Dryness decreases average growth rate and increases drought sensitivity of Mongolia oak trees in North China

Mongolian oak is a widely distributed tree species in broadleaf forests of North China, which are susceptible to increasing drought. Expansion of oak species ranges has been reported in Europe, America, and Asia with drought and heat stress, and thus, oak species are often deemed to be drought-resistant. As an expanding species in our study region, the Mongolian oak (Quercus mongolica Fisch. ex Ledeb) was used to investigate how tree growth rate and drought resistance change along a moisture gradient. In this study, we used a network of Mongolian oak tree-ring samples across a wide range of moisture gradients in North China to assess oak tree growth and its response to drought under different moisture conditions. Our results revealed that spatial variation in tree radial growth rate was highly related to the regions' moisture index, an index indicating moisture condition. Oak trees in drier portions grew slower than those in wetter portions and their growth was more sensitive to drought events. We used an ensemble of climate projections from general circulation models to predict future changes in the Ivanov moisture index, and then estimated the tree growth rate and drought-growth correlation for the period 2021-2100. Increased dryness is likely to reduce tree average growth rate markedly and increase the drought sensitivity of radial growth in the expanding dry portions of our study region, leading to substantial overall drought suppression of growth. Our results suggest that dryness will present an increasing challenge for the continuing expansion of Mongolia oak in already dry areas of North China, whereas oak trees will thrive better in high moisture conditions. The distribution of Mongolia oak trees will likely shift to regions with increased moisture availability, especially under the increasing drought.