The underappreciated importance of solar radiation in constraining spring phenology of temperate ecosystems in the Northern and Eastern United States

Spring phenology of temperate ecosystems displays high sensitivity to the recent climate change, and has generated various impacts on plant growth, biotic interactions, ecosystem productivity, and local environmental conditions. Although various prognostic models relying on environmental variables, mainly including temperature and photoperiod, have been developed for spring phenology, comprehensive ecosystem-scale evaluations over large geographical extents and long-time periods remain lacking.  Further, environmental variables other than temperature and photoperiod might also importantly constrain spring phenology modelling but remain under-investigation. To address these issues, we leveraged 20-years datasets of environmental variables (Daymet) and the spring phenology metric (i.e., the greenup date) respectively derived from MODIS and PhenoCams across 108 sites in the Northern and Eastern United States. We firstly cross-compared MODIS-derived greenup date with PhenoCams with high accuracy (R²=0.75). Then, we evaluated the three prognostic models (e.g., Growing Degree Date (GDD), Sequential (SEQ) and optimality-based (OPT)) with MODIS-derived spring phenology, assessed the model residuals and their associations with soil moisture, rainfall, and solar radiation, and improved and re-evaluated the models by including the variable contributing to high model residuals. We found that 1) all models demonstrated good capability in characterizing spring phenology, with OPT performing the best (RMSE=8.04±5.05 days), followed by SEQ (RMSE=10.57±7.77 days) and GDD (RMSE=10.84±8.42 days), 2) all models displayed high model residuals showing tight correlation with solar radiation (r=0.45-0.75), and 3) the revised models that included solar radiation significantly performed better with an RMSE reduction by 22.08%. Such results are likely because solar radiation better constrains early growing season plant photosynthesis than photoperiod, supporting the hypothesis of spring phenology as an adaptive strategy to maximize photosynthetic carbon gain (approximated by solar radiation) while minimizing frost damage risk (captured by temperature). Collectively, our study reveals the underappreciated importance of solar radiation in constraining spring phenology of temperate ecosystems, and suggests ways to improve spring phenology modelling and other phenology-related ecological processes.