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

The aim of this study was to test the potential of carbon (δ13C) and oxygen (δ18O) 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 δ18O 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 δ18O values were also highly synchronous with those of the two other coniferous species. The comparison of host and non-host ring width, δ13C and δ18O chronologies confirmed the potential of combining these isotope indicators of spruce budworm outbreaks.