Dryland Riparian Ecosystems in the American Southwest: Sensitivity and Resilience to Climatic Extremes

Abstract

Drylands make up over 40% of the earth’s land surface (Noy-Meir 1973; Reynolds and others 2007). In these arid and semiarid landscapes, surface and subsurface water flows create mesic riparian environments (Kingsford 2006; Stromberg and Tellman 2009). Dryland riparian areas sustain water-limited plants and animals that cannot withstand upland conditions year-round, thereby supporting regional diversity (Sabo and others 2005; Lite and others 2005). Dryland rivers are also characterized by occasional large, high-intensity floods that rework the bed geomorphology and increase riparian heterogeneity. This combined variation in water availability and flood disturbance creates spatiotemporally complex and unique landscapes (Soykan and Sabo 2009; Soykan and others 2012). The hydrology of desert riparian ecosystems has been extensively altered by human activities including stream diversion and groundwater extraction, and is expected to undergo further change in response to changing climates (Patten 1998). Globally, increasing temperatures associated with climate change are expected to intensify the water cycle, leading to greater rates of evapotranspiration, more water in the atmosphere, and more-frequent intense storms (Huntington 2006; IPCC 2007). More intense storms are likely to increase flood intensity over the next 50 years (Dominguez and others 2012). In the American Southwest in particular, warmer sea surface temperatures could spur stronger and more-frequent El Nino-associated winter storms (Garfin and Lenart 2007). Perhaps of greater importance are projected increases in aridity. Average precipitation is expected to increase at a global scale, but some regions, including the American Southwest, are expected to experience substantial decreases in average annual rainfall as well as increases in frequency and intensity of droughts (Seager and others 2007). As aridity increases, streams and riparian zones undergo declines in surface flow and depth to groundwater; there are declines, as well, in the extent of river segments with perennial (vs. intermittent or ephemeral) stream flow. Given that rivers and their riparian corridors function as keystone elements of arid and semiarid landscapes, climate change will have far reaching effects on regional biota (Stromberg and others 2010). Nevertheless, the extent of biotic change for riparian ecosystems in coming decades will be tempered by the fact that for desert rivers, climatic and hydrologic variation is the norm (Milly and Received 30 May 2012; accepted 13 September 2012