Ontario: Yours to Discover

Last spring, I participated in a field visit organized by the Canadian Forest Service in Ontario. The visit took us from the shores of Lake Superior to White River, with the purpose of looking at experimental trials studying the effects of organic material removal on soil productivity. Trials consisted of sites that had been harvested stem only (tops and branches left on site), whole tree (trees delimbed and branches and tops piled at roadside), or whole tree harvested AND the forest floor scraped by a bulldozer (not for faint-hearted soil ecologists). The stands were about 15 years old.

Ontario’s current guidelines for forest biomass harvesting are among my favourites because they are based on simple and easily observed criteria for identifying sensitive sites: soil depth and soil texture. Thin or coarse sandy soils are considered at highest risk of fertility loss with forest biomass harvesting.

Large areas of Ontario are located on sandy outwash plains and shallow soils and thus would be classified as risky. Forest stands on these types of sites encompass a wide range of productivity. Therefore, a refinement of the criteria seems necessary to better capture the sites at the lowest end of the productivity range, i.e., the really sensitive ones. So, in a joint effort by the Ontario Ministry of Natural Resources, the Canadian Forest Service, and other partners, trials were set up on coarse sandy or shallow soils, as well as other typical Ontario forest sites such as peatlands.

This research in Ontario is linked to a wider initiative called the Long-Term Soil Productivity program. It includes more than 100 experimental sites across the United States and Canada (with some LTSP sites in British Columbia), with trials similar to those in Ontario and covering a broad gradient of species, soils, and climates. This network of trials produces large amounts of ecological data and makes for a very powerful and credible analysis of the ecological effects of forest biomass harvesting.

So far, 15 years of post-harvest data show that in slow-growing northern forest stands such as black spruce, scraping the site of all organic material at harvest causes the regenerated trees to grow faster. This is the opposite of what we might expect or want, but occurs because such a treatment improves the microclimate for the seedlings. For example, more sun reaches the ground and warms the soil because there is no organic material cover, and that enhances root growth and produces better tree growth. This is actually in accordance with what we know about the requirements of seedlings in the first stage of stand development: growth is largely driven by the microclimate.

However, faster-growing stands such as those dominated by jack pine or trembling aspen, or those located in warmer climates, go through this initiation stage much faster. They are usually past this phase by year 15 and are reaching canopy closure; microclimate is a lot less critical for growth, and nutrient availability becomes a more important growth driver. By then, the trees on sites where more organic material was removed (i.e., whole-tree harvest and whole-tree harvest plus forest floor scraping) are starting to show signs of nutrient starvation and slower growth than those on sites that had plenty of organic leftovers (i.e., stem-only harvest).

Admittedly, the data from Ontario and the rest of the LTSP network do not show a widespread catastrophe associated with forest biomass harvesting. However, they do hint that organic material left behind at harvest may play an important role for tree nutrition and growth, but that role may not show up until later in the rotation, once the trees are past the initiation stage. This clearly demonstrates the importance of having a good long-term monitoring network of trials to follow up stands through time and see how they develop after different harvesting treatments. In this case, time is in terms of decades. The LTSP teams in Ontario and British Columbia have obviously understood this for quite a while and are providing key information for the development of sustainable forest management policies in Canada. Thumbs up!          

For more information on the ecological research going on in Ontario about forest biomass harvesting, contact Rob Fleming and Paul Hazlett at the Canadian Forest Service in Sault Ste. Marie, and Dave Morris of the Ontario Ministry of Natural Resources in Thunder Bay.

Dr. Evelyne Thiffault is a research scientist in forest biomass at Natural Resources Canada and provides Canadian Biomass with her thoughts on sustainable biomass harvesting.