Height Growth of Advance Regeneration Under an Even-Aged Bigtooth Aspen (Populus Grandidentata) Overstory

Height and age structures and recent height growth rates of white pine (Pinus strobus) and red maple (Acer rubrum) advance regeneration were examined in an even-aged bigtooth aspen ecosystem to determine if height growth inhibition in the understory is characteristic of these systems, as is suggested in general for even-aged forests. We hypothesized that the overstory was inhibiting height growth of understory stems and that the degree of inhibition would vary with overstory basal area and the relative contribution to total basal area of species differing in light-attenuating ability. The study was conducted in a mature bigtooth aspen forest on a dry-mesic site in which little density-independent mortality in the overstory had yet occurred. Correlations between age and height of advance regeneration on replicate plots were significant in 14 of 16 cases, but were highly variable and, on average, not very high. Correlations were stronger for white pine, indicating more consistent height growth than for red maple. However, red maple advance regeneration had greater mean height growth rates than white pine. Height distributions of advance regeneration, distributions of relative height increment over 5 yr, and mean 5-yr relative height increment were unrelated to plot-level measures of total overstory basal area and relative basal area of bigtooth aspen. These results indicate that height growth of advance regeneration was occurring in this forest, but height growth rates varied among individuals and species. The degree of inhibition in height growth was unrelated to plot-level measures of total overstory competition, as reflected in basal area. Further, variation in height growth rate was unrelated to the relative contribution to total basal area of canopy species differing in light-attenuating ability. The results of this study indicate that large overstory gaps are not required to promote establishment and height growth of advance regeneration in all even-aged aspen forests and that controls other than overstory structure may lead to variation in height growth rates of advance regeneration