14.6 Trends in shrub cover above treeline

Last Updated: 
October 9, 2015

This indicator measures changes in the height, biomass or percent cover of shrubs above the treeline where they form the dominant vegetation type. Shrubs can be classified as prostrate (non-erect), dwarf (up to 40 cm tall) and tall upright (40-400 cm). Shrubs generally decrease in stature and density moving northward from the treeline.

Changes to shrubs can be measured over a range of spatial scales: at local scales using field plot measurements; over landscapes using repeat aerial and ground photography; and, at regional-to-continental scales based on analysis of multi-temporal satellite data. Optical satellite imagery is particulary sensitive to tundra vegetation leaf area and above-ground biomass, making this indicator amenable to repeat measurement over large areas.

The information for this indicator was summarized by ENR, GNWT from the results of studies conducted in the NWT by many university research teams.

Shrub surveys on the Tuktoyaktuk Peninsula

Shrub surveys on the Tuktoyaktuk Peninsula. Photo: Y. Zhang

NWT Focus

Arctic shrubs can respond rapidly to improved growing conditions and, therefore, provide a sensitive indicator of climate-driven vegetation change. Shrub growth is also stimulated locally by a variety of natural and human-caused disturbances and often provides the most conspicuous and long-lasting evidence of disturbance. Expanding shrubs may overtop and outcompete lichen, which reduces the amount of lichen forage available for barrenground caribou.

Current view: status and trend

Analysis of coarse resolution satellite imagery shows the normalized difference vegetation index (NDVI) is increasing within large regions of NWT's treeline transition zone3. NDVI is sensitive to green leaf area and, in this case, is likely responding to increases in shrub cover due to climate changes and regeneration following fire. A multi-scale study of vegetation changes above the treeline in northwestern NWT found the vast majority of areas show a significant increase in NDVI at 30 m resolution since 1985. The average terrestrial NDVI across the trend surface increased from 0.49 in 1985 to 0.59 in 2011, representing a 21% relative increase4. A comparison of over 200 high resolution (~3 cm) air photo pairs taken within the Tuktoyaktuk Peninsula in 1980 and 2013 indicates these NDVI changes are the result of erect shrubs expanding and overtopping lichen4. Plot-based and unmanned aerial vehicle (UAV) measurements at sites where shrubs have expanded also support this interpretation. It is likely  vegetation changes in this region are mainly due to long-term warming trends4. Several studies have shown tundra shrub growth in NWT is enhanced locally where disturbances expose and warm soils or deepen the active layer1,2. These include wildland fires, thaw slumps, drained lakes, vehicle tracks on seismic lines and gas well sumps.

Landsat NDVI

Landsat NDVI images from 1980 and 2013 depicting deciduous dwarf shrub and heath shrub changes. Figures: Fraser et al. 20144

There are significant Landsat NDVI changes occuring between 1985 and 2011 within a region to the east of the Mackenzie Delta above treeline. Close-ups of 1980/2013 colour infrared photo pair (25 m wide) showing expansion of shrubs with concomitant lichen declines. Deciduous dwarf shrubs (primarily birch) appear red in the 1980 photo, whereas heath shrubs (primarily Labrador tea) appear orange. 

Looking around

Borth field and satellite-based studies show shrub expansion is occuring in many locations across the circumpolar low-Arctic2,5. A recent meta-analysis of tundra vegetation surveys at 46 locations across the globe has also documented increased growth of vascular vegetation under naturally warming conditions6. In addition to NWT, northern Quebec and Labrador have also shown strong increases in NDVI3,7.

Looking forward

Ecological modeling predicts large-scale northward shifts in Arctic vegetation based on climate change scenarios. More than half of Arctic vegetation is expected to change to a different physiognomic class, with woody vegetation cover increasing by 52% by the 2050s8. These vegetation changes will likely reduce the availability of lichen forage for caribou in some regions and provide additional fuels to promote increased tundra fire activity.

Find out more

NOAA produces and annual Arctic Report Card that includes a chapter on vegetation, with a major focus on shrubs and satellite-based measurements.

Technical notes

Satellite NDVI trends, both at 30 m resolution (Landsat) and >250 m resolution (AVHRR and MODIS) are derived by analyzing a multi-decade record of near peak growing season imagery. Trends are computed separately for each pixel. More details on how NDVI is measured can be found on NASA's website.

 

Found an error or have a question? Contact the team at NWTSOER@gov.nt.ca.


References:

Ref. 1 - Lantz, T.C., P. Marsh, and S.V. Kokelj. 2013. Recent shrub proliferation in the Mackenzie Delta uplands and microclimatic implications. Ecosystems 16: 47-59.

Ref. 2 - Myers-Smith, I.H. et al. 2011. Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities. Environ Res Lett 6:045509.

Ref. 3 - Pouliot, D., R. Latifovic, and I. Olthof. 2009. Trends in vegetation NDVI from 1 km AVHRR data over Canada for the period 1985-2006. Int J. Remote Sens 30:149-68.

Ref. 4 - Fraser, R.H., T.C. Lantz, I. Olthof, S.V. Kokelj and R.A. Sims. 2014. Warming-induced shrub expansion and lichen decline in the Western Canadian Arctic. Ecosystems 17: 1151-1168.

Ref. 5 - Epstein, H.E. et al. 2012. Dynamics of aboveground phytomass of the circumpolar Arctic tundra during the past three decased. Environ. Res. Lett. 7:015506.

Ref. 6 - Elmendorf, S.C., et al. 2012. Plot-scale evidence of tundra vegetation change and links to recent summer warming. Nat Clim Change 2: 453-457.

Ref. 7 - McManus, K.M., et al. 2012. Satellite-based evidence for shrub and graminoid tundra expansion in northern Quebec from 1986 to 2010. Glob Change Biol 18: 2313-23.

Ref. 8 - Pearson, R.G. et al. 2013. Shifts in Arctic vegetation and associated feedbacks under climate change. Nat Climate Change 3:673-7.