Impacts of Atmospheric and Soil Droughts on Carbon and Water Fluxes in Northern European Coniferous Forests: Evidence From Eddy-Covariance Data.

The recent decline in forest carbon (C) sink across the Nordic countries has been partly attributed to increased water limitations, that is, droughts, on forest productivity. However, empirical evidence for this remains limited. This study combines ERA5-Land reanalysis datasets with long-term C and water flux observations from Swedish and Finnish coniferous forest eddy covariance (EC) towers to assess the conditions under which atmospheric (high vapor pressure deficit, VPD) and soil droughts (low soil moisture) have affected forest carbon uptake. Using the reanalysis datasets, we then evaluate whether such conditions have become more common at the studied sites, and across northern Europe in general. EC data suggests that C and water exchange has responded only to extremely low soil moisture, whereas the response to atmospheric droughts has been more gradual and the strongest response occurred during compound droughts. At two sites, isolated drought years led to over 10% reduction in annual gross primary productivity and 20%-40% decline in annual net ecosystem productivity. However, frequent and widespread impacts on annual C fluxes were not observed. The occurrence of atmospheric droughts has increased across northern Europe during the last decade, while soil and compound droughts showed more localized changes. Beyond methodological advances in analyzing drought signals from EC data, our results shed light on how droughts may have influenced ecosystem-atmosphere carbon and water fluxes across northern European forests by integrating site-level flux responses with reanalysis datasets. The results suggest that increasing VPD may be one underlying factor for the observed decline in forest growth in the Nordic countries, while soil moisture limitations have been likely rarer and more local.