Desertification patterns induced by coupled geomorphic and vegetation interactions: the case of biological invasions

A common form of land degradation at the desert margins involves the rapid encroachment of shrubs into historical grasslands with ecohydrological and biogeochemical consequences. The conversion of native grasslands into shrublands increases soil resource heterogeneity as a result of enhancement of hydrological and aeolian soil erosion processes. Periodic fires in shrub-grass transition systems are known to enhance grass regrowth by enhancing the local scale redistribution of soil leading to resource homogenization. Here, based on a combination of extensive field experiments and a spatially explicit model, we propose a new desertification paradigm involving the invasion of stable desert shrublands by exotic annual grasses. Exotic grass invasion in desert shrublands can increase fire frequency, cause shrub mortality and alter local scale soil erosion processes thereby destroying the resource heterogeneity typical of desert shrublands. However the long-term persistence of the invasive grass cover may be restricted by recurrent droughts that displace these grass species, which may not have evolved in this regional climatic context. Drought-induced loss of invasive grasslands may be followed by a dramatic increase in functional connectivity and consequently higher erosion rates and loss of soil resources. Thus, it is proposed that desertification can be facilitated both by an increase in heterogeneity (shrub encroachment into grasslands) and in homogeneity (exotic annual grass invasion into desert shrublands) of soil resources, depending on the plant functional type inducing the change in soil resource distribution. The desertification mechanism we propose suggests that two major drivers of global environmental change, namely biological invasions and climate change, may act in concert and amplify each other’s effect on land cover and soil resources. Global climate change may be providing more opportunities for these transitions to occur by creating fluctuations in the rates and distribution of precipitation and by increasing the frequency of droughts in dryland systems.