Energy drives, atmospheric dryness modulates: multi-timescale climatic control of gross primary productivity in an...

The timescale dependence of how climatic factors influence ecosystem carbon assimilation has been a major research focus. However, our understanding of how GPP, a critical component of the carbon balance, interacts with climatic factors across timescales remains limited. To investigate the relative contributions and joint effects of photosynthetically active radiation (PAR), temperature (Ta), vapor pressure deficit (VPD), and soil water content (SWC) on GPP across timescales, we applied Wavelet Coherence (WTC) and improved Partial Wavelet Coherence (PWC) methods to an 8-year eddy covariance dataset from a temperate broad-leaved forest. WTC revealed that PAR and Ta exhibit stronger and more synchronous coherence with GPP than VPD and SWC at diurnal and annual scales, with PAR leading GPP by 0.3 ± 0.97 h and Ta leading by 3 ± 1 days, whereas no single climatic factor exerts continuous control at scales from days to months. PWC further revealed that VPD has a much stronger independent and mediating influence on GPP than SWC: removing VPD markedly weakens the coherence between GPP and PAR or Ta, especially at diurnal and annual scales, while removing SWC induces only minor changes. These patterns indicate that GPP variability emerges from coupled energy-moisture controls, where PAR and Ta provide the primary direct forcing but their effects are strongly modulated by atmospheric dryness through VPD. Our findings highlight the importance of representing cross‑timescale energy-moisture interactions, rather than energy dominance alone, when projecting forest carbon uptake.