Increased tree growth by thinning promotes hydraulic vulnerability and drought stress in pine plantations

Tree growth responses to climate depend on factors including species, site-specific conditions, and stand structure, which can be amended by the implementation of forest management practices. Among silvicultural techniques, thinning is known to proficiently enhance forest growth and physiology in seasonally dry environments, influencing tree functional and structural attributes over time. However, its impact on wood hydraulic vulnerability to drought remains unclear. To address this gap, we examined how thinning alters radial growth (BAI), wood anatomy, non-structural carbohydrates (NSC), hydraulic traits and drought resilience in two managed pine plantations (P. sylvestris (PS) and P. nigra (PN)) in a thinning trial along an altitudinal gradient with the following specific objectives: i) to assess the impact of contrasting growth dynamics by the implementation of thinning operations on xylem anatomy, NSC pools (SS and S), and hydraulic traits; and ii) to determine the extent to which thinning-induced growth patterns impacted xylem hydraulic vulnerability and stem variation-derived indices related to water stress for the two study species We found a significant relationship between tree-ring growth and NSC in needles (R² = 0.83, p < 0.001), weakening from fine roots to sapwood pools. Increased growth induced wood anatomical changes (cell number and lumen area), affecting the wood hydraulic diameter (H_D). Consequently, a greater potential hydraulic conductivity was observed for both thinned treatments, with 80.55% and 13.81% increase in PS and PN as compared to control plots, respectively. However, the percent loss of hydraulic conductivity (P₅₀) increased by up to 43.35% (PST) and 38.31% (PNT), supporting that growth follows the safety-efficiency trade-off, i.e., growth-patterns promoted hydraulic efficiency (H_D; R² = 0.79) over hydraulic safety (P₅₀; R² = 0.84). Water-stress indices derived from trunk variations showed greater sensitivity to water-shortage in thinned plots, with treatment differences of 68.67% for PS and 8.75% for PN. These insights represent the next critical step towards elevating forest management risks to a level that can contribute to our understanding of species-specific responses to thinning and the trade-offs inherent in tree physiology between growth and hydraulic vulnerability in water-scarce environments.