Forest growth resistance and resilience to the 2018-2020 drought depend on tree diversity and mycorrhizal type

1. The frequency of consecutive drought years is predicted to increase due to climate change. These droughts have strong negative impacts on forest ecosystems. Mixing tree species is proposed to increase the drought resistance and resilience of tree communities. However, this promising diversity effect has not yet been investigated under extreme drought conditions and in the context of complementary mycorrhizal associations and their potential role in enhancing water uptake. 2. Here, we investigate whether tree diversity promotes growth resistance and resilience to extreme drought and whether drought responses are modulated by mycorrhizal associations. We used inventory data (2015 - 2021) of a young tree diversity experiment in Germany, manipulating tree species richness (1, 2, 4 species) and mycorrhizal type (communities containing arbuscular mycorrhizal (AM) or ectomycorrhizal (EM) tree species, or both). For all tree communities, we calculated basal area increment (BAI) in the periods before, during, and after the drought and used the concepts of resistance and resilience to quantify growth responses to drought. 3. We found strong growth declines during the extreme 2018-2020 drought for most of the tree communities. Contrary to our hypothesis, we did not find that tree species richness per se can buffer the negative impacts of extreme drought on tree growth. However, while for EM communities, drought resistance, and resilience decreased with tree species richness, they increased for AM communities and communities comprising both mycorrhizal types. We highlight that among various tree species mixtures, only those with mixed mycorrhizal types outperformed their respective monocultures during and after drought. Further, under extreme drought, the community tends to segregate into 'winner' and 'loser' tree species in terms of diversity, indicating a potential intensification of competition. 4. While we cannot disentangle the underlying mechanisms, or clarify the role of mycorrhiza during drought, our findings suggest that mixtures of mycorrhizal types within tree communities could help safeguard forests against increasing drought frequency. 5. Synthesis: Drought resistance and resilience of tree communities depend on tree diversity and mycorrhizal association types. Mixing tree species with diverse mycorrhizal types holds promise for forest restoration in the face of climate change.