Within-patch plant diversity modulates the eco-hydrological source-sink dynamics of dryland landscapes

Source-sink processes and feedbacks are critical for ecosystem function and dynamics in dryland landscapes. In patchy-vegetation drylands, the runoff generated on bare-soil inter-patches provides resource inputs to downslope patches, enhancing plant growth and increasing patch cover, which in turn controls the size of the bare-soil areas, completing a source-sink feedback loop. Both the efficiency of vegetation patches in capturing and storing runoff-driven resources and the response of the patch vegetation to such resource inputs depend on the functional traits of the species in the patch. We hypothesized that increasing within-patch plant diversity enhances the sink function of the patch, and thus reinforces the strength of the source-sink feedback. To test such hypothesis, we established over 600 vegetation patches on a 0.5 ha bare-soil experimental slope, resulting in six replicated treatments that combined different numbers of species and individuals per patch (up to 8 species and 8 individuals per patch). Based on drone surveys conducted five years after the establishment of the vegetation patches, we estimated the area, height, volume and normalised difference vegetation index (NDVI) of each patch, as well as a variety of metrics that captured the size and shape of the upslope bare-soil inter-patch draining into each patch. We found that increasing the size of the drainage area resulted in a general increase in NDVI and patch growth. For a given patch size (number of plant individuals), increasing within-patch plant diversity increased the overall strength of the positive relationships between patch performance and drainage area. Our results demonstrate that within-patch plant diversity controls eco-hydrological source-sink dynamics in drylands, and highlight the potential of establishing functionally-diverse plant patches for the restoration of degraded drylands.