Drought impacts on tree carbon sequestration and water use – evidence from intra‐annual tree‐ring characteristics

The impact of climate extremes on forest ecosystems is poorly understood but important for predicting carbon and water cycle feedbacks to climate. Some knowledge gaps still remain regarding how drought‐related adjustments in intra‐annual tree‐ring characteristics directly impact tree carbon and water use. In this study we quantified the impact of an extreme summer drought on the water‐use efficiency and carbon sequestration of four mature Norway spruce trees. We used detailed observations of wood formation (xylogenesis) and intra‐annual tree‐ring properties (quantitative wood anatomy and stable carbon isotopes) combined with physiological water‐stress monitoring. During 41 d of tree water deficit, we observed an enrichment in (13)C but a reduction in cell enlargement and wall‐thickening processes, which impacted the anatomical characteristics. These adjustments diminished carbon sequestration by 67% despite an 11% increase in water‐use efficiency during drought. However, with the resumption of a positive hydric state in the stem, we observed a fast recovery of cell formation rates based on the accumulated assimilates produced during drought. Our findings enhance our understanding of carbon and water fluxes between the atmosphere and forest ecosystems, providing observational evidence on the tree intra‐annual carbon sequestration and water‐use efficiency dynamics to improve future generations of vegetation models.