Identifying the determinants of tree distributions along a large ephemeral river

Abstract

Although ephemeral rivers act as linear oases and play a fundamental role in sustaining regional biodiversity in dryland regions, little is known about these systems or their sensitivity to human impacts. Without such knowledge, it is difficult to manage or conserve them. Here, we conduct the first systematic investigation into the determinants of riparian tree distributions along a large ephemeral river. Adopting a macroecological approach, we test four hypotheses relating to the effects of topography, river flow, climate, and land tenure on three indices of tree distribution: species richness, occupancy, and recruitment. We also consider the effect of upstream damming. Our study site is the Swakop River in Namibia. The most common trees along the river were the invasive Prosopis spp., followed by native Faidherbia albida, Vachellia erioloba, Euclea pseudebenus, and Vachellia tortilis. We found a gradient in tree distributions along the river, with a drier climate westward associated with lower native tree species richness and increased scarcity of the dominant native species (F. albida). These patterns were seen in both pre- and post-dam samples. We also found F. albida was more likely to recruit immediately downstream of tributaries. Our results suggest that water availability (climate and river flow) is a more important determination of tree distribution along this ephemeral river than topography or land tenure and that ephemeral rivers may show a nodal organization.