The shifting states of alpine treelines in Canada
Editor's note: What does climate change have to do with treelines shifting to higher and higher altitudes? And what does this mean for animal species relying on alpine ecosystems?
This piece from Carissa D. Brown explores our diverse treelines across Canada - what keeps them alive in certain locations and what facilitates, slows down or prevents their expansion.
This article first appeared in the ACC's 2018 State of the Mountains Report. We'll continue to publish articles exploring the science on our current state of Canada's alpine on our blog throughout the year. Find them all here.
By Carissa D. Brown, Department of Geography, Memorial University.
Picture a Canadian mountain. What images come to mind? A steep slope with low vegetation, loose rocks or scree, topped with sharp, glacier scoured ridges? Backcountry skiers slicing through the unbroken snowpack, reaching the lower forested slopes where they dodge trees as if on a slalom run? Iconic images of the Rocky Mountains?
As northern latitudes like Canada continue to warm, as summers become longer, and harsh high-elevation environments ameliorate, we expect that forests that have historically been restricted to the climatic conditions of low-elevation slopes, like the ones you were just imagining, will expand upwards in elevation. With continued global change and no other barriers in place, one might expect trees to reach the summit of mountains across Canada in the coming decades.
Are trees to blame?
If that were the case, trees would overtake alpine meadows and with no other habitat for the alpine plant species to colonize, populations would be greatly reduced and some alpine species even extirpated or lost altogether. Animal species relying on alpine ecosystems, like the bighorn sheep or pika, would be under the pressure of decreasingly available habitat.
Are trees the central antagonists in a story of the conservation of alpine meadows? Yes, some treeline populations are encroaching into alpine ecosystems and the conservation concerns that come with tree invasion of alpine communities are warranted. Yet, globally, just over half of treelines are shifting in the direction predicted by recent warming(1) — upslope in mountain systems — and they are doing so more slowly than we expected. In Canada, alpine treelines are responding to recent warming in every way imaginable, where expansion rates range from slow to rapid and seem to be tree species-specific(2,3).
A canada-wide concern
The variable responses of Canadian alpine treelines is occurring not only in the iconic Rocky Mountains of Alberta and British Columbia, but in mountain systems across the country (Figure 1), from the western-most Coast Mountains in B.C., north to Kluane National Park and the Ogilvie Mountains of Yukon, all the way to our eastern most mountain range, the Appalachians, which span (in Canada) from southern Québec across the island of Newfoundland (Figure 2). Southern Québec is not the usual environment one imagines there to be a treeline, but treelines can also be studied as tree species lines – where the elevational distribution of one tree species ends, transitioning to high alpine forests (e.g., sugar maple-balsam fir alpine slopes).
What do we know so far?
What have we learned from such diverse mountain systems across Canada? Yes, a suitable climate is necessary for trees to establish beyond their current distribution along mountain slopes. Yet, we know that many things beyond a suitable climate are required for a tree to establish and survive in any given location, and alpine treeline researchers in Canada are compiling evidence of factors that may facilitate, slow, or prevent tree species from expanding their ranges upslope.
At present, we have observational and experimental evidence of non-climatic factors influencing individual trees’ abilities to establish upslope at each life stage (Figure 3). For example, nearby adult trees need to produce viable seeds that escape pre-dispersal seed predation and are then dispersed into the new, upslope habitat(4,5). That habitat needs to have the right kind of substrate (plant cover and soil) for the seed to germinate and establish(6–8). The newly emerged seedling needs to eat and drink, requiring sufficient light, soil nutrients, and soil moisture, all of which can be affected by topographical attributes such as aspect(2,9) and geomorphology(6) (Figure 4). Acquiring nutrients from the soil often involves a partnership with microbes and fungi living in the soil; if those fungi are absent, the seedling may not have access to enough nutrients to succeed(10,11). The seedling needs to survive the winter, which comes with a different set of challenges: snow scouring, exposure to strong winds, and exposure to extreme temperatures if insulating snow does not protect the seedling(12) (Figure 4). Even with all of these pieces of the puzzle in place, a seed or seedling is still at risk from enemies, be it seed predators, seedling herbivores, pathogens, or the footprint of the lesser-spotted alpine hiker (8,13).
Building upon what we know
Tree populations are not systematically moving up mountain slopes with warming; instead, treeline advance is occurring slowly on some mountains and not on others. Treeline researchers are continuing to advance our understanding of where, and under what circumstances, upward expansion of trees will occur by incorporating the many factors beyond climate that influence a tree’s ability to establish and survive.
1. Harsch, M. A., Hulme, P., McGlone, M. & Duncan, R. Are treelines advancing? A global meta-analysis of treeline response to climate warming. Ecol. Lett. 12, 1040–1049 (2009).
2. Danby, R. K. & Hik, D. S. Variability, contingency and rapid change in recent subarctic alpine tree line dynamics. J. Ecol. 95, 352–363 (2007).
3. Trant, A. J. & Hermanutz, L. Advancing towards novel tree lines? A multispecies approach to recent tree line dynamics in subarctic alpine Labrador, northern Canada. J. Biogeogr. 41, 1115–1125 (2014).
4. Kambo, D. & Danby, R. K. Constraints on treeline advance in a warming climate: a test of the reproduction limitation hypothesis. J. Plant Ecol. (2017). doi:10.1093/jpe/rtx009
5. Jameson, R. G., Trant, A. J. & Hermanutz, L. Insects can limit seed productivity at the treeline. Can. J. For. Res. 45, 286–296 (2014).
6. Macias-Fauria, M. & Johnson, E. A. Warming-induced upslope advance of subalpine forest is severely limited by geomorphic processes. Proc. Natl. Acad. Sci. (2013). doi:10.1073/pnas.1221278110
7. Lafleur, B., Pare, D., Munson, A. D. & Bergeron, Y. Response of northeastern North American forests to climate change: Will soil conditions constrain tree species migration? Environ. Rev. 18, 279–289 (2010).
8. Wheeler, J. A., Hermanutz, L. & Marino, P. M. Feathermoss seedbeds facilitate black spruce seedling recruitment in the forest–tundra ecotone (Labrador, Canada). Oikos 120, 1263–1271 (2011).
9. Dearborn, K. D. & Danby, R. K. Aspect and slope influence plant community composition more than elevation across forest–tundra ecotones in subarctic Canada. J. Veg. Sci. 28, 595–604 (2017).
10. Kernaghan, G. & Harper, K. A. Community structure of ectomycorrhizal fungi across an alpine/subalpine ecotone. Ecography 24, 181–188 (2001).
11. Wagg, C., Husband, B. C., Green, D. S., Massicotte, H. B. & Peterson, R. L. Soil microbial communities from an elevational cline differ in their effect on conifer seedling growth. Plant Soil 340, 491–504 (2011).
12. Renard, S. M., McIntire, E. J. B. & Fajardo, A. Winter conditions – not summer temperature – influence establishment of seedlings at white spruce alpine treeline in Eastern Quebec. J. Veg. Sci. 27, 29–39 (2016).
13. Brown, C. D. & Vellend, M. Non-climatic constraints on upper elevational plant range expansion under climate change. Proc. R. Soc. B Biol. Sci. 281, 20141779 (2014).