PRIMARY PRODUCTIVITY

Status and Trends

impacts on ecosystems variable and not well understood

some good datasets

emerging issue with potential for major ecological impacts

Status and Trends legend

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.

This key finding is divided into three sections:

• Key finding overview (this page)
• Primary production in Arctic lakes
• Marine primary production

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.

Changes in primary productivity on land

Trends in annual peak NDVI, 1985 to 2006

Note: trends shown are statistically significant.

Source: Ahern et al., 2010,¹ adapted from Pouliot et al., 2009³

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

Global Trends

Photosynthetic activity was estimated to have increased on about 25% of the Earth’s vegetated area and decreased over 7% of this area from 1982 to 1999. The greatest increases were in the tropics, as a result of fewer clouds and increased exposure to the sun, and in high latitudes of the Northern Hemisphere, attributed to increased temperature and water availability.⁹