Pine processionary moth defoliation affects secondary growth dynamics and tree vitality in Pinus brutia

Abstract Pine processionary moth (PPM) defoliation can reduce secondary growth and increase tree susceptibility to abiotic and biotic stressors. However, its effects on seasonal secondary growth dynamics and xylem and phloem anatomy are less understood. This study investigated the effects of PPM ( Thaumetopoea wilkinsoni ) defoliation on mature Pinus brutia trees in the Mediterranean region of Türkiye over two consecutive years. In 2018, cambial activity and secondary growth dynamics were monitored throughout the growing season in 65% PPM-defoliated and control trees using detailed histological observations. Anatomical characteristics of xylem and phloem were assessed both before wood formation started and after its cessation. In 2019, the same study was repeated on a new set of trees. PPM defoliation advanced the cessation of cell enlargement and wall-thickening phenophases, shortening wood formation by approximately two weeks. Despite this, tree-ring width reduced by only 15–20% and did not differ significantly from controls. Defoliated trees showed significantly lower earlywood to latewood (EW/LW) ratios, while other anatomical characteristics, such as cell wall thickness, were unaffected, which suggests reallocation of carbon to needle growth and storage early in the growing season. The observed decreases in the tree-ring to conducting phloem (TR/CPH) and EW/LW ratios indicate diminished vitality in defoliated trees. However, the observed reduction in ring width was far below that previously reported for 65% PPM defoliation, indicating the ability of the studied P. brutia individuals to cope with this level of defoliation stress. These findings improve our understanding of defoliation effects on secondary growth and highlight the need for more comprehensive studies to better predict forest responses to climate change and the projected increase in the frequency of PPM outbreaks.