Tuesday, 18 November 2003: 11:00 AM
Report on the use of thinning and prescribed fire to conrol fuels and wildfire threat in young Douglas-fir/Ponderosa Pine plantations
Wildfire occurrence and burn severity have increased in recent years throughout North America creating concern for the long-term sustainability of forest ecosystems. Much of the focus has been centered on the need to reduce fuels in areas containing high human and ecological values. These include homes in the wildland-urban interface, community watersheds, and threatened and endangered species habitat. Vast areas of fuel hazards exist or are constantly being created on forest lands being managed for multiple-uses including timber commodities. On these lands fuel reduction and silviculture site-preparation objectives can typically be met at 2 stages in forest development: the initial stand establishment stage following harvest, and possibly again at a more mature stage when understory burning can be safely and effectively carried out without damaging the future crop trees. The period between these 2 can be one of high to extreme hazard depending on the intensity and frequency of intermediate stand tending treatments such as juvenile spacing and pre-commercial thinning. Slash resulting from these treatments can constitute a hazard for many years. In 1989 the “Grav” fire burned 5 ha of dry ponderosa pine and Douglas-fir in the Squamish Forest District, B.C. The area was salvage logged, and planted to 1200 stems/ha. In 2001 the stand density at age 11 was >8,000 stems/ha. Simply spacing the stand to a lower target stocking of 900 stems/ha would have resulted in a high fuel hazard in an area of high to extreme wildfire threat. An experimental treatment regime of spacing and prescribed fire was applied to 1.4 ha of the plantation in the spring of 2002. The study site was divided evenly and half was spaced to 700 stems/ha and half to 500 stems/ha. Target species for retention were ponderosa pine first, followed by western larch, and Douglas-fir. The fuel reduction objective focused on significantly reducing fine fuels (1-hr and 10-hr timelag categories) and retaining the larger fuels remaining from the salvage operation. The spacing was completed in December in 2001 and the site was burned in April 2002. Fine fuels were reduced by 70% (1-hr) and 40% (10-hr), while larger fuels (>3”) were reduced by <15%. Tree mortality by the fall of 2002 was 13% for Douglas-fir, 3% for western larch, and 2% for ponderosa pine. The treatment was successful for meeting the stated objectives however further work is needed in determining follow-up fuel reduction treatments and devising ways to make the treatment cost-effective.