Tree form and anatomical determinants of maximum potential height, with focus on North American conifers

Summary Tree form can determine biomechanical strategies for attaining maximum height growth. In an earlier paper, tree height in five global forest regions was linked to increasing bending stiffness (modulus of elasticity, MOE) and, in three of those regions, height was independent of bending strength (modulus of rupture, MOR), as approximates with slender column buckling theory. But, in Latin American and North American angiosperm forests, tall trees exhibited a relationship with bending strength in addition to the relationship between height and stiffness. Here we compared forest characteristics of the five regions and determined that greater species diversity likely introduced more competition with the development of larger and broader crown size. These trees no longer approximate the slender column model and consequently result in shorter trees. Since conifers do approximate slender column buckling theory, we used the inverse relationship between tracheid length and microfibril angle (MFA), a determinate of tracheid tensile strength and stem stiffness, to show that tracheid length for 37 North American conifers correlates with current tree height. This correlation mimics that of stiffness with tree height, providing evidence that tracheid length could be used as a predictor of potential tree height. Eight conifers (four southern pines, bald cypress, sugar pine, Sitka spruce and coast redwood) with the longest tracheids revealed current maximum heights that are less than tracheid length would predict. They are currently restricted to seasonally wet habitats that fail to provide the tropical rainforest conditions from which they were excluded by angiosperms in the evolutionary past. Current rapid climate change leading to drier conditions could further reduce maximum height development in these conifer giants.