Branch growth and leaf numbers of red maple (Acer rubrum L.) and red oak (Quercus rubra L.): response to defoliation

Branch growth and leaf formation from terminal and from lateral buds of red maple (Acer rubrum L.) and red oak (Quercus rubra L.) were measured in response to simulated insect defoliation. A single large branch representative of the crown of each tree was used for enumeration of growth and of bud numbers throughout three successive years of 0, 50, 75, and 100% leaf removal for the entire tree. Leaf number per tree for both species after the last year of defoliation was reduced in direct proportion to the severity of defoliation, in comparison to the predefoliation status of the trees. Bud number per tree for red maple, but not for red oak, was also reduced in proportion to severity of defoliation. Averaged over all defoliation treatments, defoliation reduced branch growth more than leaf production. Furthermore, the reduction in branch growth and leaf production was greater in red oak than in red maple. Three years of successive defoliation reduced the mean lateral plus terminal branch growth by 40% in red oak and by 23% in red maple, while leaf number was reduced 22% in red oak and remained unchanged in red maple. In red maple, 100% defoliation caused greater branch death than the 50 or 75% defoliation treatments, and the amount of death was greater after each successive year of defoliation. In contrast to red maple, undefoliated red oak incurred a substantial amount of branch death throughout the study which was little affected by defoliation treatment.