Recent variations in the vulnerability of terrestrial gross primary productivity to compound dry-hot extremes in China

The compound dry-hot events (CDHEs) have been one of the main reasons for the loss of terrestrial gross primary productivity (GPP) under ongoing global warming. Better quantification of patterns and drivers of GPP vulnerability to CDHEs is essential for ecological protection and identifying ecologically vulnerable areas. Although there is growing evidence of a sustained increase in the frequency and intensity of CDHEs in China over the past few decades, quantitative assessments of GPP remain lacking overall. From the perspective of a short time window, this study revealed the variability of the response of summer vegetation productivity in China to four severity levels (i.e., mild, moderate, severe, and extreme) of CDHE over the last 40 years and its associated mechanisms using copula conditional probability, sliding window algorithm and multiple ridge regression models. The main findings are: (1) the resistance of vegetation depends on land cover type and climatic conditions with lagged response times of 1.62, 1.73 and 1.82 months from June to August, respectively; (2) the average GPP vulnerability of 59.0%, 56.7%, 55.3% and 53.8% of vegetation areas tended to decrease under the changing environment, respectively, indicating that vegetation gradually adapted to CDHE disturbance; and (3) the influence of the dominant drivers in GPP sensitivity is not immutable, such as the impact of the control factors on the middle and lower reaches of the Yangtze River basin is gradually strengthened with time, which weakens the resistance of vegetation to compound conditions. These findings would contribute to further understanding of the effects of compound climate on the water and carbon cycles of terrestrial ecosystems.