Early warning signs of biome shift in boreal forests

Background/Question/Methods

Global vegetation models predict that climate warming, and its amplification at high latitudes, will cause species, vegetation communities, and entire biomes to migrate poleward during the 21st century.

A biome shift at northern latitudes would affect the world's largest forest zone and potentially accelerate partial thaw of the ~500 Pg organic C currently locked in the top 1 m of Earth's permafrost soils. It could thus not only profoundly impact biodiversity and ecosystem services but also generate feedbacks to the climate system.

Here, we review studies of the biological response to historic and ongoing environmental change in boreal and tundra ecosystems to assess whether they support the notion of an initiating biome shift at high latitudes. 

Results/Conclusions

Long-term satellite observations of vegetation productivity show that today's seasonal profile of vegetation activity in high latitude zones resembles that of areas >5° further South 3 decades ago. This geographic shift in vegetation 'seasonality' has occurred in step with climate warming and has allowed establishment and accelerated growth of shrubs in tundra areas previously dominated by non-woody species.

At the same time, areas of North America's boreal forests show steady declines in vegetation productivity in the past century. In Alaska, comparisons of satellite-derived productivity patterns with tree-ring data indicate that in the warmer boreal regions, drought is increasingly limiting spruce growth. This inference is supported by observations from the boreal-temperate ecotone, where drought has resulted in increased mortality in boreal tree species

Together, these findings suggest that patterns of vegetation productivity have generally moved northward as the climate has warmed, but that moisture deficits can quickly impose new limits on plant growth and even survival. Tree-mortality as a result of large wildfire years and pest outbreaks has increased as well. Since they cause rapid but profound changes to ecosystems, disturbance events can catalyze ecological regime shifts, particularly if they are accompanied by changing environmental conditions that promote an alternative ecological regime: Severe wild fires in Alaska, for example, are causing decade-long shift towards dominance by deciduous tree species in areas previously covered by coniferous forest.

Productivity patterns of vegetation display an ongoing northward shift consistent with biome shift. However, predictions of how further climate change will redistribute the boreal biome, need to take into account multiple resource limitations to plant growth and establishment, the role of future disturbance regimes in shaping communities and the pace at which populations can migrate.