Riparian plant guilds of ephemeral, intermittent and perennial rivers

Plant functional types (or guilds) increasingly are being used to predict vegetation response to global changes. Continued human population growth coupled with projected warmer and drier climate will alter the hydrologic regimes of many arid-zone rivers, including intermittent rivers. We aimed to identify (i) woody plant guilds associated with distinct stream types of an arid region and (ii) plant traits indicating adaptation to the selective pressures of water availability and fluvial disturbance. We used hierarchical clustering to identify 11 plant guilds from floodplains, terraces and uplands of eight Arizona rivers that vary in surface flow permanence, depth to ground water and intensity of fluvial disturbance. Six guilds were riparian pioneers with small, wind-dispersed seeds, three guilds were late-seral, shade-tolerant riparian taxa with large animal-dispersed seeds, and two guilds were composed of desert xerophytes. Within the riparian pioneer and seral groups, guilds varied in water acquisition and productivity traits including wood density and rooting depth. The community-weighted traits varied or covaried with water availability and fluvial disturbance. Root: shoot ratio, canopy height and leaf area were influenced strongly by water availability, with the latter two showing a nonlinear response to changes in water table depth. Leaf length increased, and wood density decreased, as sites become wetter and more fluvially disturbed. Community-weighted seed mass, seed dispersal and spinesence varied most strongly with elevation above thalweg (an indicator of decreasing fluvial disturbance). These analyses will enable prediction of changes in the relative abundance of plant types and plant traits in response to changes in stream flow regimes, such as shifts towards greater intermittency. The distribution patterns of guilds among riparian habitat types emphasise the importance of focusing conservation efforts not only on the limited number of perennial rivers remaining in arid regions, but also on intermittent and ephemeral rivers with shallow water tables.