Seasonal reversals of upland-riparian diversity gradients in the Sonoran Desert

Flood disturbance and water resource availability vary sharply over time and space along arid-region rivers and can interact in complex fashion to shape diversity patterns. Plant diversity showed spatial patterning along a topogradient from the floodplain of the San Pedro River (Arizona, USA) to the arid upland, but the patterns shifted temporally as the suite of limiting factors changed. During two of three sampling times, spatial diversity patterns were shaped primarily by gradients of water availability, the regional limiting factor. In the summer dry season, microscale diversity (species richness per 1 m²) and mesoscale diversity (cumulative species and functional types in 20, 1-m² plots) of herbaceous plants decreased along the topogradient from floodplain to upland, reflecting the greater water availability on the low surfaces. During a summer wet season with moderate rains and flooding, diversity increased in all hydrogeomorphic zones (floodplain, terrace, upland), but the spatial pattern along the topogradient persisted. Following a very wet winter, patterns along the topogradient reversed: scour from large floods limited diversity on the floodplain and competitive exclusion limited the diversity on undisturbed river terrace, while abundant rains allowed for high microscale diversity in the upland. Disturbance and resource availability thus interacted to influence plant species diversity in a fashion consistent with the dynamic-equilibrium model of species diversity. In contrast to the microscale patterns, mesoscale diversity of species and functional types remained high in the floodplain during all sampling times, with 58% more plant species and 90% more functional types sampled in low floodplain than arid upland for the year as a whole. Species with a wide range of moisture and temperature affinities were present in the floodplain, and seasonal turnover of species was high in this zone. The floodplain zone of a perennial to intermittent-flow river thus had greater plant diversity than arid Sonoran Desert upland, as measured at temporal scales that capture seasonal variance in resource and disturbance pulses and at spatial scales that capture the environmental heterogeneity of floodplains. Although periodically limited by intense flood disturbance, diversity remains high in the floodplain because of the combination of moderate resource levels (groundwater, seasonal flood water) and persistent effects of flood disturbance (high spatial heterogeneity, absence of competitive exclusion), in concert with the same climatic factors that produce seasonally high diversity in the region (temporally variable pulses of rainfall).