The extensive afforestation efforts on the Loess Plateau, incurring hundreds of billions of CNY, trigger heightened vegetation cover, depleting soil water, and imperiling ecosystem sustainability. Widespread debate persists over the feasibility and optimal locations for afforestation. However, what has been overlooked is the potential presence of alternative stable states within ecosystems, a captivating system equilibrium behavior. This study integrates remote sensing, minimal model, and environmental data to investigate the equilibrium behavior (quantified by tree cover) of forest ecosystems on the Loess Plateau and its implications. The findings suggest a threshold relationship between tree cover and annual precipitation, with a significant increase observed up to 400 mm. Beyond this threshold, alternative stable states emerge, characterized by high tree cover (forest, >35%) and medium tree cover (open woodland, 7%~35%). The equilibrium behavior of the forest ecosystem combines thresholds and alternative stable states. Increasing spatial heterogeneity, especially the positive feedback between vegetation and precipitation, results in advancing transition thresholds with higher annual precipitation. Regime shifts from forest to open woodland increase carbon stock but decrease water yield, revealing a trade-off between carbon sequestration and water resources. This nuanced understanding of equilibrium enhances both theoretical comprehension and practical planning for afforestation on the Loess Plateau, promoting the functions and services of the forest ecosystem.

Ecosystems may exist in alternative stable states and thereby extremely differ in ecosystem structure and functions, including gross primary productivity (GPP), which is crucial for assessing an ecosystem’s ability to capture atmospheric carbon dioxide, especially under the context of climate change. This study applied alternative stable states theory to evaluate GPP in global dryland forests, and analyzed multi-year average GPP data alongside environmental factors such as the Aridity Index and mean annual precipitation. Here, we found the existence of alternative stable states of GPP along the aridity gradient. Mean GPP were 893.12 gC/m²/year and 1539.86 gC/m²/year under lower and higher branches of alternative stable states, respectively, compared to the current mean value of 1203.02 gC/m²/year. Notably, we observed striking regional disparities in GPP, with Africa and Oceania predominantly in the higher alternative stable state, while North America and Asia were in the lower alternative stable state. However, GPP along mean annual precipitation did not exhibit alternative stable states, but a significant variation during the medium range of mean annual precipitation (241-402 mm year-1). The relationships between GPP data and environmental factors were consistent across different forest types. This study sheds light on dryland forest productivity and indicates adaptive management strategies that should be used to bolster ecosystem function in the context of climate change.

Ecosystems can exhibit rich equilibrium behaviors, including alternative stable states (ASS), for which ecosystems dramatically differ in structure and functions. Therefore, it is critical to understand system equilibrium behavior. Positive feedback has been recognized as a critical process to induce ASS. However, it is largely unclear whether and to which extent increasing number of positive feedbacks will be easier to produce ASS. Using a generalized Lotka-Volterra model and two functional forms of positive feedback, this study found that positive feedback was largely necessary to produce ASS, and more positive feedbacks generally more likely produced ASS. However, above results depended on the functional form of positive feedback and interaction types. This work can help predict ecosystem dynamics under global change and intensified anthropogenic activities, both which may activate previously “dormant” positive feedbacks and make this issue more visible and important.

Ecosystems may exhibit transient (the period toward stable state) and alternative stable states, where positive feedback(s) generally plays important roles. However, the outline between transient and alternative stable states may blur when involving multiple positive feedbacks, which may shift dominance along environmental gradient. Therefore, a common theoretical framework considering multiple positive feedbacks is needed. Specifically, this study employed a mechanic model to explain a long transient state of a dryland ecosystem after restoration under effects of two positive feedbacks. We found that the long transient state might be a temporal visit of a hidden alternative stable state, maintained by the previously on-acting positive feedback and then pulled out by the upcoming feedback due to dominance shifting. This work proposes a novel perspective to study system dynamics – switch of positive feedbacks, and discovers a new type of transient state – temporal visit of hidden alternative stable states, which will benefit ecosystem interventions in practical.