Hydrologic connectivity as a predictor of degradation thresholds across semiarid sites with different vegetation patterns

Abstract

Dryland landscapes typically display a two-phase mosaic consisting of densely vegetated patches interspersed with low-cover or bare soil areas. The extent and spatial patterns of these patches have a direct effect on ecosystem function and disturbances, such as over grazing, can disrupt the original structure of vegetation and lead to degradation. This work investigates changes in the hydrologic connectivity (i.e., the degree to which areas of the landscape connect to each other) of Mulga landscapes induced by land degradation. Mulga is a keystone ecosystem of the Australian drylands and is characterised by a patchy vegetation cover, which can vary considerably from site to site. We analyse 31 plots with different degrees of degradation (or vegetation cover) in four Mulga sites with different precipitation, slope and vegetation and we quantify hydrologic connectivity combining high-resolution binary vegetation maps and DEMs. Results indicate that connectivity increases as vegetation cover decreases, but this relation is significantly non-linear with a clear threshold at 38 % vegetation cover below which connectivity (and loss of resources due to runoff out of the system) increases dramatically leading to degradation. A site with a pattern of vegetation strands concentrated along drainage lines showed consistently higher connectivity (due to longer connected paths) compared to the other sites where vegetation was more uniformly scattered or presented banded pattern perpendicular to drainage lines. Outputs from a vegetation thinning algorithm on patch edges consistent with grazing effects confirm the existence of the observed threshold in vegetation cover and the influence of vegetation patterns on connectivity. Our results indicate that connectivity is a strong indicator to detect degradation thresholds over a variety of vegetation arrangements typical of dryland systems.