The impact of overstory density on sapling height growth in the Missouri Ozarks: implications for interspecific differentiation during canopy recruitment

Abstract

Successful canopy recruitment is one of the most important components of sustainable forestry practices. For many desirable species in oak-dominated forests, insufficient sapling growth is a common limitation to successful recruitment. The objectives of this study were to (i) examine the impact of overstory density on sapling growth in the Missouri Ozarks, (ii) investigate the potential for overstory retention to promote compositional shifts via interspecific differences in sapling height growth, and (iii) compare the use of mean and near-maximum growth rates to quantify the impact of overstory density on sapling growth and height differentiation among species. We found that the periodic annual height increment of saplings decreased with increasing overstory density for all species groups in this study (red oaks (Quercus spp.), white oaks (Quercus spp.), hickories (Carya spp.), sassafras (Sassafras spp.), blackgum (Nyssa spp.), dogwood (Cornus spp.), red maple (Acer spp.), ashes (Fraxinus spp.), and elms (Ulmus spp.)). There was evidence of interspecific differentiation in growth rates during the sapling stage, and the observed differences were more pronounced at low overstory densities. Increasing overstory densities either reduced or eliminated the differences in growth among species. Although red oaks displayed the greatest maximum growth rates of all species under low overstory densities (<5 m2·ha−1), the growth advantage of red oaks was reduced with increasing overstory density. This may provide opportunities to shift species composition toward white oaks using partial harvesting regimes in the Missouri Ozarks. However, white oaks had little to no advantage in height growth over many competing species when overstory density exceeded about 10 m2·ha−1. This implies that the probability of recruitment under overstory densities greater than about 10 m2·ha−1is likely to decline for all oaks in the Missouri Ozarks. We found that using the 90th quantile of height growth rates to evaluate the impact of overstory density on sapling growth had two potential advantages over using the mean growth rate: (i) it provided better models of the limiting effects of overstory density on sapling height growth, and (ii) the focus was on the growth rates of stems that were most likely to recruit into the canopy.