TREE STORIES ABOUT A PEATLAND: TREE-RING RECONSTRUCTIONS OF WATER TABLE AND LARCH SAWFLY OUTBREAKS IN SASKATCHEWAN

Abstract

Eastern larch (Larix laricina) and black spruce (Picea mariana) are the most dominant tree species in peatlands in Canada, however interactions between peatland hydrology and species-specific radial growth are poorly understood. This study investigates the relationships between the growth/hydrological response of eastern larch and black spruce across a topographical gradient within a peatland in Saskatchewan. Tree-ring analysis revealed that peatland hydrology is the main factor driving radial-tree growth. Black spruce on the edge of the fen showed a positive relationship to increased water-table level, while eastern larch in the fen revealed a negative correlation to water-table level rise. Further analysis illustrated that radial-tree growth response to hydrology is dependent on specific water-table levels according to species and micro-site. Identified thresholds indicated that only 8 cm of variability in water-table level can greatly affect the fen forest dynamics. Once the relationship between tree ring and hydrology was established, a multiple- species regression equation was derived from tree-ring data to reconstruct past water-table levels. Results indicated that eastern larch and black spruce are suitable proxies to reconstruct hydrological variability at the site. Eastern larch ring widths are not only subjected to change by hydrology, but they can also be altered by exposure to larch sawfly outbreaks (Pristiphora erichsonii). Researchers have often been limited in their ability to draw accurate conclusions regarding the history of sawfly outbreaks in peatlands. Water-table level suppressions result in similar radial-growth patterns as when trees are defoliated by larch sawfly, making accurate diagnoses of larch sawfly outbreak a challenge. In this study I investigated the relationship between sawfly outbreaks and peatland hydrology. Five outbreaks where identified using traditional dendroecological analysis. The last outbreak identified was found to be a result of hydrological growth suppression and not sawfly defoliation. Observations indicated that periods of low water-table level may lead to increased populations of larch sawfly in northern Saskatchewan. I stress the necessity of using long-term hydrological analyses to accurately infer outbreak periods to distinguish them from water-table suppression.