Soil-atmosphere compound drought intensity overrides frequency in constraining future carbon uptake across China

Soil-atmosphere compound drought, characterized by concurrent low soil moisture (SM) and high vapor pressure deficit (VPD), poses an increasingly severe threat to terrestrial carbon sinks. Although vegetation can tolerate mild drought through physiological responses, extreme drought could still cause irreversible damage, leading to significant declines in ecological functions. However, the critical tipping points triggering ecosystem transitions from resistance to vulnerability remain poorly quantified. Here, we developed a data-driven framework to identify nonlinear response thresholds of vegetation to compound drought across China and assessed associated impacts on gross primary production (GPP) under CMIP6 scenarios. Using observations from 2001 to 2020, we found that vegetation response was not linearly related to drought occurrence; instead, a distinct drought threshold exists (mean compound drought index percentile of approximately 14.1%). Dropping below this threshold triggers a transition from resistance to vulnerability (termed ecological drought), causing a precipitous collapse in photosynthetic function where average GPP anomalies plummeted from -0.84 to -4.57 gC m⁻² mon⁻¹. Future projections (2081–2100) confirm that this threshold-driven vulnerability persists, with ecological droughts projected to occur more frequently across over 56% and 61% of vegetated areas under the two respective emission scenarios. Critically, our cross-scenario comparison reveals that the magnitude of GPP losses is governed by drought intensity rather than frequency alone. Under the high-emission SSP5-8.5 scenario, drought intensity dominates in 55.9% of the vegetated area, accelerating at a relative rate 2.32 times that of frequency. This rapid intensification drives greater average GPP losses (-28.17 ± 23.48 gC m⁻² mon⁻¹) compared to the lower-emission path (-24.59 ± 18.23 gC m⁻² mon⁻¹), resulting in higher total GPP losses (-236.53 ± 198.56 versus -199.05 ± 162.59 gC m⁻²). These findings demonstrate that drought intensity overrides frequency as the primary driver constraining terrestrial carbon uptake.