The influence of canopy gaps on overstory tree and forest growth rates in a mature mixed-age, mixed-species forest

Abstract

The death of overstory trees creates gaps in forest canopies. These canopy gaps have positive impacts on forests, including enhancing species diversity. The relative contributions to canopy gap closure made by understory trees within gaps and overstory trees at gap edges determines the future structure and species composition of a forest. Most studies of tree growth responses to gaps have focused on understory trees or canopy expansion by overstory trees. Here we focus on the stem radial growth rates of overstory trees and forest growth rates, measured as stand basal area increment. Our study area was a mature mixed-age, mixed-species deciduous forest in south-central Pennsylvania, USA. A hierarchical regression analysis found that gap-edge trees ≥20 cm dbh (diameter at breast height, 1.3 m) had 26% higher stem radial growth rates than comparable trees not located at gap edges ( P=0.004). Our hypothesis that smaller overstory trees would experience a greater growth benefit at gap edges was marginally supported ( P=0.06). A tree’s position north or south of a gap did not influence its growth response to the gap. The results suggest that overstory trees make an important contribution to canopy gap closure. Canopy gaps reduced the area occupied by overstory trees by 16%. But based on our regression model of stem radial growth rates, nearly two-thirds of the stand basal area increment lost because of gaps was offset by the enhanced growth of trees at gap edges (considering trees ≥20 cm dbh only). A simple, spatially-explicit process model of tree and forest growth in relation to gaps supported this finding. These results demonstrate that the benefits of canopy gaps come at a cost in forest growth rates that is considerably less than the gap area indicates.