Riparian plant species richness along lateral and longitudinal gradients of water stress and flood disturbance, San Pedro River, Arizona, USA

For woody species, richness within plant functional groups varied in opposing ways along the lateral gradients: hydromesic pioneer species decreased and hydromesic and xeric competitors increased with distance from or above the channel, with no overall change in species richness. Among sites, woody species richness patterns were related to water availability, but not to flood disturbance. However, richness of woody hydromesic pioneer species increased with both increasing site moisture and flood disturbance. Woody and herbaceous species richness both increased among sites as a function of increasing flood-plain width, likely due to species-area effects. Overall, results indicate that flood disturbance and water availability both influence species richness of riparian plants in the flood plain of this semi-arid region river, with the relative influence of each factor varying among plant groups and over time.