Contrasting responses of forest growth and carbon sequestration to heat and drought in the Alps

Abstract

Climate change is expected to increase both the frequency and the intensity of climate extremes, consequently increasing the risk of forest role transition from carbon sequestration to carbon emission. These changes are occurring more rapidly in the Alps, with important consequences for tree species adapted to strong climate seasonality and short growing season. In this study, we aimed at investigating the responses of a high-altitude Larix decidua Mill. forest to heat and drought, by coupling ecosystem- and tree-level measurements. From 2012 to 2018, ecosystem carbon and water fluxes (i.e., gross primary production, net ecosystem exchange, and evapotranspiration) were measured by means of the eddy covariance technique, together with the monitoring of canopy development (i.e., larch phenology and normalized difference vegetation index). From 2015 to 2017 we carried out additional observations at the tree level, including stem growth and its duration, direct phenological observations, sap flow, and tree water deficit. Results showed that the warm spells in 2015 and 2017 caused an advance of the phenological development and, thus, of the seasonal trajectories of many processes, at both tree and ecosystem level. However, we did not observe any significant quantitative changes regarding ecosystem gas exchanges during extreme years. In contrast, in 2017 we found a reduction of 17% in larch stem growth and a contraction of 45% of the stem growth period. The growing season in 2017 was indeed characterized by different drought events and by the highest water deficit during the study years. Due to its multi-level approach, our study provided evidence of the independence between C-source (i.e., photosynthesis) and C-sink (i.e., tree stem growth) processes in a subalpine larch forest.

The Role of the Intraspecific Variability of Hydraulic Traits for Modeling the Plant Water Use in Different European Forest Ecosystems

“…By adjusting the k max to better represent the transpiration response we may be able to identify the timing of important physiological processes (e.g., leaf shedding) that differ between the VC and RC. In this way, we may be able to better understand the significant changes in different ecosystem processes triggered by intense dry periods (Oddi et al., 2022).…”

The role of the intraspecific variability of hydraulic traits for modelling the plant water use in different European forest ecosystems

“…By adjusting the k max to better represent the transpiration response we may be able to identify the timing of important physiological processes (e.g., leaf shedding) that differ between the VC and RC. In this way, we may be able to better understand the significant changes in different ecosystem processes triggered by intense dry periods (Oddi et al, 2022).…”

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

“…More recently, the increasing availability of multi-year records of ecosystem productivity measurements from eddy-covariance (EC) stations in forests has allowed researchers to assess the association between inter-annual variability of aboveground woody biomass accumulation and gross or net primary productivity (GPP and NPP). Such associations have been very variable among different sites (Babst et al 2014) and studies, ranging from not significant (Rocha et al 2006, Delpierre et al 2016, Pappas et al 2020, Oddi et al 2022 to highly positively significant (Xu et al 2017, McKenzie et al 2021, Metsaranta et al 2021. In addition, Teets et al (2018) and Tei et al (2019) found significant positive associations, but observed that annual tree biomass increment lagged one year behind net ecosystem production (NEP) and GPP, respectively.…”

“…Several studies conducted in forest EC sites have used tree-ring width (TRW) to estimate biomass growth (McKenzie et al 2021, Oddi et al 2022. Tree rings are a unique source of information, providing annually resolved measures of tree radial growth over long time periods (Speer 2010).…”