Landscape variability of vegetation change across the forest to tundra transition of central Canada

Widespread increases in the productivity of tundra ecosystems and static trends - or even declines - in boreal ecosystems have been detected since the early 1980s using coarse-scale remote sensing. However, intermediate-scale Landsat studies have shown that these changes are heterogeneous and may be related to landscape and regional variability in climate, land cover, topography and moisture availability. In this study, a Landsat Normalized Difference Vegetation Index (NDVI) time-series (1984-2016) was examined for an area spanning the transition from sub-Arctic boreal forest to Low Arctic tundra in central Canada. This was supplemented by analyses of relationships with a suite of environmental variables and in situ measurements of bulk vegetation volume. Results show that NDVI trends were generally positive (i.e. increasing) across the study area but were smallest in the forest zone and largest in the northern tundra zone. More than one-quarter (27%) of un-masked pixels exhibited a significant (p < 0.05) trend and virtually all (99.3%) of those pixels exhibited an increasing, or “greening”, trend. Greening pixels were most common in the northern tundra zone and the southern ecotone zone. Random Forest modeling of the relationship between NDVI and environmental variables indicated that the magnitude and direction of trends varied across the forest to tundra transition. Areas that experienced larger increases in NDVI include: (i) areas where summer temperatures increased; (ii) areas exhibiting predominantly shrub and forest cover; and (iii) locations closer to major drainage systems, further from major lakes, and at lower elevations. Ground validation in the central portion of the study area reveals a strong relationship (R2 = 0.79) between vegetation volume and NDVI for non-tree functional groups and that alder (Alnus crispa) shrublands and open spruce (Picea mariana and P. glauca) woodland with shrubby understories were most likely to exhibit greening. These findings indicate that the largest positive and more significant NDVI trends were associated with increased productivity in shrub-dominated environments, especially at, and north of the treeline in localities with favorable growing conditions. Smaller and less significant NDVI trends in boreal forest environments south of the treeline were likely associated with long-term successional change following disturbance rather than the variables analyzed here.