Trends and Climate-Sensitivity of Phenology in China's Natural and Planted Forests

Forest is the largest carbon sink on land, and the timing of forest phenology profoundly influences carbon uptake. However, disparities in the timing and sensitivity of forest phenology, particularly between planted forests (PF) and natural forests (NF), have yet to be determined. In this study, we analyzed the phenological trends and the climate-sensitivities of China's NF and PF from 2000 to 2022 using field survey data and satellite data. We found that the start of the growing season (start of the season (SOS)) in NF and PF advanced significantly by 0.07 and 0.08 days yr(-1), while the end of the growing season (end of the season (EOS)) in both NF and PF delayed by 0.06 days yr(-1). Moreover, precipitation and air temperature jointly influenced the timing of phenological events. The pre-season precipitation delayed the EOS by 0.009 days mm(-1) and 0.006 days mm(-1) in NF and PF. Moreover, the SOS was more sensitive to precipitation in PF than in NF in arid areas (i.e., pre-season precipitation <100 mm). Air temperature significantly advanced the SOS by 0.74 and 0.95 days degrees C-1 in NF and PF and delayed the EOS by 0.89 and 0.71 days degrees C-1 in NF and PF. Additionally, the sensitivities of SOS and EOS to air temperature significantly differed between NF and PF. Our findings demonstrate a significant phenological discrepancy between PF and NF. The results suggest that NF and PF responded differently to climate variability, implying forest origin should be considered in the projection of carbon sequestration capability under changing climate.