Positive mixture effects in pine-oak forests during drought are context-dependent.

The increasing severity and frequency of droughts will play a pivotal role in shaping future forest ecosystems worldwide. Trees growing in mixtures are thought to be less susceptible to drought stress, but evidence for such positive admixture effects remains limited. This study examines how interspecific neighbourhood structures affect the growth responses of pine and oak species under recurrent drought stress in two contrasting forest ecosystems. We sampled naturally occurring, unmanaged mixed stands of Gambel oak (Quercus gambelii) and ponderosa pine (Pinus ponderosa) in semi-arid Arizona, USA, and pedunculate oak (Quercus robur) and Scots pine (Pinus sylvestris) in sub-humid Bavaria, Germany. Tree growth responses to recurrent drought events were assessed across a wide gradient of species admixture. Species admixture significantly influenced tree growth responses to drought stress, but the effects varied by species and forest ecosystem. In semi-arid Arizona, increasing species admixture buffered trees, especially Gambel oak, against drought stress. In sub-humid Bavaria, the effects of species admixture on pedunculate oak and Scots pine were more variable. Our findings emphasize the positive mixture effects in semi-arid environments, likely due to distinct niche complementarity and facilitation. Under sub-humid conditions, the effects were less consistent, aligning with the stress-gradient hypothesis. This study provides valuable insights into the complex dynamics of pine-oak interactions under drought stress and emphasizes the relevance of complementary species admixtures for climate-smart forest management in the face of climate change.