Forest Investment Account (FIA) - Forest Science Program
FIA Project Y091164

    Harvesting and site preparation treatments to develop and maintain open canopy conditions in dry-belt Douglas-fir forests: the Isobel Project
Project lead: Klenner, Walt (Ministry of Forests and Range)
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
Dry Douglas-fir forests extend over several million hectares in the Southern Interior Forest Region (SIFR) and represent a valuable resource from an economic, social and ecological perspective. The proximity of these forests to urban centers, lumber mills and valley-bottom ranches highlights the need for effective fuels management to reduce the risk of uncontrolled wildfire, and value to the timber and livestock industry. The ecological value of these sites is similarly high due to less severe temperatures and lesser snowpack in winter than at higher elevations. Concerns have been raised about management practices in dry forest habitats (Ponderosa pine and Interior Douglas-fir) grouped as Natural Disturbance Type 4 in BC (see BC Ministry of Forests and BC Ministry of Environment 1995). These dry forests generally occur at low elevations (under 1200m), often have a lower canopy closure than forests on more mesic sites, and have traditionally been characterized by the historic role of frequent low severity, “stand maintaining” fires. Over the past 50 years, numerous reports have examined and debated the role of fire and fire suppression in these ecosystems (Leopold 1924, Weaver 1951, Cooper 1960, 1961, Covington and Moore 1994). In the last decade, there has been an increasing concern regarding the shortcomings associated with current fire suppression activities (Daigle 1996, Gayton 1996, Rocky Mountain Trench Ecosystem Restoration Steering Committee 2000, 2006), and this is being reflected in local Land and Resource Management Planning. Although the reintroduction of fire is usually identified as a key requirement to returning these ecosystems to a condition within their former range of natural variability (Landres et al., 1999; Allen et al., 2002; Baker et al., 2007), uncertainty remains regarding the long-term efficacy of these treatments, the likelihood of achieving ecological objectives, and the effect of treatments on a range of forest values including the plant community, fuels, forage production and timber.

At higher elevations and more northern latitudes across the range of Ponderosa pine and in mixed stands that include Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) or lodgepole pine (P. contorta Dougl.), the historic range of natural variability for dry forests is far less certain. Shinneman and Baker (1997), Baker and Ehle (2001), Heyerdahl et al. (2001), and Sheriff and Veblen (2007) document spatially and temporally complex fire regimes in Ponderosa pine dominated forest. The occurrence of such mixed- or moderate-severity fire regimes (Agee, 1993, 1998; also termed variable severity fires in Baker et al., 2007) in more productive habitats or topographically complex areas creates uncertainty in both the need for and the nature of restoration practices. Forests that historically experienced mixed- or high-severity fire regimes are not likely to be in a structural condition that is outside the historic range of natural variability (Baker et al., 2007), and it is likely that mixed Ponderosa pine and Douglas-fir forests fall into this category (Sheriff and Veblen, 2007). However, regardless of the historic disturbance regime and the questions surrounding the need for “ecosystem restoration” in dry forests in BC, it is clear from resource planning initiatives that open forest conditions are desirable and may well be necessary to achieve a number of resource management expectations (e.g. ladder and crown fuel management, forage for native ungulates and livestock, etc.).

Historically, many of the low elevation, accessible dry forests have experienced repeated and often different harvesting treatments over the last century (Klenner and Vyse 1998, Klenner et al. 2001). A variety of harvesting practices have been applied, ranging from the unregulated selection of desired stems for specific needs, to diameter limit partial cutting or patch cutting. The primary objective of most prescriptions has been to remove timber or fuelwood, and/or to ensure regeneration and a long-term yield of merchantable timber from the site. A range of prescriptions to maintain or improve timber yields have been developed (e.g. moderate [40% of basal area] single tree selection volume removal, group selection and patch cutting, low-volume single tree removal in mule-deer winter range, etc.) and have been widely applied or tested. However, practices that favor other forest management objectives are only now beginning to receive attention. Furthermore, the efficacy of alternative treatments, and the likely effect of such prescriptions on timber yield and stand structure has largely been untested in an applied environment.

The main objective of the Isobel dry forest management project is to develop and apply prescriptions to maintain prolonged open canopy conditions in dry Douglas-fir (IDFxh2) forests. Much of the current emphasis in ecosystem restoration in these forest types has focused on the use of prescribed fire to achieve desired conditions, with few prescriptions being developed in the Southern Interior using a combination of harvesting and silviculture to achieve prolonged open conditions while maintaining long-term opportunities for timber extraction. At the Isobel project site, we have developed and applied prescriptions to maintain prolonged open canopy conditions in dry Douglas-fir (e.g. IDFxh) forests in a cost-effective manner, with a complementary objective of maintaining timber, forage and ecological values. Unlike fire which is at best difficult to manage, harvesting and related silvicultural activities have a long tradition of being used to guide stand development toward desired conditions. The work at the Isobel site applies a rigorous sampling design to the monitoring of diverse indicators of timber, forage and ecological condition. We are simultaneously evaluating several management options, including harvesting, site preparation and livestock management for their efficacy in achieving desired open conditions, and their effects on timber and ecological indicators.

The Isobel project was designed and implemented in 2003 to examine five key dry forest management issues in relation to several harvesting and site preparation treatments. The monitoring will address: 1. Conifer regeneration, 2. Understory species composition, 3. Understory productivity, 4. Fuel loading and 5. Timber productivity. In 2003, harvesting removed between 50 and 80% of the merchantable volume on 8 of the 12 blocks. There are four control (unharvested) blocks, four 50-60% volume removal blocks and four 75-80% volume removal blocks. Within each block (average = 20 ha), there is a 100 x 150m livestock exclosure, and control, mechanical spot screefing and prescribed fire site preparation treatments both inside and outside the exclosure (split-plot design). Response to the harvesting treatments is being monitored at 169 permanent sample plots systematically distributed across the site, and 2160 1m2 vegetation plots are being used to monitor vegetation and regeneration response to the site preparation and livestock exclosure treatments. Data will be analyzed as a regression design with understory light conditions as the main explanatory factor and site moisture as a co-variate since within treatment heterogeneity is high. Additional small experimental manipulations are being used to provide information on the rate of establishment or survival of conifer seedlings, the survival of transplanted bunch grass (rough fescue), tree mortality following prescribed fire, tree mortality due to windthrow and the effects of canopy gaps size on damage by western spruce budworm.
Related projects:  FSP_Y102164


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Updated August 16, 2010 

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