Ecohydrological interactions in a degraded two-phase mosaic dryland: Implications for regime shifts, resilience, and restoration

In the semi-arid rangelands of north central Kenya, the abundance of grasses has declined over recent decades. Concomitantly, the native succulent herb Sansevieria volkensii has greatly proliferated in many areas. Landscape structure has thus shifted from contiguous herbaceous cover to large, dense stands of S. volkensii with interpatches of bare or sparsely vegetated ground. Local pastoralists view this transition as highly undesirable; understanding the mechanisms and reversibility of the transition is necessary to develop management options. Because water availability is a key driver in this dryland ecosystem, we assessed the ecohydrological dynamics that may govern the vegetative spread of S. volkensii patches in a communal landholding in Laikipia District, Kenya. We assessed surface water flow patterns, soil moisture, S. volkensii proliferation, and adjacent vegetation density along the margins of dense patches of S. volkensii. We found that run-on zones along patch margins had greater soil moisture than runoff zones following a rain event. These run-on zones also had significantly higher S. volkensii ramet density, more new ramets, and greater leaf water content. New ramet density was negatively affected by the density of adjacent interpatch vegetation. We interpret these findings in the context of scale-dependent feedbacks and the cusp catastrophe model of ecosystem transitions. We hypothesize that the amount and spatial heterogeneity of plant available moisture governs the reversibility of transitions in this system and discuss the implications of this finding for management and restoration.