Background
The authors indicate that tundra (a biome where tree growth is hindered by low temperature and short growing season) and taiga (a biome characterized by coniferous forest) comprise nearly 40% of the terrestrial landscape of Canada; and they say that "the responses of these ecosystems to the regional warming over the past 20-50 years were the focus of four Canadian IPY [International Polar Year] projects."

What was done
Henry et al. present "summaries and a synthesis" of some of the major Canadian IPY projects, where warming, as they write, "has been as much as 1°C per decade over the past 20-50 years (ACIA, 2005; IPCC, 2007; Hill and Henry, 2011)."

What was learned
The nine researchers report finding "an increase in plant growth over time," and that "increased shrub cover has occurred in sites across the Arctic at both the plot and landscape scale," noting that this phenomenon "was supported by results from experimental warming," which "increased vegetation cover and nutrient availability in most tundra soils." In addition, they say that "all sites measured were sinks for carbon during the growing season," and they state that "modeled responses of a mesic tundra system to climate change showed that the sink status will likely continue for the next 50-100 years."

What it means
In addition to increasing shrub cover, which is already an ongoing phenomenon, Henry et al. opine that "areas with adequate moisture for both seed production and seedling establishment will likely see increases in tree density in the coming decades," citing Brown et al. (2010), as, we might add, the Greening of Planet Earth continues.

References
ACIA. 2005. Arctic Climate Impact Assessment. Cambridge University Press, Cambridge, United Kingdom.

Brown, C.D., Walker, X., Jameson, R., Tremblay, D. and Hofgaard, A. 2010. Reproductive potential of forest expansion in the circumpolar north (Abstract). Understanding Circumpolar Ecosystems in a Changing World: Outcomes of the International Polar Year (3-6 November 2010), Edmonton, Alberta, Canada.

Hill, G.B. and Henry, G.H.R. 2011. Responses of High Arctic wet sedge tundra to climate warming since 1980. Global Change Biology 17: 276-287.

IPCC. 2007. Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC, Cambridge, United Kingdom.