The age and height structure of red maple (Acerrubrum) populations in northern Michigan bigtooth aspen (Populusgrandidentata) forests

Abstract

Red maple (Acerrubrum L.) is often the most abundant later successional tree species recruiting in the understories of aspen and oak dominated forests on dry–mesic sites in eastern North America. Limited evidence suggests that this species is capable of recruiting to dominant canopy positions on these sites. Given the potential for increasing overstory importance of red maple in these forests, detailed population-level examinations are warranted. In this study we examined the age and height structures of red maple populations in a bigtooth aspen (Populusgrandidentata Michx.) dominated landscape in northern lower Michigan, United States. Stem analysis was used to examine relationships between establishment times, heights, and height growth rates for overstory and understory red maple from 20 replicate plots in five stands located within a 18-km2 area. Red maple was a minor overstory component in the forests of the study area. The understories of all stands were overwhelmingly dominated by red maple. The populations were composed of two clearly defined age cohorts. The first cohort contained mostly sprout-origin individuals that established concurrently with bigtooth aspen within a 10-year period, beginning 70 years prior to the time of sampling. Mean age of the sprout-origin red maple cohort was not significantly different among stands, nor did it differ from the mean age of bigtooth aspen. Mean height of the red maple sprout-origin cohort was not significantly different among stands. Within each stand, height growth rates of these individuals were highly variable. The variability was not related to differences in stem age. Recent height growth increment of the sprout-origin stems was weakly related to position of an individual in the overstory, suggesting that most red maple were competitively suppressed by taller bigtooth aspen. Some sprout-origin red maple in all stands did approach the dominant bigtooth aspen in height growth rate. These were likely stems that were never competitively suppressed. The second red maple cohort contained seedling-origin individuals that began establishing 30–35 years after stand initiation, immediately after culmination of height increment in dominant overstory bigtooth aspen and red maple. This suggests that increasing resource availability, as a result of declining overstory vigor and canopy closure, may be a factor triggering understory reinitiation in these even-aged forests. In general, heights of seedling-origin red maple were more dependent on stem age compared with sprout-origin individuals. However, height growth rates for similar-aged individuals within the seedling cohort were still highly variable. The tallest individuals generally had the greatest rates of recent height increment, and thus were at a competitive advantage within the understory environment, but these were not always the oldest stems. There was, in fact, a trend of increasing initial height growth rate over time for the fastest growing seedling-origin individuals, again suggesting that resource availability in the understory was increasing over the course of stand development. Red maple's overwhelming understory dominance and ability to reach dominant canopy positions in the stands examined suggests a potential for increasing overstory importance on dry–mesic sites. Life history attributes, including shade tolerance, vigorous resprouting potential, and the ability to respond with increased growth upon release, may foster the development and maintenance of a red maple dominated cover type in the Great Lakes region.