Mitigating fire risk to late-successional forest reserves on the east slope of the Washington Cascade Range, USA

doi:10.1016/S0378-1127(98)00274-6

Forest Ecology and Management

Volume 110, Issues 1–3, 5 October 1998, Pages 59-75

https://doi.org/10.1016/S0378-1127(98)00274-6 Get rights and content

Abstract

A fire-risk model was developed using a stand-structure approach for the forests of the eastern slopes of the Washington Cascade Range, USA. The model was used to evaluate effects of seven landscape-scale silvicultural regimes on fire risk at two spatial scales: (1) the risk to the entire landscape; and (2) the risk to three reserve stands with stand structures associated with high conservation priorities (layered canopy, large trees, multiple species). A 1000 ha landscape was projected five decades for each management regime using an individual tree, distance-independent growth model. Results suggest that a variety of silvicultural approaches will reduce landscape fire risk; however, reserve stand fire risk is minimally decreased by thinning treatments to neighboring stands. Intensive fuel reduction through prescribed burning and selection of reserve stands in favorable topographic positions provide substantial fire risk reductions.

Introduction

In recent years, considerable attention has focused on the identification and protection of late-successional forest reserves of the Western US and Canada (FEMAT, 1993). The reserves are highly valued for their biological and social values (Swanson and Franklin, 1992).

Protecting the reserves from catastrophic disturbance requires not only understanding the susceptibility of the reserve forest, but also the susceptibility of adjacent stands and the landscape as a whole. Late-successional forests are often predisposed to destructive crown fires as a consequence of their multi-layered canopies (Oliver and Larson, 1996). Many of these reserves are located within a landscape mosaic of complex ownership and management patterns in which secondary forests and managed plantations abut the protected reserves. The risk of fires in adjacent stands will depend on their structure, weather conditions, and ignition sources (Agee, 1993). If a susceptible structure is relatively common at the landscape-scale, the risk of a catastrophic fire affecting the reserve stand may be large.

Silvicultural treatments such as thinning and prescribed burning can lessen the fire-susceptibility of a given stand (Agee, 1993), however, such intensive management of reserve stands is generally prohibited. One option for lessening the fire risk to reserve stands is to decrease fire risk at the landscape-scale. Silvicultural treatments could be limited to neighboring stands or applied more widely across the landscape.

In this paper, we evaluate the effectiveness of differing landscape-scale silvicultural regimes in mitigating fire risk for both the entire landscape and several fire-susceptible late-successional reserve stands. Seven landscape-scale silvicultural regimes ranging from no treatment to intensive thinning with prescribed burning are simulated for five decades. We use an individual tree, distance-independent growth model to project future stand conditions and mimic silvicultural manipulations. Projected stand conditions are evaluated using a fire-risk model developed for the eastern slopes of the Washington Cascade Range, USA. The fire-risk model incorporates equations that evaluate the potential for crown-fire ignition and crown fire spread based on forest stand conditions.

Section snippets

Mechanics

The fire-risk model was built in Microsoft Access, a database management program, using output from the Landscape Management System (LMS; McCarter et al., 1996; McCarter, 1997), a computerized system that integrates landscape-scale data, stand-scale information, and growth models [in this case FVS¹ Northern Idaho variant (Wykoff et al., 1982)] to project changes through time across forested landscapes (Oliver and McCarter, 1996). Fig. 1 details the flow of

No treatment (none)

Landscape risk can be evaluated by examining the proportions of landscape area that fall in each risk category. In the no treatment scenario, landscape risk increases with time (Fig. 4). In presenting the results we refer to stand fire risk scores of 1 or 2 as `low risk' and scores of 6 or 7 as `high risk'. In the first decade, 15% of the landscape is composed of high-risk stands, and ≈40% is low risk. By the third decade stands with a risk of 1 occur on less than 10% of the landscape. By the

Discussion

The seven scenarios presented above address a wide spectrum of management intensities ranging from no treatment to various degrees of thinning and prescribed burning to total treatment at the landscape scale. The fire risk for the landscape decreases steadily as the intensity of management increases. The no treatment scenario has nearly 30% of the landscape in high-risk categories (6 and 7) by the fifth decade, whereas the intensive treatment of all stands has 100% of the landscape in the

Conclusion

Management of landscape fire risk involves trade-offs between individual stand- and landscape-scale considerations (Oliver, 1992). In this analysis individual stand- and landscape-scale risk were given equal weight. Our analyses suggest that even light treatments across a portion of the landscape provide a considerable reduction in overall landscape fire risk, although they may not lower the risk to unmanaged reserve stands with large trees and multi-layered canopies. Increasingly intense

Acknowledgements

We would like to thank Jim Agee and Bob Keane for reviews of earlier versions of the manuscript. Jim McCarter provided growth model support, helped to refine ideas, and assisted with Microsoft Access programming. Jeff Moffett provided GIS support. This research is part of a cooperative project between the Silviculture Laboratory (C.D. Oliver, Principle Investigator), College of Forest Resources, University of Washington and the U.S.D.A. Forest Service, Pacific Northwest Research Station. The

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