Impacts of severe droughts on species interaction in forests

Global change-type droughts increasingly endanger sustainable forest functioning in semi-arid and also in temperate European forests. Generally, mixed forests comprising different tree species are considered more resistant towards droughts, however, little is known about changes in species interactions (i.e. facilitation and competition) under increasing drought severity. In particular, knowledge on the regulation of ecohydrological processes, such as tree water fluxes, is lacking. We investigated responses during both natural extreme drought in 2018 and 2022 in a pine forest and experimental drought and competition treatment in a cork-oak forest between 2017 and 2020.

The heavily impacted Scots pine (Pinus sylvestris L.) forest at the ICOS ecosystem site Hartheim in the upper Rhine valley, Germany, hit a tipping-point during the 2018 drought showing very negative leaf water potentials, and over 47 % tree mortality in 2019. Net carbon exchange indicated slow recovery of NEE and a vegetation shift to broadleaved understory trees.

The combined precipitation exclusion and shrub invasion (Cistus ladanifer L.) experiment in a Mediterranean cork oak (Quercus suber L.) ecosystem in Portugal showed that the combination of imposed drought and shrub invasion amplified stress effects during an extreme drought, with strongly reduced tree transpiration. Contrarily, the imposed drought reduced the competitiveness of the shrubs in the following recovery period, which buffered the negative effects of shrub invasion on Q. suber.

Further a literature review on the impact of species interactions on tree resilience underlined that interactions can shift with increasing drought severity: beneficial species interactions, i.e. improved water relations, were prevalent under mild droughts. However, with increasing drought, negative effects, such as interspecific competition increased. These prevailed under extreme droughts, where even trees with complementary resource use strategies competed for water resources.

Moreover, under extreme droughts, competition effects and reduced recovery of some species were observed, which can strongly compromise tree resilience. Our results demonstrate the highly dynamic and non-linear effects of interacting stressors on ecosystems and urges for further investigations on biotic interactions in a context of climate change induced alteration.