Genetic basis of growth reaction to drought stress differs in contrasting high-latitude treeline ecotones of a widespread conifer

Climate change is increasing the frequency and intensity of drought events in many boreal forests. Trees are sessile organisms with a long generation time, which makes them vulnerable to fast climate change and hinders fast adaptations. Therefore, it is important to know how forests cope with drought stress and to explore the genetic basis of these reactions. We investigated three natural populations of white spruce (Picea glauca) in Alaska, located at one drought-limited and two cold-limited treelines with a paired plot design of one forest and one treeline plot. We obtained individual increment cores from 458 trees and climate data to assess dendrophenotypes, in particular the growth reaction to drought stress. To explore the genetic basis of these dendrophenotypes, we genotyped the individual trees at 3000 single nucleotide polymorphisms in candidate genes and performed genotype-phenotype association analysis using linear mixed models and Bayesian sparse linear mixed models. Growth reaction to drought stress differed in contrasting treeline populations. Therefore, the populations are likely to be unevenly affected by climate change. We identified 40 genes associated with dendrophenotypic traits that differed among the treeline populations. Most genes were identified in the drought-limited site, indicating comparatively strong selection pressure of drought-tolerant phenotypes. Contrasting patterns of drought-associated genes among sampled sites and in comparison to Canadian populations in a previous study suggest that drought adaptation acts on a local scale. Our results highlight genes that are associated with wood traits which in turn are critical for the establishment and persistence of future forests under climate change.

Automated summary

Climate change is causing more frequent and intense drought events in boreal forests, which have significant impacts on tree growth and survival. Understanding how forests cope with drought stress and the genetic basis of these reactions is crucial for future forest management and conservation. In this study, we investigated white spruce populations in Alaska and found that the growth reaction to drought stress varied among populations located at different treeline positions. We also identified genes associated with drought tolerance traits, highlighting their importance for the establishment and persistence of future forests under climate change. These findings suggest that drought adaptation acts on a local scale and that different populations may be unevenly affected by climate change.