Neighbor effects on tree architecture: functional trade-offs balancing crown competitiveness with wind resistance.

David W. MacFarlane, Brian Kane

Each tree has a unique architecture defined by different arrangements of the major parts: trunk, branches, leaves. Tree architecture is partially defined by what species the tree is; an expression of its genetics. For example, pine trees have their branches arranged in whorls around the stem. However, tree architecture is also plastic—trees grow adaptively to match the changing environmental conditions that the tree is exposed to over its life span. So, there is a lot of variation within species that is not well documented. It is generally understood that shading from other trees and wind pressure are major forces shaping tree architecture. Trees must grow taller and arrange their branches and leaves to capture enough light energy to survive, in competition with neighbors. However, they must also adopt a wind-stable growth form or risk stem breakage or toppling over and uprooting under wind pressure. We were interested in determining the tradeoff between optimizing architecture for light capture versus wind resistance, looking at trees growing in the open versus growing in the forest with differing levels of crowding from neighbors. To study this we analyzed measureable tree- and branch-level architectural traits of four temperate, broad-leaved, deciduous tree species of differing shade tolerance and wood strength from multiple locations across the northeastern United States.

Our results show that differences within species, due to the effects of crowding from other trees, were much stronger than differences between species and locations. Open-grown trees developed relatively large crowns and branches and a squat growth form suitable to resist greater wind exposure. By contrast, increasing light competition from neighboring trees caused forest-grown trees to become increasingly more spindly in the main stem, with slender branches sparsely distributed over a disproportionately large crown volume—presumably to maximize light capture. Though the latter is an intrinsically less wind-stable form, it can be adopted to increase light capture, because neighboring trees reduce exposure to the wind, which should greatly reduce the likelihood of stem breakage or uprooting under critical wind pressures.

Image caption: The architecture of a tree is shaped by its genetics (center tree is a maple) and the influence of surrounding trees. Photo by David W. MacFarlane.
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