Higher Drought Sensitivity of Autumn Photosynthetic Phenology in Planted Forests Than in Natural Forests of Tropical and Subtropical China

Severe droughts advance autumn phenology, reducing terrestrial ecosystem productivity and carbon sequestration. Approximately 25% of China's tropical/subtropical forests are planted for climate mitigation, yet differences in drought sensitivity of autumn phenology between planted and natural forests remain unclear. In this study, we used four phenological fitting methods to extract end-of-photosynthetic-growing-season (EOPS) dates in China's tropical/subtropical forests over the period 2001–2020, and employed ridge regression to assess the difference in response of EOPS to drought (the standardized precipitation evapotranspiration index, SPEI) between natural and planted forests. The results showed that planted forests exhibited significantly later mean EOPS, with day of year (DOY) of 274 versus 269, greater interannual variability measured by standard deviation (SD) of 7.1 versus 6.3 (p < 0.05), and comparable delaying trends of 0.33 versus 0.32 days per year, relative to natural forests. Importantly, EOPS sensitivity to SPEI was double higher in planted forests (0.12 vs. 0.06 per unit, p < 0.01). Projections for 2021–2100 based on partial least squares regression indicate that planted forests will experience a ~5-day later mean EOPS, greater interannual variability (by 0.8 and 1.4 days under the SSP245 and SSP585 scenarios, respectively), but a slower delaying trend (0.05 and 0.06 days year⁻¹) compared to natural forests. These results reveal planted forests' heightened drought sensitivity threatens to shorten growing seasons under intensifying droughts, undermining carbon sequestration efficiency in afforestation programs.