Coevolving patterns of flowpaths, soils, landforms and vegetation in semiarid regions: links between spatial organization, hydrologic connectivity and function (Invited)

Abstract:

Semiarid landscapes exhibit highly nonlinear interactions between coevolving physical and biological processes. Coevolution in these systems leads to the emergence of hydrology, soil, landform and vegetation patterns that are intrinsically linked to their healthy function. Growing concern over ecosystem resilience to climate and land use perturbations that could result in irreversible degradation has imposed a pressing need for research, aiming at elucidating the processes, feedbacks, and dynamics leading to these coevolving patterns. This is particularly important in drylands, since degradation has been frequently linked to feedback effects between water, biota and erosion processes.

We will discuss results based on remote sensing data for several sites in Australia that suggest the presence of a critical degradation threshold, associated to loss of vegetation cover. Below this threshold landscapes display high rainfall use efficiency and low hydrologic connectivity, and can therefore be considered functional. Above this threshold, we find landscapes that display high hydrologic connectivity and low rainfall use efficiency and can be classified as dysfunctional. We also observe landscapes that exhibit vegetation pattern transitions to alternative states that can be still considered functional but with less productivity.

We further analyse these transitions, which arise from coevolving hydrologic, soils, landforms and vegetation processes through modelling experiments. The model captures the feedbacks through the coupled nature of the processes included in the hydrology-landform-vegetation modules. Hydrologic connectivity is dynamically prescribed by the emergence of areas with low infiltration in unvegetated soil patches (surface crusting) and high infiltration rates in the vegetated soil patches (improved soil aggregation and macroporosity). This dynamic hydrologic connectivity determines areas of soil erosion and deposition. In this way, model simulations allow us to track the dynamics of hydrologic connectivity emerging from these non-linear feedbacks and understand its role in determining ecosystem health and resilience.

The relevance and implications of these results for the successful reclamation of water-limited environments, in which vegetation stability largely depends on the redistribution of the scarce water resources, will be discussed. These implications are illustrated using data and observations from agricultural sites in central Australia and reclaimed mining sites in Spain. Finally, the need for additional work considering other alternative relevant mechanisms will also be discussed and will hopefully trigger further debate that can be later pursued in the synthesis workshops and discussions.