Soil water content mediates heat wave impacts on stem water status and growth of Neltuma affinis in the Espinal

Heat waves can constrain tree growth and, under extreme conditions, may trigger tree mortality. We evaluated their effects on the water status of Neltuma affinis in a xerophytic forest of the Espinal. Stem radius variations were recorded at 15-min intervals in six trees over a complete annual cycle (July 2024–June 2025). Soil temperature and volumetric water content (VWC) were monitored simultaneously. Tree water status was assessed using dendrometer-derived metrics, including normalized maximum daily shrinkage (MDS norm ) and normalized tree water deficit (TWD norm ). Daily aggregated and normalized series were analyzed using regularized dynamic regression models that incorporated temporal autocorrelation, lagged hydroclimatic predictors, and system memory. The most severe heat waves of the past 90 years were concentrated in the last decade (2015–2025), and heat wave frequency was significantly higher during 1995–2025 than during 1935–1965 (0.79 vs. 0.37 events yr −1 ; p = 0.011). A severe summer drought from mid-December 2024 to February 2025 caused sustained stem dehydration, which intensified during three concurrent heat wave episodes. These events were characterized by high TWD norm values, reaching mean maxima of 1.178 ± 0.246, low MDS norm values, VWC close to the permanent wilting point, and high evaporative demand. Under severe soil water deficit, extreme positive temperature anomalies amplified reductions in stem radius. Conversely, higher VWC during extreme heat buffered thermal effects and reduced stem contraction. This study shows that soil water availability is the key modulator of heat wave impacts on N. affinis , determining whether extreme temperatures translate into severe stem dehydration and growth decline.