Age-related patterns and climatic driving factors of drought-induced forest mortality in Northeast China

Given the significantly increasing frequency and intensity of droughts in Northeast China under climate change, it is necessary to understand drought-induced mortality in forest ecosystems. The magnitude of drought-induced forest mortality strongly depends on climate variability and forest age, which are both important driving factors of tree mortality. However, the age-related patterns and climatic driving factors of drought-induced forest mortality at large scales are poorly understood. This study identified the age-related pattern and the key climatic driving factors of drought-induced mortality for 17 dominant tree species in Northeast China using rain-use efficiency (RUE), standardized precipitation evapotranspiration index (SPEI), and tree-list information (species and diameter for every tree). Considering that climate variability and forest age are important factors of drought-induced mortality, species-specific analysis was conducted for describing age-dependent mortality patterns. The results showed that the mean annual rate of the drought-induced mortality of forests in Northeast China was 0.49%, with relatively high mortality rates in the Changbai Mountains and the eastern part of Liaoning Province. The sensitivity of age-dependent mortality patterns to climatic drivers of drought exhibited considerable variability. Resolving the contributions of precipitation deficits and heatwaves on the drought-induced mortality of trees, almost 91.60% of the forest region in Northeast China was found to be primarily affected by heatwaves (temperature anomalies), suggesting the important role of temperature extremes in forest mortality. The findings provide deeper insight into the mechanisms behind the species-specific formation of age-dependent patterns in Northeast China. This study provides a basis for the formulation of drought adaptation measures for forest species across successional stages and highlights the potential of remote sensing indices in identifying the patterns and climatic drivers of large-scale drought-induced forest mortality.