Background
The authors write that "land cover change is a major response of arctic and subarctic terrestrial ecosystems to climate warming," and they say that "under warmer temperatures, open-tundra ecosystems will likely be colonized by tree and shrub species," citing Euskirchen et al. (2009), yet they indicate that "in situ evidence of erect shrub species expansion in response to climate is scarce."

What was done
In a study that provides such evidence, Tremblay et al. compared two series (1964 and 2003) of vertical aerial photos from the vicinity of Kangiqsualujjuaq (58°42'39" N, 65°59'43" W) in Eastern Nunavik (Canada), where long-term climatic data (1948-2008) from the nearest available meteorological station (160 km to the southwest) indicate the occurrence of "a strong warming trend over the last two decades, consistent with observations from several other Canadian Eastern Arctic sites (Allard et al., 2007)."

What was learned
The three Canadian researchers report that "during the 40 years spanning the two photo series, erect shrub and tree cover increased markedly on more than half of the land surface available for new colonization or infilling." More specifically, they say that "areas with dense shrub and tree cover (>90%), increased from 34% to 44%, whereas areas with low cover (<10%) shrank from 45% to 29%." And they note that "this increase in cover of trees and shrubs occurred throughout the landscape regardless of altitude, slope angle and exposure."

The main shrub species involved in this increase was Betula glandulosa Michx. (dwarf birch), which was present in 98% and dominant in 85% of the 345 plots that were studied. In addition, numerous seedlings and saplings of Larix laricina (eastern larch) were found above the treeline (25% of plots), suggesting, in Tremblay et al.'s words, that "the altitudinal tree line might shift upslope in the near future."

What it means
The three scientists say their results provide evidence of "dramatic increases in shrub and tree cover at a Low Arctic site ... contributing to a growing number of observations of woody vegetation change from various areas around the North," which suite of observations provides evidence for the Greening of the Earth that has been ongoing everywhere in recent decades, as a result of the historical and still-ongoing increase in the air's CO2 concentration that has been driven by mankind's ever-increasing usage of coal, gas and oil.

References
Allard, M., Fortier, R., Sarrazin, D., Calmels, F., Fortier, D., Chaumont, D., Savard, J.P. and Tarussov, A. 2007. L'impact du rechauffement climatique sur les aeroports du Nunavik: caracteristiques du pergelisol et caracterisation des processus de degradation des pistes. Final Report to Ouranos, Natural Resources Canada and Transports Quebec, Quebec: Universite Laval, Centre d'etudes Nordiques.

Euskirchen, E.S., McGuire, A.D., Chapin, F.S., Yi, S. and Thompson, C .C. 2009. Changes in vegetation in northern Alaska under scenarios of climate change, 2003-2100: implications for climate feedbacks. Ecological Applications 19: 1022-1043.