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Dry Forest Understory

Subject Areas Dry forests of ponderosa pine (Pinus ponderosa) and interior Douglas-fir (Pseudotsuga menziesii var glauca) occur along the valleys at low-to-mid elevations within the southern interior of British Columbia.

Prescribed fire

Fire suppression, overgrazing, and selective logging in these forests are believed to have caused forest encroachment on grasslands and ingrowth within open forests. Encroachment is tree ingrowth in previously treeless openings. Ingrowth is excessive tree recruitment, primarily by shade-tolerant species, such as interior Douglas-fir, within low-density, open forests 6

Forest cover changes over a 40 year period near Premier Lake, BC provide an example of forest encroachment onto grasslands. The photo on the left (below) shows forest cover in 1954 and the photo on the right shows forest cover in 1994. It has been estimated that 1500 to 3000 ha of open forest and grassland are lost annually to ingrowth and encroachment in the East Kootenay region of BC. 7

Forest cover changes 1954 - 1994

Ingrowth often results in the retrogression of understory plant communities, the loss of habitat for sensitive wildlife species, and an increased risk of catastrophic wildfires. Changes in forest structure within ingrown forests reduces forage availability for wildlife and livestock. For example, a lack of light and increased competition from pinegrass may limit the abundance of important forage species such as bluebunch wheatgrass and rough fescue. Habitat value for grassland and open-forest dependent wildlife species such as the badger is also diminished with ingrowth.

Restoration of ingrown stands usually begins with a harvest pass to remove merchantable timber and reduce overstory stocking to between 76 and 400 stems per ha. Slashing is used to eliminate excess intermediate layer trees that cannot be safely removed in a prescribed fire.

Untreated and treated stand

In the East Kootenay region of BC, restoration prescriptions are based on land use guidelines set by the Kootenay-Boundary Land Use Plan. The Rocky Mountain Trench Restoration Program estimates that 135 000 ha of ingrown forest will be restored to grassland or open forest by the year 2030.

Subject Areas
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Last Modified: 2006 Sep 7. Ministry contact: Reg Newman.
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