The Influence of Climate and Diplodia sapinea on the Radial Growth of Scots Pine

Scots pine (Pinus sylvestris), traditionally considered a drought-tolerant species, is increasingly threatened by climatic stress and the opportunistic pathogen Diplodia sapinea in Central Europe. Using linear mixed models, this study investigated the basal area increment (BAI) of 164 trees in Northern Bavaria using tree-ring analysis and the detection of D. sapinea in shoots to investigate whether pathogen detection is associated with radial growth and climate sensitivity. Climate–growth analyses (based on chronologies spanning 82–127 years) identified the summer climatic water balance (CWB, June–August), previous-year spring CWB (March–May), and current-year spring temperatures (February, March) as the main growth drivers. Model results demonstrate a strong size-dependency during the 2003–2023 study period: while small trees in which D. sapinea was detected in shoots (basal area (BA) ≤ 800 cm2) appear to exhibit higher climatic sensitivity than trees without detectable D. sapinea, larger pines (BA ≥ 1500 cm2 ≙ DBH ≥ 43.7 cm) exhibited trends pointing toward a near-complete climatic decoupling. These larger trees exhibited a significant growth deficit of −3.69 cm2/year during cold spring conditions (−2 °C) compared to healthy trees. In the largest size class (BA ≥ 2200 cm2), this productivity loss reached its peak at −10.83 cm2/year under high lagged spring water availability (+95 mm CWB in the previous year’s spring). While healthy trees reached an annual BAI of 17 cm2/year under these favorable conditions, infected trees were limited to 10 cm2/year, representing a reduction of approximately 41% and confirming the profound disruption of climatic growth stimuli in mature trees. Our findings indicate that mature pine stands are more vulnerable to productivity losses than predicted by purely abiotic models, although fixed effects explain a limited portion of overall growth variation (marginal R2 = 20.81%, compared to a conditional R2 = 71.99% including random effects). Consequently, despite high individual-tree and site-level variability, these size-dependent associations with D. sapinea shoot colonization must be increasingly integrated into forest resilience assessments as crucial population-level trends.