Differential thresholds of net ecosystem productivity in karst and non-karst regions for identifying their potential carbon sinks areas.

Within ecosystems, habitat influences structure, and structure determines function, forming a habitat-structure-function framework (HSFF). Net ecosystem productivity (NEP) is a key indicator for assessing regional or global carbon dynamics. However, the response thresholds of NEP to habitat and structural factors, along with management strategies based on these thresholds, remain under-explored. Therefore, this study examines the response thresholds of NEP to habitat and structural factors in the karst and non-karst regions of southwest China, which exhibit strong surface heterogeneity, based on the HSFF using a restricted cubic spline method. The results are, (1) The interannual NEP increase rate and carbon storage per unit area were notably greater in karst regions than in non-karst ones. However, compared to non-karst regions, karst regions show greater NEP variability and lower stability. (2) Significant nonlinear relationships were identified between NEP and nine habitat factors and six structural factors. NEP thresholds due to habitat and structural factors were smaller in karst regions than in non-karst regions. (3) Habitat factors had greater relative importance and marginal contribution than structural factors in karst and non-karst regions, with energy and water as the main influences on NEP. (4) Using the potential carbon sinks areas determined by the threshold, the karst areas in the entire study will play an important role in carbon sinks in the future. Overall, this study not only deepens the understanding of the differences in ecosystem NEP between karst and non-karst regions, but also provides new perspectives and strategies for optimizing ecosystem management based on habitat and structural characteristics.