Variations in growth and architecture in the range limit of Salix humboldtiana, the willow native to South America

Abstract

The conditions faced by edge populations of a tree species may affect the architecture of individual trees. The latitudinally broad range of Salix humboldtiana Willd. encompasses tropical to cold-temperate climatic regions, with its southernmost population in central Patagonia. We expected S. humboldtiana trees from the southern-edge population to develop smaller shoots and a lower level of axis differentiation than trees of this species from a more northern population, and we expected edge trees to outperform non-edge trees when growing in a common garden at an intermediate latitude. For juvenile individuals of S. humboldtiana from edge and non-edge natural populations, the trunk diameter, height, branching angle, as well as the length, number of nodes, diameter, leaf area, and specific leaf area of the main branch and the trunk annual shoots were measured. The size and extension rates of shoots developed by trees of both origins in a common garden were also evaluated. In natural populations, the shoot leaf area and the specific leaf area were higher for the edge population than for the non-edge population. In a common garden, the shoot extension period and the length were higher for non-edge plants than for edge plants. The growth-period differences between populations in a common garden may support the hypothesis of stress adaptation in the southernmost S. humboldtiana trees.