Quantifying changes in temperature sensitivity of spring phenology over recent decades

Understanding the factors driving leaf-out times in temperate and boreal trees is increasingly vital in the context of rapid climate change, as it is crucial for predicting ecosystem dynamics. This study investigates if and how the sensitivity of spring phenology to temperature (S_t) has evolved in recent decades, especially given the ongoing debates about the extent and direction of future phenological shifts. Leaf-out times are influenced by a mix of spring warming, winter chilling, and day length, with their significance varying across different species. However, climate change could reduce S_t due to warmer climates and earlier spring onset, potentially enhancing winter chilling and day length constraints. To evaluate the relative importance of these factors, we conducted a comprehensive analysis using both long-term ground observations and satellite-derived phenology data, covering a span of six decades. Our approach included simulations to better understand the mechanisms behind variations in species-level T_s. This research aims to determine whether the phenological responsiveness of trees to climate change is decreasing, taking into account both environmental and physiological factors. Our findings challenge the prevailing belief of a consistent decline in St sensitivity, underscoring the need for a more nuanced understanding of the drivers behind leaf-out phenology in a changing climate. This study adds to the ongoing discourse on how global warming shapes natural phenomena and is pivotal for forecasting how ecosystems will respond to environmental changes.