On the Adaptive Significance of Leaf Height in Forest Herbs

Based on the premise that competition for light is the most important selection pressure on leaf height in forest herbs, I present a game theoretic model for the evolution of this trait which balances the structural costs of a given height increment over an opponent with the resultant photosynthetic benefits. Herbs growing in areas with sparse herbaceous cover are unlikely to be next to and thus under an opponent. Hence, they reap little advantage by being taller, whereas the opposite is true in areas of dense cover. Therefore, the evolutionarily stable strategy favors greater leaf height in productive herb communities having dense coverage than in less productive communities with sparse coverage. In deciduous, cool temperate forests, winter photosynthesis may be limited more by leaf temperature than by light intensity, and thus may favor recumbent basal leaves in evergreen or wintergreen species and seasonal morphs. These ideas are confirmed by data on plant height and coverage from a study along a topographic gradient from a dry oak woods, through a mesic forest, to a floodplain forest. Maximum leaf height roughly doubles with each 7% increase in herbaceous cover, corresponding rather closely to quantitative predictions based on independent measures of the allometry of support tissue as a function of leaf height. The low stature of evergreen herbs favored by winter conditions appears to restrict their occurrence to dry, sterile sites with low coverage by summer competitors, and may be another factor favoring an association between evergreenness and soil poverty in temperate forest herbs. Patterns of leaf height among orchids native to the northeastern United States correspond to the qualitative predictions of the model. Extensions of the model are sketched for herbs in open habitats; woody plants; higher leaves in reproductive individuals; selection on flower or fruit height; evolutionary advantages of the herbaceous habit in relation to height growth; and the evolution of altruism along relatives. If a species occurs in herbaceous communities of high density only when its neighbors are close relatives, selection may favor the evolution of lower, more "altruistic" leaf heights than with unrelated competitors. Aster macrophyllus, which usually has basal leaves even though it occurs in dense herb communities, is advanced as a possible case of altruism in that its stands are nearly monospecific and appear to be maintained by allelopathic compounds.