Unusal equilibrium behavior of forest ecosystems in the Loess Plateau

Ecosystems may exhibit various equilibrium behaviors (e.g., linear and threshold), which will dramatically affect how we understand and regulate ecosystem dynamics under different environmental conditions, thus reshaping ecosystems’ sustainable development. Studying equilibrium behaviors is particularly crucial for the Loess Plateau because hundreds of billions of Chinese Yuan have been paid to alter ecosystem structure and thereby to reduce soil erosion. Resultant increasing vegetation, however, exhausts soil water, which heavily threatens the sustainability of ecosystem function and services therein. It has been a widespread and long-lasting controversy over whether and where to afforest. However, one of the most fascinating equilibrium behaviors of alternative stable states, which permits more than one states under the similar conditions, has largely ignored in the framework. By integrating remote sensing products, a minimal model, and environmental data, this study explored alternative tree-cover states and its effects on functions and services of forest ecosystems in the Loess Plateau. The equilibrium behavior along annual precipitation gradient appeared to a threshold-type (uni-stability) combined with a fold bifurcation. That is, tree cover showed a threshold-type uni-stability when annual precipitation was lower than 400 mm, beyond which alternative stable states of high tree cover (forest, >35% tree cover), and medium tree cover (open woodland, 7%~35% tree cover) co-existed. Increasing spatial heterogeneity, and especially vegetation-precipitation positive feedback would advance the thresholds of transitions between alternative states towards to higher annual precipitation. Furthermore, regime shift from forest to open woodland states increased carbon stock, while reduced water yield, i.e., carbon and water formed a trade-off. This unusual balancing behavior not only enriches our understanding theoretically but also substantially benefit afforestation planning in the Loess Plateau practically, thereby promote forest ecosystem functions and services of forest ecosystems.