Intra-annual decoupling of climate-growth responses and saturation thresholds in a Pyrenean mixed conifer forest

Montane forests near the Mediterranean bioclimatic boundary are increasingly exposed to warming-induced water stress even under hyperhumid conditions, yet how seasonal growth phases respond to climatic limitation remains poorly understood. We combined intra-annual growth partitioning with non-linear modelling to investigate climate-growth responses in four co-occurring conifers ( Picea abies , Pinus sylvestris , Abies alba , and Larix decidua ) in a mixed afforestation stand at 1195 m a.s.l. in Canfranc, central Spanish Pyrenees (>1500 mm annual precipitation; +1.5 °C since 1950). Species-specific earlywood and latewood chronologies (1950–2020) were analyzed through Pearson correlations, multiple regression, Generalized Additive Models (GAMs), moving-window correlations, and pointer year analysis. The three evergreen species shared a strong common signal dominated by summer precipitation, with earlywood and latewood responding to distinct temporal windows: earlywood reflected a legacy of previous-year water availability, whereas latewood was tightly coupled to current-year summer precipitation. GAMs consistently outperformed linear models for latewood, revealing saturation responses ranging from approximately 100 mm in Picea (August) to 200-300 mm in Pinus and Abies (July-August). This non-linearity produces a structurally asymmetric growth function in which dry-year penalties outweigh wet-year gains, amplifying net growth losses under declining summer precipitation. Larix decidua showed a markedly divergent behavior, shifting from thermal to hydraulic limitation over the study period and exhibiting synchronous growth collapses. These results demonstrate that whole-ring analyses obscure mechanistically distinct seasonal signals, and that even historically humid montane stands are vulnerable to hydraulic limitation once precipitation falls below species-specific thresholds.