The spatio-temporal dynamics of four plant-functional types (PFTs) in alpine meadow as affected by human disturbance, Sanjiangyuan region, China

Abstract

Overgrazing, along with small mammal outbreaks, has been considered a primary reason for the increased degradation of alpine meadows in the Sanjiangyuan region on the Qinghai-Tibetan Plateau (QTP). Plant communities have been dramatically affected by such degradation. Field-based studies of plant communities affected by degradation are inevitably short-term and local, and therefore struggle to reveal the spatio-temporal dynamics of plant-functional types (PFTs) during and after degradation. Such problems can, at least partly, be resolved using spatially explicit simulation modelling drawing on contemporary understanding of these ecosystems. A grid-based model was developed, based on field studies and existing literature, to assess controls upon grassland degradation and prospective restoration strategies using model experiments. The model suggests that at small scales the PFTs show aggregated spatial distributions. After 500 simulation years, grasses and forbs dominated in the landscape under a very low grazing level (< 20% stocking rate), grasses and sedges under a low grazing level (20-40%), sedges and grasses under a moderate grazing level (40-60%), while degraded ground is prevalent under high grazing levels (> 60%). The model suggests that degradation will be ongoing under the current stocking rate of around 70%. Severely degraded areas (Heitutan) did not form under very low, low and moderate grazing intensities, but formed after 370 simulated years at the high grazing level, and after around 21 years at the very high grazing level. The timeframe for Heitutan self-restoration varied under different grazing intensity regimes from 50 years or less, at a very low grazing intensity, to effectively un-restorable, at very high intensities. A grazing intensity of around 60% stocking rate represents a biotic threshold condition for the formation of Heitutan. A burrow density of around 4500 burrows/ha represents an abiotic threshold condition for the formation of Heitutan. With a stocking rate greater than 60% the grassland loses its resilience and adopts an alternative stable state (i.e. Heitutan). Three alternative stable states with high resilience were identified in the alpine meadows on the QTP: grass-dominated grassland, sedge-dominated with sub-dominance by grasses grassland and Heitutan dominated by degraded ground.