LAUSR

The occurrence of hot drought, i.e. low water availability simultaneous with high air temperature, represents a severe threat to ecosystems. Here, we investigated how the 2018 hot drought in Central Europe caused a tipping point in tree and ecosystem functioning in a Scots Pine (Pinus sylvestris L.) forest in South-West Germany. Measurements of stress indicators, such as needle water potential, carbon assimilation and volatile organic compound (VOC) emissions of dominant P. sylvestris trees were deployed to evaluate tree functioning during hot drought. Ecosystem impact and recovery were assessed by ecosystem carbon exchange, normalized difference vegetation index (NDVI) from satellite data and tree mortality data. During summer 2018, needle water potentials of trees dropped to minimum values of -7.5 ± 0.2 MPa, which implied severe hydraulic impairment of P. sylvestris. Likewise, carbon assimilation and VOC emissions strongly declined after mid-July. Decreasing NDVI values from August 2018 onwards were detected along with severe defoliation in P. sylvestris, impairing ecosystem carbon flux recovery in 2019, shifting the forest into a year-round carbon source. 47% of all monitored trees (n = 368) died by September 2020. NDVI recovered to pre-2018 levels in 2019, likely caused by emerging broadleaved understorey species. The 2018 hot drought had severe negative impacts on P. sylvestris. The co-occurrence of unfavourable site-specific conditions with recurrent severe droughts resulted in accelerated mortality. Thus, the 2018 hot drought pushed the P. sylvestris stand towards its tipping point with a subsequent vegetation shift to a broadleaved dominated forest.