Dr. Jill Johnstone

Changing climate, disturbance, and vegetation succession in northern boreal forests

Dr. Jill Johnstone, freelance researcher

Dr. Jill Johnstone is a plant ecologist based in Whitehorse, Yukon, Canada, where she conducts research on plant ecology in affiliation with Yukon University, University of Alaska Fairbanks (USA), and University of Saskatchewan. Much of her research focuses on northern ecosystems, such as boreal forest and tundra, that are currently experiencing rapid rates of climate change. She uses field measurements and experimentation combined with statistical or simulation modeling to examine the dynamics of plant community responses to environmental change. She is particularly interested in how disturbances may catalyze ecosystem changes and the role of plant-soil interactions in stabilizing different potential configurations of ecosystems. Dr. Johnstone has been involved in a number of international research collaborations that have developed our understanding of the mechanisms and patterns of how forests are responding to changing fire and climate.

Abstract: Interactions of disturbance with climate change play a pivotal role in driving forest dynamics during the coming century because of the disruption of ecological legacies that sustain ecosystem resilience to change. In northern boreal forests, changes in climate are leading to multiple changes in environmental factors that affect forest growth, usually in the direction of increasing environmental stress. However, the substantial inertia of intact forest communities means that large scale changes in forest structure are most likely to occur in association with disturbance. In addition, the characteristics of a disturbance itself can push a system in different directions, acting as an additional driver of change. Here I discuss results from studies of post-fire vegetation succession across the northern boreal region that help identify conditions that support forest recovery to pre-fire conditions (=resilience) versus those that may be pushing the system into new configurations. Material legacies associated with organic layer accumulation and propagule availability are a key factor supporting resilience across fire cycles. Unusual disturbance conditions that disrupt these legacies, such as changes in fire return interval or severity, support the initiation of alternative trajectories of vegetation succession that are maintained by a new set of internal ecosystem feedbacks. Once established, these alternative successional trajectories have cross-scale effects on ecosystem processes of regional importance such as carbon sequestration, fire behaviour, and wildlife habitat quality. As a consequence, system responses of boreal forests to climate change will very likely be sensitive and non-linear in response to changing disturbance regimes, with widespread ramifications for the boreal region and beyond.