Pines compensate for winter insect defoliation with longer and more photosynthetically efficient needles

Insect outbreaks are major biotic disturbances in forest ecosystems. Defoliating insects reduce leaf area, limiting photosynthetic capacity and resource availability for physiological functions and foliage regrowth. This is particularly critical in evergreen conifers that store resources in older foliage and have high leaf construction costs. Understanding structural and physiological adjustments in new foliage following defoliation is essential to assess tree resilience and ecosystem recovery. We examined the effects of pine processionary moth (Thaumetopoea pityocampa (Denis & Schiffermüller)) defoliation on Pinus nigra J.F.Arnold leaf functional traits in two stands in north-central Spain. In 2023, we collected branches from a total of 38 trees with contrasting degrees of defoliation and measured needle length, width, number of stomatal rows and stomatal density on needles formed following the defoliation event. We also recorded physiological traits, including the maximum quantum yield of photosystem II (Fv/Fm; July and September), stomatal conductance (gs) and net photosynthetic assimilation rate (An; September; n = 18 trees). Additionally, we measured needle length, width and Fv/Fm of needles produced prior to the defoliation event (1-year-old needles). Linear mixed models and a structural equation model were employed to analyze both direct and indirect effects of defoliation on structural traits, while general and generalized linear models were used to assess physiological changes. Defoliated trees produced longer new needles, suggesting a strategic allocation of resources toward crown recovery. Stomatal rows and density did not vary with defoliation but were instead related to needle width. Fv/Fm was unaffected by defoliation. Finally, defoliated trees showed higher An values without concomitant changes in gs, indicating a mechanism of photosynthetic upregulation that is not hydraulically mediated but might be driven by greater resource allocation. Together, these results suggest that P. nigra can partially compensate for canopy loss through structural and physiological adjustments, although complete defoliation may compromise tree resilience.