Legacies from early-season hot drought: how growth cessation alters tree water dynamics and modifies stress responses in Scots pine.

Tree responses to drought are well studied, but the interacting effects of drought timing on growth, water use, and stress legacy are less understood. We investigated how a widespread conifer, Scots pine, responded to hot droughts early or late in the growing season, or to both. We measured sap flux, stem growth, needle elongation, and leaf water potential (Ψ) to assess the impacts of stress timing on drought resilience in Scots pine saplings. The early summer hot drought had peak temperatures of 36.5 °C, while the late summer hot drought peaked at 38.2 °C. Soil water content during both periods declined to ca. 50% of control values. The early-season hot drought caused growth cessation already at Ψ - 1.1 MPa, visible as an almost 30 days earlier end to needle elongation, resulting in needles 2.7 cm shorter, on average. This reduction in leaf area decreased productivity, resulting in a reduction of 50% in seasonal transpiration. However, the reduced water use of early-stressed saplings appeared to enhance resistance to a late-season drought, as reflected in a smaller decline in Ψ and lower tree water deficit compared to saplings that did not experience early-season stress. In summary, we observed persistant drought legacy effects from early-season hot-drought stress, as evident in a 35% reduction of leaf area, which impacted tree water use, stress resistance, and productivity. These structural adjustments of leaf development and reduced bud mass from early-season stress could be critical in evergreen conifers, whose long-lived foliage influences future water use and growth potential.