Tree-ring response of jack pine and scots pine to budworm defoliation in central Canada

• Impacts of jack pine budworm defoliation on radial growth of host trees were studied.
• Host and non-host comparison revealed five major outbreak periods.
• Tree-rings with thin latewood followed by narrow rings matched recorded outbreaks.
• Outbreaks were associated with cool spring and dry summers.
• Absence of outbreaks since the 1980s could be due to altered phenological synchrony.

Insect outbreaks constitute major disturbances and global climate changes are expected to increase their frequency and severity. In Canada, an increase in outbreak severity of the jack pine budworm is expected as a consequence of more frequent droughts associated with climate changes. In this study, the impact of jack pine budworm defoliation on radial growth was assessed on two host species: jack pine (Pinus banksiana Lamb.) and scots pine (Pinus sylvestris L.). Standard tree-ring chronologies were developed for each host species in thirteen plantations established in the early 20th century and located in Spruce Woods Provincial Forest (central Canada). Radial growth suppressions caused by jack pine budworm defoliation were identified using a host and non-host comparison and calibrated against historical outbreak records. Five periods of major growth suppression were identified (1956–1958, 1966–1968, 1974–1977, 1979–1980 and 1984–1986) that matched historical jack pine budworm outbreaks. An annual tree-ring signature made up of a tree ring with thin latewood followed by a narrow ring most often characterized these growth suppressions. The occurrence of missing rings also increased during outbreaks. Based on the timing of suppression, jack pine was the initial host with scots pine often showing a one year lag in suppression. However, scots pine may be more sensitive to jack pine budworm defoliation as indicated by the abundance of missing rings during outbreak years. In the study area, jack pine budworm outbreaks were generally associated with the occurrence of dry summers and cool May temperatures. No outbreak occurred in the study area since the mid-1980s. The occurrence of droughts that were not synchronized with cool May temperatures suggests that warmer springs associated with climate changes could alter the phenological synchrony between the jack pine budworm and its host trees species. Future research should attempt to (i) relate the results of this study to natural forest stands where management practices (and non-native tree species) have not influenced the natural jack pine budworm population dynamics, (ii) assess the spatial dynamics of these outbreaks using a network of tree-ring chronologies and (iii) reconstruct outbreaks prior to historical surveys. Such research would help develop a better understanding of insect population dynamics and subsequent impacts on both European and North American forests under future climate changes.