Recurrent hot droughts cause persistent legacy effects in a temperate Scots Pine forest

Recent hot-dry events have caused significant impacts and legacy effects in temperate ecosystems. Here, we investigate legacy effects of the 2018 hot drought on a Pinus sylvestris L. forest in southwestern Germany and the effects of post-2018 recurrent hot-droughts on ecosystem carbon fluxes. We combined ecophysiological, remote sensing (Enhanced Vegetation Index, EVI) and micrometeorological (Net Ecosystem Carbon Exchange, NEE) measurements to assess past and present ecosystem functioning. We found strong and persistent legacy effects and high tree mortality of P. sylvestris, with deciduous understorey trees slowly replacing P. sylvestris. After 2018, EVI clearly followed the pattern of a deciduous-dominated forest, indicating changes in canopy structure, type and seasonality in NEE. Significant legacy effects in NEE were found and the ecosystem shifted from a carbon sink (NEE = -391 ± 204 g C m^-2 year^-1, 2003-2006) to carbon neutral (NEE = +13 ± 28 g C m^-2 year^-1) in 2021, a cold and wet year. All other years post-2018 were hotter and drier than the long-term average (1991-2020), and the ecosystem was turning into a carbon source, with highest values in 2022 (NEE = +329 ± 19 g C m^-2 year^-1). These compound events of atmospheric and edaphic drought led to strong ecosystem carbon release post-2018. Our data show that the ecosystem most likely experienced strong drought legacy effects, such as 2018, at species, community and ecosystem scales. These negative effects were further exacerbated by recurrent atmospheric and edaphic droughts, shifting the ecosystem to a net carbon source after 2018.