The Adaptive Significance of Tree Height

A game-theoretical model of tree growth is developed to evaluate the adaptive significance of height growth. The model balances the advantages of height for light interception against height-related costs, such as increased maintenance respiration, that reduce the energy available for stem growth. The model predicts an evolutionarily stable strategy (ESS) for trees of even-aged stands. This ESS consists of a prolonged interval of height growth that terminates when the trees reach 87% of the theoretical break-even height, at which stem maintenance and root and leaf renewal costs require all available photosynthate, leaving none for wood production. Tests of the model with data from forest yield tables indicate that (1) averagesized trees of even-aged stands follow the predicted ESS until reaching at least 70%-90% of their maximum height; (2) trees that are larger than average have thicker-than-expected trunks to withstand disproportionately greater wind forces in the upper canopy; (3) height growth may cease in very old stands, as predicted; and (4) height growth appears to cease gradually rather than suddenly. Features that are not in the model but might favor a gradual cessation of height growth include the greater wind exposure of the upper canopy and unpredictable environmental variation. The general success of the model in explaining patterns of tree growth suggests that competition for light is the primary factor responsible for the evolution and maintenance of the arboreal life form. The resulting evolutionarily stable growth pattern maximizes the competitive ability of the individual but reduces the collective wood production of older stands because of costs associated with height. This result is consistent with other analyses of individual versus group selection that have found productivity to be lower in individual-selected than in group-selected populations.