Adaptation to climate in many tree species of temperate and boreal forests involves balancing selection between high growth rates and stress resistance. In climatically mild environments high growth rates are favored over cold tolerance, while the opposite governs adaptation in climatically harsh environments. This growth-cold tolerance trade-off has been well documented for conifers but evidence in broadleaf deciduous species is scarce. In this study, we evaluate clinal responses of growth to climate and assess evidence of the growth-cold tolerance trade-off in five eastern North America broadleaf deciduous species: Acer rubrum (red maple), Betula alleghaniensis (yellow birch), Juglans nigra (black walnut), Prunus serotina (black cherry), and Quercus rubra (northern red oak). We use data from range-wide provenance tests established in the USA between 1960 and 1975, and we focus on the mean tree height of populations as a fitness trait. To evaluate clines of growth potential, we use observations for those populations transferred to test sites where the climate was as mild or milder than home climate, as trees growing without adverse environmental stress can express the innate growth capacity of the populations. We evaluate cold-limited responses as a proxy for cold tolerance. To do so we use the observations for those populations tested in climates colder than the home climate, thereby assuming that the height expressed on such sites would be limited by a trade-off with cold tolerance.
Results/Conclusions
Black walnut and black cherry showed steep clines in association with temperature variables. In these species, clines explained as much as 44% of the total tree height variation after correcting for test effects. Observed clines were less pronounced for red maple, with clines explaining as much as 36% of the total tree height variation. Population differentiation in northern red oak and yellow birch was weak and clines were detectable for only a few temperature variables. We document strong evidence for the growth-cold tolerance trade-off in broadleaf deciduous species: all growth potential clines had slopes that were opposite to those of cold-limited growth clines. The results indicate that these species have different strategies to accommodate environmental variation, and that different genetic growth responses should be expected as climate warms.