P.A., Christensen, A.H., Hoffmann, J.P., 2002. New field method to determine stream flow timing using electrical resistance sensors. Vadose Zone J. 1, 289e299. Bowers, J.E., 2005. Effects of drought on shrub survival and longevity in the Northern Sonoran Desert. J. Torrey Bot. Soc. 132, 421e431. Brown, D.E. (Ed.), 1994. Biotic Communities: Southwestern United States and Northwestern Mexico. University of Utah Press, Salt Lake City. Brusca, R.C., Wiens, J.F., Meyer, W.M., et al., 2013. Dramatic response to climate change in the Southwest: Robert Whittaker's 1963 Arizona mountain plant transect revisited. Ecol. Evol. 3, 3307e3319. Burquez, A., Martinez-Yrizar, A., Nunez, S., Quintero, T., Aparicio, A., 2010. Aboveground biomass in three Sonoran Desert communities: variability within and among sites using replicated plot harvesting. J. Arid Environ. 74, 1240e1247. Datry, T., Larned, S.T., Tockner, K., 2014. Intermittent rivers: a challenge for freshwater ecology. BioScience 64, 229e235. Davis, J.M., Baxter, C.V., Crosby, B.T., Pierce, J.L., Rosi-Marshall, E.J., 2013. Anticipating stream ecosystem responses to climate change: toward predictions that incorporate effects via land-water linkages. Ecosystems 16, 909e922. de Soyza, A.G., Killingbeck, K.T., Whitford, W.G., 2004. Plant water relations and photosynthesis during and after drought in a Chihuahuan desert arroyo. J. Arid Environ. 59, 27e39. Dominguez, F., Canon, J., Valdes, J., 2010. IPCC-AR4 climate simulations for the Southwestern US: the importance of future ENSO projections. Clim. Change 99, 499e514. Ehleringer, J.R., Cooper, T.A., 1988. Correlations between carbon isotope ratio and microhabitat in desert plants. Oecologia 76, 562e566. Facelli, J.M., Chesson, P., Barnes, N., 2005. Differences in seed biology of annual plants in arid lands: a key ingredient of the storage effect. Ecology 11, 2998e3006. Free, C.L., Baxter, G.S., Dickman, C.R., Leung, L.K.P., 2013. Resource pulses in desert river habitats: productivity-biodiversity hotspots, or mirages? Article Number: e72690 PLoS One 8. Friedman, J., Rubin, M.J., 2015. All in good time: understanding annual and perennial strategies in plants. Am. J. Bot. 102, 497e499. Fu, B., Burgher, I., 2015. Riparian vegetation NDVI dynamics and its relationship with climate, surface water and groundwater. J. Arid Environ. 113, 59e68. Garssen, A.G., Baattrup-Pedersen, A., Voesenek, L.A.C.J., Verhoeven, J.T.A., Soons, M.B., 2015. Riparian plant community responses to increased flooding: a meta-analysis. Glob. Change Biol. 21, 2881e2890. Garssen, A.G., Verhoeven, J.T.A., Soons, M.B., 2014. Effects of climate-induced increases in summer drought on riparian plant species: a meta-analysis. Freshw. Biol. 59, 1052e1063. Golodets, C., Sternberg, M., Kigel, J., Boeken, B., Henkin, Z., Seligman, N.G., Ungar, E.D., 2015. Climate change scenarios of herbaceous production along an aridity gradient: vulnerability increases with aridity. Oecologia 177, 971e979.
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Contents lists available at ScienceDirect
Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv
Riparian vegetation of ephemeral streams Juliet C. Stromberg a, *, Danika L. Setaro a, Erika L. Gallo b, Kathleen A. Lohse c, Thomas Meixner b a
Arizona State University, Tempe, AZ 85287-4501, USA University of Arizona, Tucson, USA c Idaho State University, Pocatello, USA b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2016 Received in revised form 17 November 2016 Accepted 2 December 2016
Ephemeral streams are abundant in drylands, yet we know little about how their vegetation differs from surrounding terrestrial zones and about their projected response to regional warming and drying. We assessed plant communities at seven ephemeral streams (and terrestrial zones) distributed among three climatic settings in Arizona. Compared to terrestrial zones, riparian zones had similar herbaceous cover but greater woody vegetation volume. They supported more plant species, with several woody taxa restricted to the ephemeral zone (consistent with the idea that herbaceous plants are rain-dependent while riparian trees rely on runoff stored in stream sediments). Their herbaceous communities had high compositional overlap with terrestrial zones and may sustain regional diversity as droughts intensify. Presumably owing to periodic flood disturbance, riparian plant communities had greater evenness than terrestrial zones, many of which were dominated by Eragrostis lehmanniana. Patterns along the climatic gradient suggest that increasing aridity will reduce the number of herbaceous (and total) plant species within riparian zones (110 species per stream in semihumid settings, 88 in semiarid, 48 in arid) and drive compositional shifts from perennials grasses and forbs to annuals. Hotter and drier conditions will drive sharp declines in herbaceous cover, converting riparian savanna to xeroriparian scrubland. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Aridity Climate change Eragrostis lehmanniana Savanna Species richness Riparian vegetation
1. Introduction In hot dryland regions, the groundwater-dependent riparian ecosystems that border perennial to intermittent rivers and streams have substantially more biomass and greater productivity than the surrounding terrestrial vegetation (Scott et al., 2014). They are vegetated by distinct suites of fast-growing and flood-adapted wetland plant species (obligate riparian taxa), while also providing habitat for many plant species that are typical of more xeric habitats (facultative riparian taxa). They maintain levels of plant species diversity that are greater or lesser than in adjacent desert habitat, depending on context, and increase regional diversity (Sabo et al., 2005). Ephemeral streams are the predominant stream type in desert regions but are understudied relative to their wetter counterparts. By definition, ephemeral streams are decoupled from regional
* Corresponding author. E-mail address: [email protected] (J.C. Stromberg). http://dx.doi.org/10.1016/j.jaridenv.2016.12.004 0140-1963/© 2016 Elsevier Ltd. All rights reserved.
groundwater and flow only in response to major storm runoff events (Meinzer, 1923). Their plant communities-drought-adapted taxa including small-leaved shrubs and short-canopies trees-have been referred to as xeroriparian (Warren and Anderson, 1985; Johnson et al., 1984). The trees grow in narrow linear bands, sustained by water supplied by periodic floods (run-on events) that recharge the often-sandy stream sediments and/or create shallow perched water tables (Atchley et al., 1999; de Soyza et al., 2004; Rassam et al., 2006). Soils often are deeper than on alluvial fans. The extent to which the plant communities of these ephemeral streams differ in biomass, species composition, and species diversity from the terrestrial vegetation remains little studied. While many parts of the world are becoming warmer and wetter, many arid and semiarid regions are becoming hotter and drier (Dominguez et al., 2010; Vicente-Serrano et al., 2012). Analyses of long-term data sets in the American Southwest have documented recent drought-related declines of desert shrubs and upward elevational range shifts of various taxa (Bowers, 2005; McAuliffe and Hamerlynck, 2010; Brusca et al., 2013). Vegetation
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patterns along spatial aridity gradients reveal yet other changes that may occur through time as climate changes, including decreases in above and below ground biomass, shifts in plant growth form, and declines in species richness (Schulze et al., 1996; Munson et al., 2013; Ulrich et al., 2014). Plant communities in riparian ecosystems are changing in response to direct effects of temperature and precipitation and to indirect effects of climate change on watershed processes that regulate flows of surface water and groundwater (Perry et al., 2012; Davis et al., 2013; Garssen et al., 2014; Kløve et al., 2014). For groundwater-dependent-ecosystems (GDEs) in dryland regions, declining groundwater inflows will induce shifts from perennial flow to intermittent flow in susceptible river reaches (Seager et al., 2013). This will cause riparian vegetation to transition from tall, broad-leaf riparian forests to more deeply-rooted and smallerleaved shrublands (Stromberg et al., 2010), and drive initial increases in species richness followed by declines to lowest levels as flow becomes highly infrequent (Katz et al., 2012). The effect of increasing aridity on ephemeral stream vegetation remains little studied. Particularly for streams draining small watersheds, effects may mimic those in the adjacent terrestrial vegetation. There has been a recent surge of interest in understanding and ~ a et al., conserving ephemeral streams in desert landscapes (Acun 2014; Datry et al., 2014). Our goals were to increase understanding of the ways in which vegetation along ephemeral streams differs from the desert matrix and how vegetation will change under a climate scenario of increasing aridity. Focusing on the American Southwest, we asked, (1) to what extent do biomass, diversity, and composition differ between ephemeral streams and adjacent terrestrial zones, and (2) how do plant community attributes vary along an aridity gradient? 2. Materials and methods We selected seven ephemeral streams (colloquially, washes or arroyos) in central and southern Arizona, spanning three aridity zones (Table 1; Appendix 1, electronic version only). The aridity gradient is created by elevational change, and encompasses changes in mean annual precipitation and mean annual temperature. All sites have a summer wet season (JulyeSeptember) and winter wet season (December through March), although the westerly sites receive a higher percentage of rainfall in winter (56%) relative to summer (44%) compared to other sites (42% and 48% respectively). Study streams were functionally Type I (Shaw and Cooper, 2008) in that they drain small watersheds and are regulated more by the local climate than by that of distant mountains. We determined the percentage of time that surface water was
present in study streams by instrumenting one 200-m stream length per site with three electrical resistance sensors with exposed leads (ER, Tidbit v2 UTBI-001 data logger, Onset Corporation, Bourne, MA) (Blasch et al., 2002). (The 200-m stream length also served as the base area for vegetation sampling). The sensors logged a resistance signal every 10 min over a two-year period. For one stream, Sauceda Wash, surface flow data were obtained from USGS stream gaging station 09519760. Collectively, these data indicated that study streams had surface flow for only a few days per year (Table 1; Fig. 1). We calculated aridity using the de Martonne Aridity Index (mean annual precipitation in mm divided by the sum of mean annual temperature in C plus a constant of 10) (Quan et al., 2013). A value of <5 is arid, 5e10 is semiarid, 10e20 is semihumid, 20e30 is humid, and >30 is perhumid. Black Gap Wash and Sauceda Wash, within the Barry M. Goldwater Air Force Range near Gila Bend, Arizona, were arid, with Aridity Index of 3. Both are in the Lower Gila River Basin, on semi-consolidated alluvial basin fans. Their matrix vegetation is Sonoran Desertscrub (Lower Colorado Valley subdivision) (Brown, 1994). Two unnamed streams in the Santa Rita Experimental Range (currently administered by the University of Arizona College of Agriculture), Santa Cruz River Basin, had a semiarid climate with Aridity Index of 8. Both are center-of-basin braided unconsolidated sandy channels. Their matrix vegetation is degraded semidesert grassland (a mix of cacti, grasses, and woody plants). The three ephemeral streams with the greatest precipitation and lowest temperature (semihumid climate, Aridity Index from 11 to 15) drained the foothills of the Huachuca Mountains near Sierra Vista, Arizona, within the San Pedro River Basin. These piedmont streams have semi-consolidated alluvial channels. Two of the three were on land managed by the Department of Defense (Fort Huachuca) and the third was on the Coronado National Forest. For convenience, we named these three streams based on the name of the closest canyon. Their matrix vegetation is semidesert grassland. We delineated the riparian zones from the terrestrial zones based on vegetation cues including changes in size of facultative riparian shrubs (i.e., those that also grow in the dryland). We determined the boundary between the channel and riparian zone using hydrological cues (recent evidence of flow) and vegetation cues (differences in cover). The combined riparian/channel zone width ranged among sites from 15 to 58 m. We established a terrestrial zone of comparable length and width at each site, with a buffer of at least 15 m between the riparian and terrestrial zones. Given the importance of edaphic factors to vegetation, we collected soils or sediments in the channel, riparian, and dryland zones from the 0e5 cm depth using a core sampler (n ¼ 5 replicates
Table 1 Attributes of ephemeral streams. Precipitation and temperature values are 30-year means (US Climate Data; http://www.ncdc.noaa.gov/). A ¼ arid, SA ¼ semiarid, SH ¼ semihumid. DoD ¼ Dept. of Defense; USFS ¼ U.S. Forest Service. Site name
Goldwater sites Black Gap Wash Sauceda Wash Santa Rita sites Small Santa Rita Large Santa Rita Huachuca sites Nr. Huachuca (DoD Nr. Garden (DoD) Nr. Ramsey (USFS)
Lat. and long. in decimal degrees
Mean annual temp.( C)
Mean annual precip.(mm)
Winter precip. as percent of total
Coefficient of variation in precip.
Aridity Index
Aridity zone
Elev-ation(m)
Surface flow (% of time)
Catchmentarea (km2)
32.711123, 112.831066 32.878405, 112.752874
22.3 21.6
97 97
56% 56%
0.48 0.48
3.0 3.1
A A
324 258
0.6 1.1
10 326
31.885414, 110.88042 31.880545, 110.883672
19.0 18.1
227 227
42% 42%
0.23 0.23
7.8 8.1
SA SA
947 952
1.1 2.0
2 18
31.540278, 110.334113 31.506705, 110.316744 31.468538, 110.294548
16.7 16.6 16.3
293 335 397
42% 42% 42%
0.34 0.34 0.34
11.0 12.6 15.1
SH SH SH
1453 1494 1533
1.9 2.0 1.3
1.3 0.5 0.3
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Fig. 1. Streamflow presence through time for ephemeral streams in three climatic settings. Data points are the means for two to three sites per climatic setting.
per stream and zone). In the lab, soils were passed through a 2 mm sieve before determining particle size (percent sand, silt, and clay). We calculated gravimetric soil moisture seasonally, after drying the soils at 105 C. We sampled vegetation during 2010, 2011, and 2012. As an index of above ground biomass, we measured vegetation volume using the vertical line intercept method: We determined the number of vegetation intercepts at decimeter intervals along a telescoping pole, at ten random points in each riparian and terrestrial zone. For plant species richness, we collected incidence data in ten, 2 5 m (10 m2) plots in each riparian zone (70 total) and each terrestrial zone (70 total). We estimated herbaceous ground cover of vascular plants, by species, in ten randomly placed 1 m2 quadrats in each riparian zone (70 total) and terrestrial zone (70 total). We sampled for species richness and herbaceous cover four to five times per site to capture temporal variation (March/April and September/October at the Goldwater sites, May/June and September/October for other sites). Precipitation was above average during 2010, for both winter and summer, and below average during 2011 and 2012 (https:// www.ncdc.noaa.gov/sotc/and http://www.prism.oregonstate.edu/ explorer/). We identified vascular plants to species using Kearney and Peebles (1960), the Vascular Plants of Arizona Project (http://nhc. asu.edu/vpherbarium/vpap.html), and other regional references, and deposited voucher specimens in the Arizona State University herbarium. We grouped species as woody (trees, shrubs, woody vines, and stem succulents in the Cactaceae) or herbaceous. We further classified herbaceous species as predominantly perennial or predominantly annual, using data from the USDA Plants Database (http://plants.usda.gov/) in conjunction with field knowledge. The few biennials were classified as perennials. We used general linear models to determine whether vegetation volume, species richness (totaled across four sampling times), and herbaceous cover and Simpson's E (averaged across sampling times) differed between riparian and terrestrial zones (categorical variable) and with Aridity Index (continuous variable). To determine if the numbers of species exclusive to the riparian zone differed as a function of aridity, we pooled the incidence data from two sites and four collection times per climate setting (using the two Fort Huachuca sites for the semihumid setting) and then classified species as riparian-exclusive (at least five occurrences
with 100% of these in riparian zone), terrestrial-exclusive (at least five occurrences with 100% of these in terrestrial zone), or infrequent (less than 5 occurrences out of 160 possible). We calculated community similarity between zones using Sørensen similarity coefficients, for data pooled across two sites and four seasons. We consider values to be of probable biological significance if P 0.10. 3. Results 3.1. Soils The riparian soils had greater water content than terrestrial soils only during periods with above-average flood runoff (e.g., 2010) (Appendix 2). They had a slightly higher percentage of sand compared to the terrestrial zone (Appendix 3). The percentages of silt, sand, and clay were relatively constant across the aridity gradient, although semihumid sites had slightly more clay (across topographic zones) than arid sites. 3.2. Vegetation volume and cover Woody (and total) vegetation volume were significantly greater in riparian zones than terrestrial zones, but values for herbaceous volume and cover did not differ (Fig. 2., Fig. 3, left panel; Table 2). Species contributing substantially to woody riparian vegetation volume were the legume trees Olneya tesota, Parkinsonia florida and Prosopis velutina (arid setting), P. florida and P. velutina (semiarid), and P. velutina (semihumid). As sites became hotter and drier, woody vegetation volume changed little in riparian and terrestrial zones, whereas herbaceous vegetation volume and cover decreased sharply. Total vegetation volume thus also decreased. At arid sites, herbaceous cover was essentially zero during dry seasons. There were no trees in the terrestrial zone of the arid site. 3.3. Species richness Aggregate numbers of herbaceous, woody, and total species sampled per site were greater in riparian zones (vs. terrestrial), with the between-zone differences considerably greater for
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Fig. 2. Vegetation volume by stratum, parsed by woody and herbaceous, for riparian zones of seven ephemeral streams and adjacent terrestrial zones.
herbaceous species than for woody (Fig. 3, right panel; Table 2). Plot-level differences emerged during wet seaons. At the semihumid site during a summer in which over 30 cm of rain fell during the five months prior to sampling, the riparian zones had up to seven more species per m2 than terrestrial zones. Plot values were uniformly low in riparian and terrestrial zones during dry as well as very hot sampling times (Fig. 4). Species richness patterns across the aridity gradient also differed between herbaceous and woody vegetation. As sites became hotter and drier, numbers of herbaceous species site decreased, while woody species number remained similar for riparian and terrestrial zones alike. Total vascular plant species richness decreased two to three-fold along the aridity gradient (Fig. 3, right panel). Evenness of herbaceous species in the community was higher in riparian zones (vs. terrestrial) (Table 2), with differences most evident in the semihumid-climate streams during the summer wet season. In the terrestrial zone of the near-Huachuca stream, for example, the perennial grass Eragrostis lehmanniana was a clear dominant (Appendix 4). In the riparian zone, in contrast, E. lehmanniana shared dominance with four annual graminoids (Cyperus odoratus, Eriochloa acuminata, Panicum hirticaule, Urochloa arizonica). 3.4. Species composition Compositional similarity between riparian and terrestrial zones
was high in all climatic settings (Sorenson coefficient ¼ 0.69 for arid; 0.79 for semiarid; 0.71 for semihumid). Similarity did differ by plant type, however. The Sorenson community coefficient averaged 0.75 for the herbaceous subset of species and 0.51 for the woody subset. Although the percentage of species that were annuals (vs. perennials) did not differ greatly between topographic zones, it doubled as sites became more arid (Fig. 5). Eight-seven percent of species in the riparian zones of the arid setting were annuals, with the remainder being small trees, shrubs, or succulents. Of these annuals, most were cool-season species (e.g., Amsinckia tesselata) that germinated during the winter wet season; only a few, including Pectis papposa, were present during the hot summer wet season. At the semiarid sites, annuals comprised 53% of the species, and were a mix of cool-season (e.g., Pectocarya recurvata) and warm-season (e.g., Amaranthus palmeri) species. At semihumid Huachuca, annuals comprised only 35% of species (with most, such as Bidens leptocephala, growing during the summer wet season); herbaceous perennials were the prevalent form. The percentages of species in the grass family (Poaceae) differed sharply between the arid setting (3%) and the other two climate settings (22% and 19%). The percentage of species exclusive to the riparian zone was greatest in the arid setting, with many of these being shrubs or trees. Sixteen percent of species sampled at the two Goldwater sites combined, including Ambrosia salsola, Lycium andersonii, Olneya tesota, Parkinsonia florida, and Prosopis velutina, were sampled exclusively in a riparian zone (Fig. 6; Appendix 5). Only one species,
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Fig. 3. Vegetation volume and plant species richness in relation to Aridity Index, for riparian zones of seven ephemeral streams and adjacent terrestrial zones. Value for richness are totals of four sampling seasons per site. Also shown are best-fit regression lines and adjusted r2 values.
an annual, was terrestrial-exclusive. For the two sites at semiarid Santa Rita, four percent of species including Anisacanthus thurberi were riparian exclusive and none were terrestrial exclusive. At semihumid Fort Huachuca, six percent of species, including
Amaranthus palmeri and Epilobium canum, were exclusive to the riparian zone and two percent were terrestrial-exclusive (Fig. 7; Appendix 6).
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Table 2 Effects of aridity and topographic zone (riparian vs. terrestrial) on vegetation volume and plant species richness (total across four seasons, as sampled in 10 m2 plots) based on general linear models (df ¼ 1,12). Aridity Index
Vegetation volume-total Vegetation volume-herb. Herbaceous cover Vegetation volume-woody Species richness- total Species richness- herb. Species richness- woody Herbaceous evenness
Topographic zone
F-ratio P value F-ratio
P value
10.4 20.7 58.9 <0.01 24.4 31.4 0.9 <0.01
0.02 0.94 0.79 0.01 0.05 0.06 0.09 0.10
0.01 <0.01 <0.01 0.97 <0.01 <0.01 0.35 0.91
7.0 <0.01 0.1 9.3 4.7 4.3 3.5 3.2
Model adj. r2
0.61 0.65 0.84 0.46 0.72 0.76 0.29 0.22
infiltrates deeply into the porous sediments of the stream bed and banks. This allows for survivorship of the small legume trees (Olneya, Prosopis, Parkinsonia) that provide the major biomass structure of many ephemeral desert streams in southwestern USA (Smith et al., 1995). Indeed, one study noted that woody plants along an arroyo showed drought-alleviation in response to a channel-filling flow, but did not show ecophysiological response simply to rainfall (de Soyza et al., 2004). In sharp contrast to the woody vegetation, herbaceous plants did not differ in mean cover or volume between topographic zones. Shallow-rooted herbaceous plants respond to events that wet surface soils (Weltzin and McPherson, 2000; Ogle and Reynolds, 2004; Throop et al., 2012), and seasonally-pulsed response to precipitation was evident for herbaceous plants in the washes and drylands alike.
Fig. 4. Mean herbaceous species richness in relation to prior precipitation and temperature (five-month blocks) for riparian and terrestrial zones. Data are shown for all three climatic settings.
4. Discussion 4.1. Conservation value of ephemeral streams in drylands: productivity Individually and collectively, the many small, unnamed ephemeral streams (aka washes) in dryland regions have high conservation value (Ludwig, 1987). Despite their infrequent stream flow, the riparian zones of ephemeral streams are ‘hot spots’ of productivity. The riparian zones we studied supported greater vegetation volume than the surrounding deserts or semidesert grasslands, consistent with observations of greater above-ground biomass, cover, or net primary productivity along ephemeral streams versus adjacent plains or hillsides (Ehleringer and Cooper, 1988; Burquez et al., 2010; Free et al., 2013). The seasonal differences in surface soil water content between ephemeral stream margins and the matrix vegetation are small except during periods of above average runoff, at which time water
4.2. Conservation value of ephemeral streams in drylands: species diversity Ephemeral stream riparian zones sustain many plant species. Our multi-season and multi-year sampling yielded higher species numbers (values of 39e130 among individual streams) than typically reported for ephemeral streams. Most studies to date have captured only one growing season or year or have excluded plant types such as forbs (e.g., Leitner, 1987; Shaw and Cooper, 2008). In addition to supporting more plant species, the ephemeral streams supported more unique plant species than the adjacent piedmont or alluvial plain. Potential explanations for the higher richness include differences in resources (e.g., water) and in dispersal (e.g., more seed flow in the stream network). Differences in diversity between riparian and terrestrial zones are context-dependent and not always apparent. Leitner (1987), for example, found similar numbers of plant species in canyon slopes as in riparian zones of ephemeral streams in Punto Cirio, Mexico, presumably owing to
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Fig. 5. Percent distribution of plant species by lifespan and growth form. Data are based on four sampling times at two streams per climatic setting (the two semihumid streams are on Fort Huachuca, DoD). T ¼ Terrestrial, R ¼ Riparian. Values on the right are total numbers of plant species.
the abundant water stored in the fractured bedrock of the rocky slopes. Ephemeral streams increase regional diversity by supporting a small number of riparian-affiliated species (many of these being woody taxa). And, given the high compositional overlap in herbaceous taxa between ephemeral streams and terrestrial zones, they also may influence regional diversity by providing seed reservoirs for plants that are declining in the dry terrestrial zones. As droughts become more frequent or severe, ephemeral streams may become increasingly important as seed reservoirs for drought-sensitive regional species. Composition of the matrix plant community influences that of desert washes (Levi and Fehmi, 2014) and the converse is presumably true, as well. For example, herbaceous species including Sphaeralcea ambigua and Eriogonum fasciculatum may become locally dependent on washes for persistence (and subsequent spread) in areas of the Colorado Desert that are experiencing extreme drought (Miriti et al., 2007) as may species of Bouteloua in the Chihuahuan Desert (Snyder and Tartowski, 2006). Source-sink dynamics of these ecosystems warrant further study.
4.3. Increasing aridity Aridity is a product of precipitation and temperature. Vegetation of ephemeral streams and terrestrial zones in the American Southwest will be influenced by the interactive effects of declining precipitation and rising temperatures. Per unit of precipitation decline, there is less biologically available soil water under hotter conditions owing to higher rates of evapotranspiration (Fu and Burgher, 2015). High temperatures severely restrict vegetation abundance when water is limiting, as seen in the limited response to summer storm events by the ephemeral stream herbaceous plant community in the hot, arid setting. Extreme resource limitation reduces productivity and the potential for coexistence of multiple plant species (Huston, 2014; intermediate productivity hypothesis). Indeed, we observed a sharp decline in plant species richness with increasing aridity. Most
species in the arid setting were annuals, a life history strategy that is favored by unpredictable environments such as typifies hot deserts (Free et al., 2013; Friedman and Rubin, 2015). Ground cover is sparse for much of the year in arid settings, along ephemeral streams and adjacent deserts alike (Reynolds et al., 2004). Following episodic storm events, short-lived annual plants emerge from storage in the soil as cued by appropriate temperature and moisture conditions (Facelli et al., 2005; Gremer and Venable, 2014). As aridity (and climatic variability) increases in the American Southwest, the riparian zones of ephemeral streams and the adjacent deserts the herbaceous vegetation will have less biomass and fewer plant species and will support a single plant functional type (annual species). Vegetation changes often are non-linear. Mean annual precipitation of 180 mm (a value in between our arid and semiarid sites) has been identified globally as a threshold at which vegetation composition differs sharply (Ulrich et al., 2014). Indeed, we observed stark difference in growth form between riparian ecosystems of the arid climate zone (xeroriparian scrub with seasonally abundant annuals) and the semiarid and semihumid climatic zones (xeroriparian savannahs with abundant perennial and annual grasses). In sharp contrast to herbaceous species, the woody plant community of the ephemeral streams maintained constant abundance and richness (although changing in dominant species) along the spatial aridity gradient. As aridity increases in the American Southwest, the grassland component of ephemeral streams will diminish, and xeroriparian savannahs will be replaced by xeroriparian scrublands. We refrain from making specific projections as to how individual sites will be effected by climate change. Space-for-time substitutions are useful models, but do have limitations. Our different geographic sites are not a perfect analogy for temporal changes because of the existence of some inter-site differences in geomorphology and in seasonal weather patterns (winter vs. summer rain percentages; which may itself change in the future).
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Fig. 6. Number of occurrences of plant species in riparian and terrestrial zones of arid Goldwater. Maximum possible occurrence per species is 80 (two sites four times 10 plots). Plants with less than five occurrences are omitted. Species are listed from least to most abundant within the riparian zone. Woody species are shown separately in inset figure.
4.4. Flood disturbance Water serves as a resource and an agent of disturbance, and climatic models predict an increase in intensity of rain events (Dominguez et al., 2010; Garssen et al., 2015). Well known to influence diversity patterns in large river floodplains, periodic flood scour and sediment transport also create patches available for colonization in ephemeral streams. In our study, the riparian zone had greater evenness of herbaceous species compared to the terrestrial zone, presumably owing in to reduction in competitive
exclusion processes following periodic flood scour. Patterns of Eragrostis lehmanniana exemplify these differences. This Africanorigin grass, viewed by some as a nuisance species, was widely seeded onto overgrazed rangelands in the mid 1900s. Its populations have fluctuated over recent decades but it continues to be a dominant species in many desert grasslands (Bagchi et al., 2012; Morris et al., 2013). The many small ephemeral streams that fragment the semidesert grasslands allow a variety of species to coexist in high numbers with this and other perennial grasses.
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Fig. 7. Number of occurrences of plant species in riparian and terrestrial zones of semihumid Huachuca Mountain foothills. Maximum possible occurrence per species is 80 (two sites four times 10 plots). Plants with less than 10 occurrences are omitted. Species are listed from least to most abundant within the riparian zone. Woody species are shown separately in inset figure.
4.5. Broader context Drylands and their embedded ephemeral streams are being influenced by many local to global changes, in addition to climate. For example, CO2 fertilization may offset projected declines in grass cover resulting from aridity increase (Notaro et al., 2012). In contrast, grazing, a common land use in drylands, has disproportionally negative effects on grass abundance. Grazing can convert
mesic riparian vegetation to a more xeric condition, necessitating careful management of rangelands (and ephemeral streams) to prevent degradation from increasing aridity (Allsopp et al., 2007; Beschta et al., 2013; Golodets et al., 2015). On a more local scale, urban encroachment can exacerbate effects of regional aridity on ephemeral streams by increasing temperatures or can mitigate them by increasing urban runoff (Hutmacher et al., 2014).
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