Community Composition and Functional Traits of Riparian Vegetation on the Effluent Dependent Santa Cruz River Along a Water Availability Gradient

Abstract

As aridity intensifies globally, riparian ecosystems are facing heightened vulnerabilities due to their dependence on water availability. We investigate how surface flow permanence and depth to groundwater influence plant community composition and functional traits over time, revisiting sites from a 2008 study to assess changes in response to altered water availability. Our research reveals that surface flow permanence plays a significant role in shaping plant community metrics such as total cover, species richness, diversity, and wetland indicator status, whereas depth to groundwater has a less consistent impact due to its disassociation with surface water due to effluent release. Sites with sustained or increased surface flow between 2008 and 2023 demonstrated notable increases in species richness and diversity, emphasizing the critical role of maintaining surface flow for riparian vegetation and associated ecosystem services. Additionally, we apply a functional trait framework, analyzing specific leaf area (SLA), leaf dry matter content (LDMC), and plant height to understand shifts in resource-use strategies. Our findings indicate a transition towards resource conservation, evidenced by decreased SLA and plant height, alongside increased LDMC as water availability diminishes. The narrowing ranges of trait values under limited water conditions highlight a stronger filter on species persistence and a trend towards reduced diversity. These results demonstrate the ongoing shifts in both species composition and functional traits driven by increasing aridity, illustrating the need for continued monitoring and management to preserve riparian ecosystems in aridland regions.