Identifying the determinants of tree distributions along a large ephemeral river

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 westwards 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.