Dr. Jill Johnstone
University of Saskatchewan
Research areas
Ecological research led by Johnstone in the boreal forest as part of CCRN focused on two main study areas: BERMS (SK) and Wolf Creek (YT). Major outcomes from this research are summarized below for each of these locations, as well as for related research on the resilience of boreal forests to changing fire and climate.
BERMS: CCRN-funded research in the southern boreal forest of Saskatchewan, near the Boreal Ecosystems Research and Monitoring Sites (BERMS), has highlighted the sensitivity of these forests to interactions between climate variations and disturbances, such as fire, insects, and disease (Ireson et al. 2015). Even drought-tolerant species such as jack pine show decreased growth when dry periods coincide with widespread outbreaks of defoliating insects or the parasitic plant, dwarf mistletoe. Analyses of past growth of jack pine obtained from tree rings indicate that neither cooler nor wetter habitats in the Boreal Plains ecozone provide a refuge for jack pine from the compound effects of drought and disturbances. Insects that attack pine trees during a dry period appear to trigger a slow decline in tree growth than can cause trees to die several decades later (Mamet et al. 2015). Similarly, severe infection by dwarf mistletoe causes a decline in tree growth in pine trees that persists for decades and alters the way the tree responds to subsequent variations in climate (Horachek 2016). Our investigations of patterns of tree growth in response to natural climate variations experienced in Saskatchewan over the past half century suggests that climate may have relatively minor effects on forests when acting alone, but can lead to large changes forest vigor when disturbances and dry period occur together.
Wolf Creek: Monitoring of tundra vegetation (Mamet et al. 2016) in the alpine zone of Wolf Creek indicates relatively little change in low stature tundra over the past two decades. Likewise, aging of trees at and above the forest limit suggests that trees are responding slowly to recent warming and the 21st century will see only minor changes in the position of treeline in this area. In contrast, woody shrubs appear to be rapidly expanding in size and distribution, suggesting that alpine regions with moderate to heavy snow cover may develop greater shrub cover and height within just a few decades. [These data have not yet been published.]
Boreal forest: The emerging picture of boreal forest sensitivity to changing climate indicates that changing disturbances will play a major role in shaping the future of Canada’s forests. Climate directly affects the severity, frequency, and size of disturbances such as fire, insects, and windthrow, and interacts with disturbances to affect the ability of forests to recover from such events (Johnstone et al. 2016). Trees growing in cold, northern environments with permanently frozen ground appear to be just as vulnerable as trees in southern regions to drought stress (Walker et al. 2014, 2015) and drought-stressed trees are less likely to regenerate after fire (Walker et al. 2017). Predicting when and where boreal forests are likely to undergo rapid changes in response to changing climate and disturbance is a major focus of future research that will build off the CCRN legacy (Turetsky et al. 2017).