Adaptation to climate in five eastern North America broadleaf deciduous species: Growth clines and evidence of the growth-cold tolerance trade-off

Clines in genetic variation along climate gradients documented for many species in temperate and boreal forests illustrate adaptation of forest tree populations to climate. Adaptation tends to involve balancing selection between high growth rates and stress resistance: a trade-off in resource allocation between stress tolerance and competitive ability that determines fundamental life history strategies. The growth-cold tolerance trade-off, in particular, has been well documented for many conifers but evidence in broadleaf deciduous species is scarce. Using populations’ mean total tree height and provenance climate from range-wide provenance tests established in the USA between 1960 and 1975, we evaluate and model growth clines along climatic gradients and assess evidence of the growth rate-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). Number of populations and provenance tests in this study varies by species, ranging from 31 to70 and 4 to 8, respectively. For each species, observations were separated into two groups, those where the population was moved to a test site climate milder than at its origin, and those where the population was moved to a colder test site climate. Clines of growth potential along climate gradients were evaluated using the group of observations in which the populations had been transferred to a milder test climate, while cold-limited responses were evaluated using the observations in the group of populations moved to colder test climate. Results showed steep clines in association with temperature variables for black walnut and black cherry and less pronounced clines for red maple. Population differentiation in northern red oak and yellow birch was weak. Growth potential clines had slopes opposite to the cold-limited growth responses, providing strong evidence for the growth-cold tolerance trade-off in broadleaf deciduous species. The results show that genetic growth responses to climate will differ considerably among species and thereby demand different management strategies to mitigate climate change impacts.