干旱区生态环境知识资源中心

Resilience theory suggests that ecosystems can persist for long periods, before changing rapidly to a new vegetation phase. Transition between phases occurs when ecological thresholds have been crossed, and is followed by a reorganization of biotic and environmental interactions, leading to the emergence of a new vegetation phase or quasi-stable state. Savannas are dynamic, complex systems in which fire, herbivory, water and nutrient availability interact to determine tree abundance. Phase and transition has been observed in savannas, but the role of these different possible drivers is not always clear. In this study, our objectives were to identify phase and transition in the fossil pollen record, and then to explore the role of nitrogen and fire in these transitions using delta(15)N isotopes and charcoal abundance. We present palaeoenvironmental data from the Kruger National Park, South Africa, which show transition between grassland and savanna phases. Our results show transition at the end of the ninth century A.D. from a nutrient- and herbivore-limited grazing lawn, in which fire was absent and C(4) grasses were the dominant and competitively superior plant form, to a water-, fire- and herbivory-limited semi-arid savanna, in which C(4) grasses and C(3) trees and shrubs co-existed. The data accord with theoretical frameworks that predict that variability in ecosystems clusters in regions of higher probability space, interspersed by rapid transitions between these phases. The data are also consistent with the idea that phase transitions involve switching between different dominant driving processes or limiting factors.