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

Abstract

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.

Revealing how intra- and inter-annual variability of carbon uptake (GPP) affects wood cell biomass in an eastern white pine forest

“…Furthermore, inter-annual variability of EW CWA was associated to spring and early summer GPP, while LW CWA was associated to GPP from July to the beginning of September, suggesting that cell biomass variability was directly related to the C uptake in the season. Finally, the common intra-annual variations of GPP and intra-ring variation of CWA observed in specific years suggests that concurrent heat and drought events can rapidly decrease C sequestration during the wood formation, as observed in a recent study on xylem formation monitoring in Norway spruce (Martínez-Sancho et al 2022).…”

What drives forest carbon storage? The ramifications of source–sink decoupling

“…Extreme summer drought in 2015 resulted in dramatically different seasonal patterns of woody growth and C sequestration (Fig. 1; Martínez‐Sancho et al ., 2022). During a 41‐d period from mid‐June to late July during which stem radial shrinkage indicated physiological water stress, rates of cell enlargement and thickening were slowed, resulting in bimodal growth and C sequestration curves with peaks before and after the drought.…”

“…To reduce uncertainty regarding the future impact of global change on forest C sequestration, we need to develop comprehensive understanding regarding: (1) when, where and why decoupling occurs; (2) the spatial and temporal scales at which it operates; and (3) the implications for how we understand and model the forest C cycle. Given the complexity of tree physiology and forest C cycling, answering these open questions necessitates cross‐scale and cross‐methodology approaches, spanning from wood formation (Martínez‐Sancho et al ., 2022) to dendroecology, eddy covariance and modelling.…”

“…For instance, models would be likely to overestimate the sensitivity of ANPP woody to relatively short droughts (e.g. Martínez‐Sancho et al ., 2022) but underestimate its sensitivity to longer droughts (Kannenberg et al ., 2022), while overestimating ANPP woody of temperate deciduous forests under warming spring temperatures (Dow et al ., 2022). To reduce uncertainty regarding the future impact of global change on forest C sequestration, we need to develop comprehensive understanding regarding: (1) when, where and why decoupling occurs; (2) the spatial and temporal scales at which it operates; and (3) the implications for how we understand and model the forest C cycle.…”

“…sink limitation) across species and biomes (K€ orner, 2015). In this issue of New Phytologist, Mart ınez- Sancho et al (2022;pp. 58-70) exemplifies the type of research needed to improve our ability to predict forest C sequestration under climate change, using an innovative approach to describe the seasonal course of C sequestration in tree stem growth and how it is affected by drought.…”