Abstract:

Grasslands are globally threatened by climate changes and unsustainable land-use, which often cause transitions among alternative stable states, and even catastrophic transition to desertification. Spatial vegetation patch configurations have been shown to signify such transitions at large spatial scale. Here, we demonstrate how small-scale patch configurations can also indicate state transitions.

The whole spatial series of degradation successions were chosen in alpine grasslands characterized as seven typical communities. Patch numbers, and perimeter and cover of each patch were recorded using adjacent quadrats along transects in each type of the communities. Species abundance within each patch was measured.

Across seven grazing-induced degradation stages in the world’s largest expanse of grassland, from dense ungrazed turf to bare black-soil crust, patch numbers and perimeters first increased as patch cover decreased. Numbers and perimeters then decreased rapidly beyond an intersection point at 68% of initial continuous vegetation cover. Around this point, the vegetation fluctuated back and forth between the sedge-dominated grassland breaking-up phase and the forb-dominated phase, suggesting impending shift of grassland state. This study thus demonstrates how ground-based small-scale vegetation surveys can provide a quantitative, easy-to-use signals for vegetation degradation, with promise for detecting the catastrophic transition to desertification.

Key words: grassland degradation, state transition, Tibetan Plateau, trade-off, vegetation patchiness