KEY FINDING 18. Primary productivity has increased on more than 20% of the vegetated land area of Canada over the past 20 years, as well as in some freshwater systems. The magnitude and timing of primary productivity are changing throughout the marine system.
Primary productivity is the conversion of the sun's energy into organic material through photosynthesis. On land, it is driven by temperature and availability of water and nutrients modified by land use. In aquatic ecosystems, primary productivity is driven by the availability of nutrients and light and, to a lesser extent, by temperature and other factors. Primary productivity is important because it is the process that forms the foundation of food webs in most ecosystems.
Primary productivity increased significantly on 22% of Canada's vegetated land area between 1985 and 2006 and decreased on less than 1% of land.1 This trend in primary productivity is based on changes in the normalized-difference vegetation index (NDVI), a remote-sensing based measurement of photosynthetic activity – it is a good indicator of the amount of healthy green vegetation.2-4
The largest increases in primary productivity were found in the North where temperatures have risen the most. Changes in vegetation that correspond with this "greening" in northern Canada include a transition to shrubs and grasses where lichens and mosses once dominated,5 and changes in tree growth and density at mountain and northern treelines.6-8
In southern Canada, increases in primary productivity are likely more strongly related to changes in land use than they are to climate change.3 For example, increases in primary productivity in the Prairies are related to increases in crop area.3 The small decreases in primary productivity seen in some areas may be associated with urban and industrial development, or, as in interior British Columbia, forest insect infestations. Some increases in primary productivity may also be associated with fire, as burns can have positive or negative NDVI trends,depending on the age of the burn.3