Background
The authors write that "global vegetation models predict rapid poleward migration of tundra and boreal forest vegetation in response to climate warming," while noting that "local plot and air-photo studies have documented recent changes in high-latitude vegetation composition and structure, consistent with warming trends." Therefore, "to bridge these two scales of inference," as they put it, they ...

What was done
... used fine-scale Landsat observations to quantify vegetation changes within and among plant cover types over the past quarter-century," focusing on "a latitudinal transect across the forest-tundra biome boundary in northern Quebec, a region that has experienced rapid warming in both winter and summer seasons during the satellite era," carefully selecting "time series of peak-summer Landsat data to evaluate changes in vegetation composition and structure rather than changes in phenology," i.e., temporal changes in the initiation and termination of the growing season.

What was learned
McManus et al. state that "30% of the observable (cloud-free) land area experienced a significant (P < 0.05) positive trend in the Normalized Difference Vegetation Index (NDVI)," with low shrub and graminoid (grass-like) tundra contributing "preferentially to the greening trend." Across the entire transect, for example, they report that "the area-averaged LAI increase was ~0.2 during 1986-2010," while "a higher area-averaged LAI change (~0.3) within the shrub-tundra portion of the transect represents a 20-60% relative increase in LAI during the last two decades."

What it means
In the words of the eight scientists involved in the study, "the observed association between shrub cover types and increased NDVI is generally consistent with the concept that woody plants can take advantage of warmer conditions and grow more vigorously."