Knowledge on ecological genetics and the extent of local adaptation in tree species is commonly gained from provenance trials by interpreting growth and survival of tested genotypes as a proxy of fitness. However, the nature of adaptation remains unknown without measuring adaptive traits and their associations with environmental variables directly. Genecology research addresses this gap, revealing the underlying mechanisms of local adaptation by studying population differentiation in adaptive traits on the landscape level. Adaptive traits are not conventional demographic traits, but they are essential components that influence long-term survival and population dynamics.
In our study, we investigated adaptation to climate in populations of two widespread tree species across a range of contrasting environments in western Canada. In a series of common garden experiments, bud phenology, cold hardiness and seedling growth traits were assessed for 254 populations in the interior spruce complex (Picea glauca, P. engelmannii, and their hybrids) and for 281 populations of lodgepole pine (Pinus contorta). We use phenotypic expression in adaptive traits to delineate genetically homogenous groups of populations based on geographic and climatic criteria so that they can be used to manage adaptive variation under current and expected future climates. We further quantify among and within-population diversity using different variance partitioning methods to assess how closely adaptive strategies of populations correspond to environmental gradients.
Complex multitrait adaptations to different ecological regions such as boreal, montane, coastal, and arid environments accounted for 15 to 20% of the total variance. This population differentiation could be directly linked to climate variables through multivariate regression tree analysis. In single traits, population variation for cold hardiness showed the strongest geographic differentiation in interior spruce, explaining 41% of variation, while budset was the most highly differentiated trait in lodgepole pine (22%). Nevertheless, our results suggest that adaptation to climate does not always correspond linearly to temperature gradients. For example, opposite trait values (e.g. early versus late budbreak) may be found in response to apparently similar cold environments (e.g. boreal and montane). This study investigated the 'ingredients' that create landscape demographic processes, which are also the ingredients for demographic change under climate change. While we can't make direct inferences on landscape demography, observed changes to demographic processes in long-term forest inventory plot data could likely be linked to geographic variation in adaptive traits that define locally adapted populations in this study.