# Interannual Regime Shifts and Driver Thresholds of Terrestrial Ecosystem Vulnerability in Northwestern Sichuan of China Based on an XGBoost-SHAP Model

## TL;DR

This study uses advanced modeling to show that ecosystems in northwestern Sichuan experience sudden shifts in health, highlighting the need for better management to protect biodiversity and water security.

## Contribution

The study introduces an XGBoost-SHAP model to identify nonlinear thresholds and abrupt shifts in ecosystem vulnerability in northwestern Sichuan.

## Key findings

- 47.96% of the area experienced abrupt shifts in ecosystem vulnerability.
- Grazing intensity and soil moisture showed distinct thresholds affecting ecosystem health.
- 37.89% of previously restored areas began deteriorating since 2010.

## Abstract

Northwestern Sichuan is a key ecological function zone in China, providing water conservation and biodiversity protection services and helping to safeguard water security in the upper reaches of the Yangtze and Yellow Rivers. Previous studies often oversimplified how ecosystems change, missing sudden shifts in their health. This study used advanced computer modeling to analyze the vulnerability of these terrestrial ecosystems to damage. We found that nature does not change in a straight line, while nearly half the region experienced abrupt shifts. Alarmingly, more than 37% of the previously restored territory has begun to deteriorate again since 2010. This suggests that early restoration efforts, like planting trees, may have reached their limits. We discovered specific “tipping points” where factors like grazing and soil moisture change from being helpful to harmful. For example, while moderate grazing supports grasslands, excessive grazing damages them. These findings are vital because they show that simply expanding vegetation quantity is not enough. To protect these diverse landscapes, society must shift focus to improving ecological quality and strictly managing these risk thresholds. This approach will better safeguard regional biodiversity and water security for the future.

TENS constitutes a critical ecological barrier on the southeastern margin of the Qinghai–Tibet Plateau, providing essential services such as water conservation and biodiversity protection and helping to safeguard water security in the upper reaches of the Yangtze and Yellow Rivers. Thus, elucidating its vulnerability dynamics is paramount for regional security. Integrating multi-source spatiotemporal data with an interpretable XGBoost–SHAP framework, we quantified interannual variation in vulnerability and the nonlinear threshold responses of key drivers. The results showed pronounced nonlinear phase changes in vulnerability, with 47.96% of the area experiencing abrupt shifts. Notably, 37.89% of TENS reversed from decreasing to increasing vulnerability. TENS underwent an intensive transition during 2010–2015. Interannual variability was dominated by the coupled influence of human disturbance, soil moisture, and atmospheric water, accounting for nearly 60% of the variation, and showed distinct thresholds. Grazing intensity < 0.90 SU/ha was a moderate disturbance, reducing vulnerability, but it became a stressor above this level. Soil moisture showed an inflection point at 79 mm, while vapor pressure deficit (VPD) < 0.39 kPa enhanced resilience, revising the view of VPD as solely a stress factor. Different ecosystems exhibited distinct driving mechanisms. Grasslands were controlled by shallow soil moisture and grazing, forests by hydrothermal balance, and wetlands by low-intensity anthropogenic disturbance (NTL as a proxy; e.g., tourism development or urban expansion). These findings highlight the risk of abrupt shifts in vulnerability regimes (turning points and trend reversals) and support management that emphasizes quality improvement and threshold-based risk management.