Stem water content is crucial to support fruit tree functioning during heatwaves in a Mediterranean climate

Droughts are expected to intensify in the Mediterranean region due to climate change, yet their effects on local trees remain largely unknown. To study the effect of heatwaves in irrigated orchards, where soil drought can be mitigated, we propose a heatwave definition that focuses on atmospheric stress and its consequences by relating the intensity of high VPD events to losses in tree stem-water storage (StWS). Unlike common threshold-based heatwave definitions, this approach combines sub-daily VPD percentiles with a physiological filter based on declines in stem water content, ensuring that identified events are both meteorologically extreme and relevant to the tree. We show that the sensitivity and resilience of StWS to heatwaves are species specific and differ between contrasting water-management strategies (isohydric orange versus anisohydric mango). Navel orange trees were sensitive to heatwaves starting at the 80th VPD percentile in early spring. After irrigation began, stem water content (StWC) increased slightly over the dry season, despite harsh Mediterranean temperatures. In contrast, Shelly mango trees showed a net seasonal reduction in StWC, accompanied by an increase in heatwave sensitivity from the 90th to the 80th percentile. Across events, absolute StWS losses increased with the Cumulative Heatwave Unit (CHU), a metric that integrates the intensity and duration of statistically high VPD, with a much steeper slope in mango than in orange. Using CHU to quantify heatwaves allowed us to characterize each species’ sensitivity according to event rarity and their resilience over key seasons. Although our experiment was performed in a Mediterranean climate, hotter droughts are rising globally, and the framework developed here for quantifying and measuring heatwave effects can be broadly applied across geographic locations.