Exploring the role of functional traits in regulating the spatial and temporal variability in land surface phenology across temperate forests

Plant phenology, the study of recurring plant life history events’ timing, is a key indicator of global environmental change and significantly impacts ecosystem functions and services. Land surface phenology (LSP) characterizes plant phenology by monitoring seasonal plant canopy structure dynamics via satellites. Numerous studies have demonstrated that ecosystem-scale LSP variability is mainly driven by climate and environmental conditions across different ecosystems. However, significant spatial and temporal phenological variations are still observed within local landscapes where environmental conditions are relatively similar. This suggests that biotic factors may be important to regulating LSP variability, but their role in determining phenological variability has been underexplored. To address this knowledge gap, we selected four temperate forest sites with minor topographic relief to ensure the homogeneity of environmental conditions and examined how functional traits regulate intra-site spatial and temporal LSP variability. We combined plant functional traits derived from remote sensing data with multi-year Harmonized LandSat-Sentinel-2 (HLS) data to investigate the effects of functional traits on phenological variability. For spatial LSP variability, we assessed the extent to which functional traits could explain the variation in the start of season (SOS) and end of season (EOS). We found that functional traits showed a substantial explanatory power for spatial phenological variability across all the study sites, with cross-validation correlations (cv) ranging from 0.50 to 0.85. For temporal LSP variability, we used multi-year series of the two band Enhanced Vegetation Index (EVI2) to calculate the cumulative deviation of EVI2 values from their long-term means, which served as an indicator of temporal phenological variability. Functional traits also significantly contributed to the temporal variability across all sites, with cv ranging from 0.46 to 0.71. Furthermore, our results show that plant traits related to vegetation competitive ability and productivity (e.g., canopy height, plant area index, and leaf mass per area), are crucial to explaining intra-site phenological variability, but their relative contributions vary among different sites. Collectively, these results demonstrate that functional traits play a critical role in regulating intra-site spatial and temporal LSP variability, and plants employ diverse strategies to cope with the environment, which ultimately impacts various ecological processes.