Abstract
1. Studies have shown that differences in height growth rates are characteristic of tree species that differ in understory tolerance and relative successional status. Less-tolerant species have greater juvenile height growth rates than more-tolerant species and thus, in a mixed-species forest, the former gain an early competitive advantage after a stand-initiating disturbance. Species-level height stratification is the predicted outcome under this model. 2. Silvicultural evidence suggests that a species-level differential growth rate model is idealistic because height stratification among species differing in understory tolerance is not consistently expressed within an even-aged forest. Stratification may fail to develop if individuals of a relatively tolerant species are competitively uninhibited, establish earlier than less-tolerant species, or are of vegetative-origin. 3. The establishment and height growth histories of three species differing in understory tolerance were examined within two forests in northern Michigan, USA. Objectives of the study included: (i) determining if species height stratification paralleling understory tolerance was a characteristic of these forests; (ii) assessing the degree of height variation within species populations; and (iii) determining if species age distributions, changes in individual competitive environments, or regenerative origin influenced population height structures. 4. In both forests, Populus grandidentata, an intolerant, early successional species was typically taller than Quercus rubra, a mid-tolerant, later successional species, which in turn was taller than Acer rubrum, a tolerant, later successional species. However within Quercus and Acer populations, heights were often highly variable, with some individuals equalling the heights of Populus. Age differences had no influence on height patterns. 5. Growth rate differences among surviving stems determined species stratification patterns and height variation within populations. Populus typically outgrew both Quercus and Acer, although some individuals of the latter two species had height growth rates equivalent to Populus. Most Quercus and Acer appeared to be of vegetative origin, as were all Populus ramets. For individuals of the former two species, potential juvenile height growth rates were likely greater than that of true seedlings, thus allowing some individuals to match growth rates of Populus. 6. Populus numerically dominated the forests because a large proportion of growing space was occupied by its root systems and ramets, relative to Quercus and Acer. Vertical domination by Populus was apparently the result of competitive inhibition of most, but not all, Quercus and Acer, not because of inherent, species-specific differences in juvenile height growth rates.