Oecologia (Berlin) (1983) 57:14-19 Oecologia Springer-Verlag 1983 C02 assimilation of primary and regrowth foliage of red maple (Acer rubrum L.) and red oak ( Quercus rubra L.): response to defoliation G.H. Heichel and N.C. Turner* The Connecticut Agricultural Experiment Station, New Haven, CT 06504, USA Summary. The CO2 assimilation of primary foliage of red maple (Aeer rubrum L.) and red oak (Quercus rubra L.), and of regrowth foliage produced in response to simulated insect defoliation, was measured throughout the season by infrared gas analysis: parallel measurements of leaf conduc- tance were obtained by ventilated diffusion porometry. The rate of net photosynthesis, measured at a quantum flux density of 1,150 gmol m-2s - 1, of primary foliage of both species increased from slightly negative values to about 5 gmol m- 2s- ~ by early June. Thereafter the rate of photo- synthesis of maple slowly declined to about 4 gmol m-2s 1 before onset of a senescent decline in early September, while that of oak slowly increased to about 8 gmol m-2s - ~ before onset of senescence. Manual defoliation to simulate insect attack in mid-June elicited refoliation proportional to the severity of defoliation in early July. After 100 % defoliation, fully expanded regrowth foliage of maple, but not of oak, had a rate of net photosynthesis from mid-July through September that was about 50% higher than in the primary foliage of undefoliated trees. A 30 to 60% enhancement of photosynthesis of residual primary foliage remaining on 50 and 75% defoliated trees during July was also observed. The seasonal patterns of CO2 exchange of primary and regrowth foliage, and the enhancement of CO 2 assimilation in residual foliage, was paralleled by similar changes in leaf conductance to water vapour. Carbon budgets of leaf canopies of each species showed that the net assimilation of the leaf canopy of both species ranged from 19 to 67% more than what would have been expected solely from replacement of leaf area. This response was greater in maple than in oak, presumably a reflection of the high rate of COz assimilation of regrowth maple foliage compared with that of the undefoliated control in maple. The increased COz assimilation of regrowth maple foliage and the increases in COz assimilation of residual primary foliage after defoliation offer evidence that hereto- fore unanticipated physiological mechanisms may be im- portant to perennial species coping with herbivory. Offprint requests to: G.H. Heichel, USDA-ARS, Plant Science Re- search Unit, 1509 Gortner Avenue, St. Paul, MN 55108, USA * Present address: CSIRO Division of Plant Industry, POB 1600, Canberra City, ACT 2601, Australia Introduction Periodic defoliation of deciduous hardwood forests by phy- tophagous insects may have important consequences to species composition of the forest (Stephens 1971; Campbell and Sloan 1977), and may cause death of species intolerant of leaf loss (Kulman 1971; Campbell and Sloan 1977). Insect defoliation of trees can alter the light environment (Collins 1969) and the nutrient and water relations (Ste- phens et al. 1972; Kitchell et al. 1979; Swank et al. 1979) within the forest ecosystem. Thus, defoliating insects can play an important role in regulating competition, succes- sion, death, nutrient cycling and long term productivity in forest ecosystems (Mattson and Addy 1975). In the northeastern U.S.A., the climax vegetation is red maple (Acer rubrum L.) and red oak (Quercus rubra L.) (Stephens and Waggoner 1980). The primary defoliators of this vegetation are gypsy moth (Lymantria dispar L.) and elm spanworm (Ennomos subsignarius Hbn.). Defolia- tion usually occurs in mid-June with little subsequent leaf loss, and whole sections of the forest are usually totally defoliated. The lepidopterous larvae preferentially defoliate red oak compared with red maple (Campbell and Sloan 1977; Stephens 1981). Compared to knowledge of the impact of the herbivore, little is known of how deciduous trees adapt to the stress of insect defoliation. Our previous research on the response of red oak (Quercus rubra L.) and red maple (Acer rubrum L.) to simulated insect defoliation (Heichel and Turner 1976) showed that severe defoliation hastened refoliation in the same season and budbreak in successive seasons com- pared with zero or mild defoliation. Species differences in the numbers of regrowth leaves formed, areas of individual regrowth leaves, and canopy growth in successive years of defoliation were also evident. The resistance to water vapour of regrowth foliage of red maple was much less than that of primary foliage on undefoliated trees, while no such difference was evident in red oak (Turner and Heichel 1977). This latter result suggested that leaf proper- ties associated with CO2 assimilation may have conse- quences in the growth and survival of deciduous trees when defoliated. Therefore, we examined the CO2 assimilation characteristics of foliage from defoliated and undefoliated trees and developed simple carbon budgets of leaf canopies to demonstrate the importance of photosynthetic rates and replacement of leaf area in response to defoliation in both maple and oak.