Background climate determines the response of spring leaf‐out to climate change—Results from a national‐scale twig‐cutting experiment

Global warming has significantly advanced spring leaf‐out in recent decades, impacting terrestrial carbon and water cycles. Spring temperature is recognized as the primary driver of leaf‐out in temperate regions, influenced by both the magnitude of warming and the region's background temperature. However, how warming interacts with background climate to influence leaf‐out remains unclear, largely due to the lack of large‐scale experimental evidence. Here, we conducted a large‐scale manipulative experiment with multiple temperature treatments by collecting twigs of Ginkgo biloba from 17 sites across China, spanning latitudes from 25° N to 40° N. We found that higher temperature significantly advanced leaf‐out, with temperature sensitivity being greater under cooling treatments (7.8 days/°C, i.e. lower temperature treatments relative to the background temperature) than under warming treatments (4.3 days/°C, higher temperature treatments relative to the background temperature). More importantly, each 1°C increase in background temperature led to an additional 2.7‐day advancement in leaf‐out. By combining background temperature effects with future warming scenarios, we project that Ginkgo leaf‐out in China will advance by an average of 19.7 days by the end of the 21st century under high‐emission scenarios, with more pronounced changes in southern regions. Synthesis . Our results highlight the interactive effects of climate warming and background temperature in regulating leaf‐out, which should be incorporated into phenology models to improve our understanding of ecosystem responses to climate change.