Strengthened sink control under heatwaves contributes to changes in the phenological sequence of photosynthesis and growth

Climate change is increasing the frequency and intensity of heatwaves, with profound consequences for forest carbon cycling. Photosynthesis (source) and radial stem growth (sink) are two key processes underpinning this carbon cycle, yet their coordination under heat stress remains unclear. We quantified heatwave-induced changes in source-sink dynamics at 19 European forest sites during the record-breaking heatwaves of 2015 and 2018. We combined FLUXNET-derived gross primary production (GPP, representing photosynthesis) with high-resolution dendrometer data (radial stem growth), and applied the convergent cross-mapping (CCM) method to quantify the causal relationships between photosynthesis and radial stem growth under heatwaves. Our results showed that in evergreen needleleaf forests (ENF), GPP typically peaks before radial growth (4.7 days), but heatwaves reversed this phenological sequence, with growth peaking earlier than photosynthesis (–10.0 days). The CCM analysis further showing that bidirectional source-sink causal effects were strengthened during heatwaves, with the sink-to-source causal effect increasing more in ENF. This heatwave-induced phenological shift was driven primarily by plant functional type (21.4%), with additional contributions from increased summer temperature (11.4%) and enhanced sink-to-source feedback (9.4%). Overall, our study demonstrates that tree carbon allocation strategies are plastic under extreme climate events and that plant functional types differ fundamentally in the source-sink regulatory responses, with important implications for predicting forest resilience to future climate change.