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 b University of Arizona, Tucson, USA c Idaho State University, Pocatello, USA article info Article history: Received 14 June 2016 Received in revised form 17 November 2016 Accepted 2 December 2016 Keywords: Aridity Climate change Eragrostis lehmanniana Savanna Species richness Riparian vegetation abstract 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. 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 di- versity (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 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, sus- tained 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 di- versity 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). Ana- lyses 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 * Corresponding author. E-mail address: jstrom@asu.edu (J.C. Stromberg). Contents lists available at ScienceDirect Journal of Arid Environments journal homepage: www.elsevier.com/locate/jaridenv http://dx.doi.org/10.1016/j.jaridenv.2016.12.004 0140-1963/© 2016 Elsevier Ltd. All rights reserved. Journal of Arid Environments 138 (2017) 27e37