Tree-ring widths and isotopes of artificially defoliated balsam firs: A simulation of spruce budworm outbreaks in Eastern Canada

Abstract

Defoliation by insects is a major disturbance influencing the forest dynamics in many ecosystems and can affect forest productivity worldwide. The main objective of this research was to further investigate the potential use of tree-ring widths and isotopic compositions to identify different degrees of past spruce budworm defoliation episodes. A secondary objective was to understand the responses of trees to defoliation episodes using carbon isotopes as a proxy to provide insights into subsequent physiological changes. Tree-ring widths, carbon and oxygen isotopic compositions in wood cellulose and gas exchange measurements were compared among 288 balsam fir (Abies balsamea Mill.) seedlings grown in a controlled experiment that involved different intensities of defoliation. Observations were performed over four growing periods. Moderate to heavy-defoliated seedlings showed reduced radial growth and enriched their cellulose carbon isotopic composition probably as a result of mobilized stored carbohydrates enriched in 13C. Less severely defoliated seedlings did not show significant reductions in growth and 13C enrichments. The gas exchange observations and wood cellulose oxygen isotope compositions do not suggest photosynthetic compensation in the remaining needles although a positive trend in the response of both assimilation rate (A) and stomatal conductance (gs) to defoliation was observed in the first growing period. Thus it remains open as to which mechanisms were employed to compensate for the reduced carbon source in the mildly defoliated seedlings. While further investigations are advised, the results of this study still help promote the utilization of tree-ring widths in combination with carbon isotopic compositions for reconstructing severe past defoliation events.