Carbon and oxygen stable isotopes from tree-rings to identify spruce budworm outbreaks in the boreal forest of Québec

Abstract

The aim of this study was to test the potential of carbon ( δ 13C) and oxygen ( δ 18O) isotopes as indicators of spruce budworm outbreaks. It was hypothesized that defoliation induced by insects would trigger a 13C enrichment of Abies balsamea and Picea mariana tree-ring α-cellulose through higher photosynthetic compensatory rate, while δ 18O would remain constant. The hypothesis was based on observations of increased photosynthetic rate induced by defoliation, as a compensatory mechanism ([Little, C.H.A., Lavigne, M.B., Ostaff, D.P., 2003. Impact of old foliage removal, simulating defoliation by the balsam fir sawfly, on balsam fir tree growth and photosynthesis of current-year shoots. For. Ecol. Manag. 186, 261–269], [Lavigne, M.B., Little, C.H.A., Major, J.E., 2001. Increasing the sink:source balance enhances photosynthetic rate of 1-year-old balsam fir foliage by increasing allocation of mineral nutrients. Tree Physiol. 21, 417–426]). Comparison of the two host species, A. balsamea and P. mariana with a non-host one, Pinus banksiana, revealed carbon isotope enrichments during both the 1950s and 1970s spruce budworm outbreaks which did not occur in the non-host species. Carbon and oxygen isotope values showed high synchronicity not only within species but also between species ( A. balsamea and P. mariana) and sites. P. banksiana δ 18O values were also highly synchronous with those of the two other coniferous species. The comparison of host and non-host ring width, δ 13C and δ 18O chronologies confirmed the potential of combining these isotope indicators of spruce budworm outbreaks.