How beech ecophysiology shapes temperate forest gross primary productivity – Part 2: Identifying critical timeframes across phenological stages

Seasonal processes fundamentally shape forest carbon uptake, yet their timing and sensitivity remain poorly resolved. Using a 24-year eddy-covariance record from a maturing beech forest (FR-Hes), we developed three annual ecophysiological indicators (IRise, IPeak and IDrop) of gross primary productivity (GPP) and assessed their environmental controls using phenology-aligned sliding correlations across multiple window lengths and start dates relative to the start of season (SOS). This framework allowed us to identify precise seasonal timeframes in which climate drivers exert disproportionate influence on ecosystem productivity. Early-season growth rate (IRise) emerged from the interaction between reserve availability, leaf ontogeny and early-spring (SOS+10 to SOS+31) light/temperature conditions. Peak productivity (IPeak) was strongly shaped by canopy structural development and, critically, by a 1 week precipitation window around bud-set (SOS+56 to SOS+63) in the previous year, highlighting a developmental bottleneck that governs next-year canopy potential. Mid-season decline (IDrop) was driven overwhelmingly by atmospheric demand: two short VPD-sensitive windows (SOS+92 to SOS+106, corresponding to stress buildup before drop onset, and SOS+107 to SOS+114, reflecting atmospheric stress after drop onset) determined the intensity of the summer drop, with flash-drought years exhibiting earlier and sharper declines when these windows coincide with rapid early-summer warming. Extreme summers produced a second striking pattern: when soil water remained available, peak GPP increased proportionally to temperature and radiation, suggesting active acclimation via thermotolerance, stomatal cooling and structural adjustments. Thinning effects, by contrast, were modest and transient. These findings demonstrate that beech forest productivity is governed by brief, phenologically constrained time windows that integrate physiology, developmental history and atmospheric forcing. By resolving these windows, our approach provides a mechanistic foundation for phenology-explicit carbon-cycle models and sharper predictions of forest responses under increasing climatic variability.