Community and Ecosystem Dynamics of the Yukon Boreal Forest

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The boreal forest occupies half of Canada’s landmass and is undergoing marked change because of human-induced climate change and natural variation. Our research in the southwestern part of the Yukon will address the underlying causes for these key changes and their long-term ecological impacts. This part of Canada is a global hotspot for rapid climate change. Physical changes in temperature and rainfall are relatively easy to document and measure, but ecological changes are more complex. The boreal forest is a boom-and-bust ecosystem, driven by cycles in plant and animal production. Our research focuses on three levels. Plant primary production is predicted to be most immediately driven by changes in temperature and rainfall. We developed models that use summer rainfall and temperature to predict the annual crop size of berries (crowberry, bearberry, red bearberry, cranberry), white spruce cones, and above-ground mushrooms. We will develop similar models to predict the growth of the two major shrubs (grey willow, dwarf birch) from rainfall and temperature. If we are to understand the impacts of climate change we must develop these kinds of environmental models for the dominant species of plants and then test the models with subsequent field data. Animal populations in the boreal forest are strongly affected by the 9-10 year cycle of snowshoe hares, the dominant herbivore. We have shown that the hare cycle is driven by a suite of predators that fluctuate in synchrony with the hares, and these fluctuations set the ecological pulse of the boreal zone. The peak density of the snowshoe hare cycle has been slowly collapsing since 1971, a general trend found also in many small rodent cycles in Europe. This collapse has ecosystem consequences with hare browse in winter being reduced, allowing increased shrub growth in the forest understory. One explanation for the collapse of cyclic peaks has been increased predation in winter due to a mismatch in coat colour and snow onset and melt. If hares turn white too soon in the autumn, they become highly visible to predators, with the reverse problem in the spring. We will test this hypothesis with radio-collared hares in collaboration with ecologists working in Montana on the same problem. An alternative hypothesis is that predation has intensified as additional predators (marten, weasels) have become more common in the boreal forest and are increasing the mortality rate of hares. Arctic ground squirrels were a major herbivore in the Yukon boreal forest until 2000 when populations collapsed to virtual extinction. We are studying the landscape ecology of this collapse over a region of 15,000 km2, since ground squirrels remain abundant in some but not all alpine areas but not in forested areas. The alternative hypotheses are that they are held down by predators in the forest or that they are being affected by climate change via winter icing of hibernation sites. By reintroducing ground squirrels into previously occupied habitats we will test the predation model directly. Previous research has determined that stress from repeated unsuccessful predator attacks reduces reproductive rates in snowshoe hares over the population cycle. This work will be followed up by replicating the experiments carried out in the previous 10-year cycle and extending the results to measure how stress affects epigenetic changes in DNA and subsequent brain function in hares. Since the hare cycle is into the increase phase in 2013 we are poised over the next 5 years to advance these studies at the molecular and physiological level. Our overall research objective is to increase our ecological understanding to how boreal forest plants, herbivores, and predators are responding to rapid climate change in northwestern Canada.