Finding plants adapted to multiple climate extremes: Using genecology and ecophysiology to identify drought resistant and cold hardy populations of Douglas-fir
Drought and freeze events are two of the most common forms of climate extremes that result in tree damage or death, and the frequency and intensity of both stressors may be altered with climate change. Consequently, identifying populations of species that are (and are not) genetically well adapted to both desiccation and cold stress (i.e., stress hardiness) may be useful for successful regeneration of ecologically and economically important species such as coast Douglas-fir (Pseudotsuga menziesii var. menziesii) in the Pacific Northwest, USA. Douglas-fir populations are highly genetically differentiated with respect to stress tolerance due to climate-related natural selection. To assess and model genetic variation among populations in stress hardiness, we measured ecophysiological traits associated with drought resistance and cold hardiness on 35 populations of Douglas-fir growing in two common gardens. We then used principal components analysis to merge drought resistance and cold hardiness trait data into master ‘stress hardiness’ traits (PC1 and PC2). Principal component scores were modeled as a function of the historical climate at the location that seeds were collected (seed-source climate) using multiple regression; the equations were then used to map genetic variation in cold hardiness across the Douglas-fir range.
PC1 and PC2 explained 69 and 20% of the variation in the data, respectively. Higher PC1 values were indicative of greater drought resistance and cold hardiness, and related with cooler winter minimum temperatures of the seed-source climate. Therefore, populations originating in mountainous regions with colder winter climates were well adapted to cope with both cold and drought stress. Higher PC2 values were indicative of greater drought resistance at the expense of cold hardiness, and related with greater summer aridity and fall maximum temperatures, which indicated that climatic differences in summer and fall can confer relatively greater drought resistance or cold hardiness. Overall, we demonstrate that populations adapted to multiple climate extremes are readily identifiable based on climate. Populations originating in regions with cold winter, dry summer and warm fall seasons will be best adapted to survive current and future drought stress while maintaining a high level of cold hardiness, and therefore may be ideal to integrate into crop improvement programs or assisted migration strategies to manage forests for future climate.