Extreme climatic events, especially heatwaves, are increasing in frequency and intensity due to climate change. Heatwaves often coincide with drought, compounding their negative effects on forests. The 2003 European heatwave drastically reduced ecosystem productivity. Drought stress leads to stomatal closure, inhibiting photosynthesis and depleting tree water reserves. Stem water reserves can temporarily mitigate drought's impact, but prolonged drought causes hydraulic failure and mortality. High-resolution dendrometers, measuring stem diameter variations, provide valuable insights into tree growth and water status. The 2018 summer was exceptionally hot and dry in northwestern Europe. While studies using satellite imagery, carbon fluxes, and models have assessed forest responses, they lack tree-level physiological detail. This study used a large-scale dendrometer network to analyze tree-level responses to the 2018 heatwave, hypothesizing that (1) the heatwave would reduce annual growth and increase water deficit, depending on site conditions, and (2) conifers would show less water deficit than broadleaves due to their conservative water use strategy.

The literature extensively documents the detrimental effects of heatwaves and drought on forest ecosystems. Studies highlighting the impact of the 2003 heatwave on European forest productivity and the mechanisms of drought-induced tree mortality are frequently cited. The use of high-resolution dendrometers to monitor tree water status and growth has gained traction. Previous research using this methodology, coupled with climatic data, has demonstrated its effectiveness in understanding tree responses to varying environmental conditions. While satellite imagery and ecosystem-level measurements have been applied to the 2018 heatwave, a tree-level analysis of water stress and growth across species and sites has been lacking until this study.

High-resolution dendrometer data (15–60 min intervals) from 377 trees across 53 sites in Central and Atlantic Europe were analyzed. Data covered 21 broadleaf and conifer species, with focus on *Fagus sylvatica*, *Quercus* spp., *Picea abies*, and *Pinus sylvestris*. Daily cumulative growth (GRO) and daily minimum and maximum tree water deficit (TWD) were calculated. Data from 2018 were normalized against 2016–2017 to account for individual tree variability and site conditions. Linear mixed-effect models were used to analyze the relationships between TWD, GRO, vapor pressure deficit (VPD), and relative extractable water (REW). Separate analyses were conducted for annual and daily scales and for major species. The hydrometeorological space encompassing varying TWD ratios (2018 relative to control years) across VPD and REW was mapped.

The 2018 heatwave resulted in almost double the minimum TWD compared to the preceding two years, with no significant difference between broadleaves and conifers. However, annual GRO did not show consistent reduction across species and sites, though a non-significant trend toward lower growth was observed at sites with higher VPD and lower REW in 2018 compared to the previous years. A negative correlation between TWD and GRO was found. Analyses in the hydrometeorological space showed that conifers exhibited larger minimum TWD ratios compared to broadleaves under comparable conditions. The sub-daily TWD amplitude (difference between minimum and maximum TWD) was significantly higher in broadleaves, indicating better night-time stem rehydration. Oak trees displayed exceptional drought resilience, while Norway spruce and Scots pine showed moderate to high TWD across the hydrometeorological space.

The lack of consistent growth reduction in 2018 despite increased TWD is attributed to the timing of the heatwave (late July), after the main period of wood formation. Favorable spring conditions likely stimulated early growth but were followed by drought effects on soil and stem water status, which affected the overall annual carbon uptake. While annual growth was not significantly affected, the increased stem dehydration suggests potential longer-term consequences. Conifers’ lower rehydration capacity compared to broadleaves highlights species-specific vulnerability to drought. This study confirms that oak trees are remarkably drought-resistant, whereas Norway spruce and Scots pine show greater sensitivity. Differences in stomatal regulation and hydraulic safety margins likely underlie the observed differences in water use strategies.

This study demonstrates the value of high-resolution dendrometer data in understanding tree-level responses to heatwaves. The 2018 heatwave caused widespread stem dehydration, but consistent growth reduction was not observed due to the timing of the event. Conifers, especially Norway spruce and Scots pine, exhibited less resilience to drought than broadleaves. Future research should integrate dendrometer data with other measurements (transpiration, root traits, hydraulic traits) to refine our understanding of forest responses to climate change.

The study focused mainly on Central and Atlantic Europe, limiting the generalizability to other regions. The analysis relied on existing dendrometer networks, which might not be fully representative of all forest types and species. While the study analyzed a substantial dataset, including multiple species, it could benefit from data from a broader range of species to solidify some of the interspecific comparisons.