Vegetation Dynamics Control Hillslope Asymmetry In Semiarid Ecosystems

The non-uniformity in insolation leads to the development of aspect-controlled ecosystems characterised by heterogeneity in vegetation. Vegetation dynamics is strongly influenced by the availability of soil moisture in opposing hillslopes in semi-arid landscapes; which in turn is responsible for the water balance, soil moisture dynamics as well as the type and rate of erosion, and landscape evolution. The available soil moisture becomes the limiting factor for vegetation growth in water-stressed regions, especially in mid-to-high latitudes, where poleward-facing slopes tend to develop denser vegetation cover, providing more erosion protection than on the equatorward-facing slopes. Over the longer period of time, this difference in erodibility caused by soil moisture and vegetation dynamics leads to asymmetry in opposing hillslopes on the coevolving landscapes. This asymmetry in hillslope can be measured by Hillslope Asymmetry Index (HAI), a metric that is calculated as the ratio of the median slope angles of opposite hillslopes. In this study, we present a novel approach to investigate the relationships of HAI with climatological, geomorphic and ecologic variables at a global scale. Here, we analysed these relationships using DEM data (to compute HAI) and existing data on vegetation and climatology for 80 different catchments across the world, in which aspect-controlled vegetation has been reported in the literature. Further, we also perform similar analyses for a few hundred young cinder cones, with ages that have been radiometrically dated as less than 1.5 Ma, to understand how HAI vary with time after the formation of cinder cones. The regression analysis shows that latitude, mean topographic gradient, and aridity index, which are taken as proxies for different climates, are the leading factors controlling HAI in semi-arid catchments, possibly because of their key role on the modulation of incoming solar radiation and their effect on vegetation density. On the other hand, hillslope asymmetry in cinder cones is mostly affected by latitude and aridity index. These results improve our understanding of the main factors contributing to hillslope asymmetry and show important implications for the analysis and modelling of coevolving landscapes in water limited ecosystems.