Drought resilience of coastal Douglas-fir is influenced by competition but not genetic selection

Abstract

Coastal temperate forests in western North America are projected to experience more frequent episodes of extreme heat and drought resulting from anthropogenic climate change. The impact of these episodes on tree growth is uncertain and is of great economic and ecological importance. Coastal Douglas-fir (Pseudotsuga menziesii var. menziesii) is a widespread and economically valuable tree species in western North America, and tree breeding programs have produced fast-growing seed for reforestation through genetic selection for stem volume. However, it is important to know if this selection process has affected Douglas-fir’s susceptibility to extreme heat and drought, and how such effects are modulated by competition. This study uses tree-ring data from five replicated realized gain trials in coastal British Columbia (BC), Canada, each consisting of four planting densities and three levels of genetic gain for stem volume. By comparing tree-ring widths of 1190 trees across all factorial combinations, we evaluated the effects of selection and competition on growth patterns following years with extreme drought (2015) or heat (2021). We found that a low planting density increased resistance (capacity to reduce impact of the extreme event) and resilience to drought, but had no effect on recovery (capacity to regain pre-event growth rates) after drought. In addition, we found no difference in resilience between operational planting densities and the lowest planting density, while operational planting densities produce higher-volume per hectare and higher-quality wood. Second, we found that genetic selection has not decreased the resilience of Douglas-fir to either heat or drought. With planting density as the primary management driver of Douglas-fir growth variability, the use of operational planting densities could ensure that future Douglas-fir forests are resilient to climatic extremes and can continue to provide their ecological, economic, and cultural functions.