American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

Historical Perspective: The Adaptive Geometry of Trees revisited”

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Thomas J. Givnish (June 2020)

Horn’s Adaptive Geometry of Trees had an important influence on plant ecology, but its central model must now be recast

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Abstract

The Adaptive Geometry of Trees had an important conceptual influence on plant ecology, and helped inspire many new approaches to understanding succession, plant adaptation, and plant competition. Its central model provided an elegant potential explanation for how optimal canopy form should shift with ecological conditions, change those conditions through time, and thus help drive succession and be a consequence of it. Yet, upon close examination, this deeply inspirational model does not lead to the predictions for which it is widely known. Here I show that the Horn model actually favors monolayer canopies over multilayers under all light conditions if relative growth rate (growth per unit investment) is maximized. Horn’s conclusion that multilayers would be favored over monolayers in brighter sites is an artifact. I propose that self-shading multilayers might gain an advantage in brightly lit sites by reducing water loss, reducing the costs of branch construction and maintenance, reducing photoinhibition, increasing light capture in side-lit microsites, and increasing water and nutrient supplies, or leaf longevity, when combined with one or more of the previous potential advantages. I conclude with a brief discussion connecting Horn’s model to other conceptual frameworks in plant ecology, and outlining possible future extensions.