Soil seed bank dynamics under the influence of grazing as alternative explanation for herbaceous vegetation transitions in semi-arid rangelands

Ecological studies have frequently stressed that the availability of seeds in the soil is important for the recovery of semi-arid rangelands. However, the crucial role of soil seed banks has not been incorporated into rangeland models to understand vegetation states and transitions in semi-arid rangelands. We developed and evaluated a novel model to show that the availability of seeds in the soil seed banks as a function of plant cover can trigger transitions from perennial to annual grasses and from annual grasses to bare soil with increasing grazing pressure. The model indicates that when grazing pressure is low, a high cover of perennial grasses and a large soil seed bank of these grasses may be present, whereas annual grasses with their seeds in the soil appear with increasing grazing. When grazing pressure further increases, vegetation cover and the soil seed bank size decline. We found that the positive feedback between plant cover and the size of the soil seed bank depends on seed traits, i.e., longevity and germination rate. This positive feedback is an alternative explanation for a sudden vegetation changes in rangelands, which are often explained by the positive feedback between plant cover and the infiltration rate of rain into the soil. In contrast to this latter positive feedback, our model can explain shifts in vegetation from perennials to annuals and vice versa on different soil types, which are often seen in semi-arid rangelands. Our model contributes therefore to the understanding of vegetation dynamics for the proper management and possible restoration of degraded semi-arid rangelands.