Biodiversity buffers the response of spring leaf unfolding to climate warming

Understanding the sensitivity of spring leaf-out dates to temperature (ST) is integral to predicting phenological responses to climate warming and the consequences for global biogeochemical cycles. While variation in ST has been shown to be influenced by local climate adaptations, the impact of biodiversity remains unknown. Here we combine 393,139 forest inventory plots with satellite-derived ST across the northern hemisphere during 2001–2022 to show that biodiversity greatly affects spatial variation in ST and even surpasses the importance of climate variables. High tree diversity significantly weakened ST, possibly driven by changes in root depth and soil processes. We show that current Earth system models fail to reproduce the observed negative correlation between ST and biodiversity, with important implications for phenological responses under future pathways. Our results highlight the need to incorporate the buffering effects of biodiversity to better understand the impact of climate warming on spring leaf unfolding and carbon uptake. The authors combine 393,139 forest inventory plots with satellite data to understand the impact of biodiversity on the sensitivity of spring leaf-out dates to temperature (ST). They show that high diversity significantly weakens ST, a relationship that Earth system models largely fail to reproduce.