A degradation threshold for irreversible loss of soil productivity: a long-term case study in China

1. During the past three decades, conservation and restoration biologists have increasingly recognized that ecological communities are likely to exhibit threshold changes in structure. However, because long-term monitoring data are generally lacking, little is known about the consequences of such ecological thresholds for the processes of ecosystem degradation and recovery. 2. To identify whether a degradation threshold exists that defines the boundary between the possibility of natural recovery and the need for artificial restoration of an ecosystem and to use this knowledge to support the development of a suitable strategy for environmental restoration, we performed long-term monitoring of vegetation recovery in China's Changting County since 1984. 3. A major problem was identified, which we refer to as the 'irreversible loss of soil services'; when vegetation cover decreases below a degradation threshold, this leads to sustained degeneration of the vegetation community, erosion of the surface soil and declining soil fertility. These changes represent a severe and long-lasting disturbance that will prevent ecosystem recovery in the absence of comprehensive artificial restoration measures. 4. Synthesis and applications. We identified a degradation threshold at about 20% vegetation cover suggesting that for some sites, vegetation cover can serve as a simple proxy for more sophisticated approaches to identifying thresholds; restoration must start with the restoration of soil fertility and continue by facilitating vegetation development. Our results support the concept of ecological thresholds (specifically, for soil services in a warm and wet region) and provide a model to inform restoration strategies for other degraded ecosystems.