MECHANISMS PROMOTING RECOVERY FROM DEFOLIATION IN GOLDENROD (SOLIDAGO-ALTISSIMA)

Plant responses to defoliation were examined using Solidago altissima and a leaf-chewing beetle (Trirhabda sp.). Plants were exposed to five intensities of defoliation (ranging from 0 to 85% leaf area loss) and effects on carbon gain, vegetative growth, and flowering were determi ned. Defoliated plants partially restored their capacity for carbon ga in in the following ways: (i) activity of damaged leaves remaining aft er defoliation was increased via delayed senescence and enhanced photo synthetic rates and (ii) regrowth leaves on damaged plants had higher specific leaf area (leaf area per leaf mass) than comparable leaves on undamaged plants, but photosynthetic rates per unit area were equival ent to controls; thus, these leaves covered more area for a given inve stment in biomass with no loss in activity per unit area. Delayed leaf senescence and compensatory photosynthesis are commonly observed foll owing defoliation, but increased specific leaf area is not generally r ecognized as a mechanism contributing to plant regrowth. In spite of t hese changes, total carbon gain capacity of defoliated plants was stil l less than that of controls after 3 weeks of regrowth. Overall plant performance was reduced by defoliation. Defoliated stems grew at a slo wer rate early in the season, added fewer new leaves in the first few weeks after defoliation, and had fewer lateral stems throughout the se ason. Damaged plants delayed flowering and maintained height growth la ter into the season than undamaged plants. Damaged stems reached heigh ts comparable with undamaged stems by the end of the season, but they were thinner and their flower production was lower. Declines in plant growth and flowering were linear functions of the percentage leaf area loss, suggesting that even low levels of insect feeding are likely to affect plant performance.