Persistent drought legacy effects in a Scots Pine forest after years of concurrent drought and heat

In Central Europe, re-occurring compound events (drought and heat) have caused substantial damage to forest ecosystems with significant changes in carbon and water fluxes. Here we investigate the impact of the 2018 compound event and following drought years on net ecosystem carbon exchange (NEE) and vegetation dynamics of a Scots Pine (Pinus sylvestris) forest in SW-Germany (ICOS Site DE-Har, Hartheim, Germany). The compound event of 2018 caused severe hydraulic damage to trees, which led to high mortality rates of Scots Pine trees (>60% until 2024). While the forest ecosystem was a strong annual carbon sink in the past (up to -603 g C m^-2 year^-1), the ecosystem shifted to almost carbon neutral in a cold and wet year (2021). All other years since 2018 were hotter (and drier) than the long-term average, which led, in combination with legacy effects of 2018, to an annual carbon release with maximum values of +298 ± 12 g C m^-2 year-¹ in 2022. These values correspond to a difference in NEE of up to +901 g C m^-2 year^-1compared to conditions before 2018.

Concurrently, the vegetation composition of the ecosystem is slowly shifting from an evergreen coniferous forest to a mixed/deciduous forest. Deciduous trees in the understory expressed a higher resilience (higher water potentials and sap flux density) towards compound events compared to Scots Pine, potentially due to microclimatic buffering effects. This vegetation shift was clearly visible in the enhanced vegetation index (EVI) of the site, which increased in summer and decreased in winter, indicating an ongoing shift in canopy type and greenness towards deciduous species since 2018.

In conclusion, the compound event of 2018 caused significant legacy effects at the ecosystem and community scales in the studied Scots Pine forest. These effects were further exacerbated by recurrent atmospheric and edaphic drought conditions after 2018, which led to a significant ecosystem carbon release since then. If climate extremes do occur with the same frequency as in 2018-2024, this could significantly delay or even prevent ecosystem recovery, putting more ecosystems in Central Europe at risk.