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Author(s):
Jonathan D. Bakker, Charles B. Halpern, Richy J. Harrod, Lauren S. Urgenson, Allison K. Rossman, David W. Peterson
Year Published:

Cataloging Information

Topic(s):
Fuels
Fuel Treatments & Effects
Mechanical treatments
Prescribed Fire-use treatments
Recovery after fire

NRFSN number: 16991
Record updated:

Fuel-reduction treatments have been used effectively in dry, fire-adapted forests to reduce risk of high-severity crown fire, but it is less certain if they achieve their ecosystem restoration objectives. To date, there has not been a comprehensive assessment of how the spatial and temporal dimensions of ecological assessments may influence our understanding of the effectiveness of fuel treatments (mechanical thinning, prescribed burning) in meeting ecosystem restoration objectives. We addressed this gap in knowledge through two study components: (1) a systematic review of the peer-reviewed literature that explicitly considers the temporal or spatial aspects of vegetation response to fuel-reduction treatments; and (2) remeasurement of a long-term experiment followed by a multi-scale assessment of how spatial scale of observation, time since treatment, and pre-treatment conditions interact to shape vegetation responses to thinning and prescribed burning. Our review (Component 1) identified 224 studies examining vegetation responses to thinning and/or burning treatments in western North America. Of these, 46% were ‘long-term’ examining responses ≥ 5 years since treatment. Consideration of spatial variation was limited and included multi-scale sampling approaches (8% of studies), comparisons of responses among sites differing in biophysical conditions (e.g., soil type; 17%), measures of variability within treatment units (9%), and analyses of spatially explicit patterning in the overstory or understory (e.g., clump-size distributions; 12%). Very rarely did studies consider responses at both multiple temporal and spatial scales. Additionally, only 33% of long-term studies included both control and pre-treatment data as benchmarks for interpreting ecological responses to treatments. Our analyses of long-term experimental data (Component 2) indicated that species richness was enhanced by burning and, to a lesser extent, by thinning, although the timing, duration, and strengths of these effects varied with spatial and temporal scale, and also varied among components of the understory. Annuals showed an early and persistent increase after burning at the larger scale, but a lagged response at the smaller scale. In contrast, perennial herbs showed lagged responses to thinning at smaller scales and to burning at larger scales, suggesting slower rates of colonization than annuals. Pre-treatment richness was a significant predictor of responses at both spatial scales but had no effect on colonization rate and did not interact with treatment to affect post-treatment richness. Non-natives were unresponsive to treatments, likely because they were uncommon in the landscape. Conclusions about treatment effectiveness can be influenced by basic aspects of the analytical approach, including whether to aggregate data, to treat thinning as a nominal or continuous variable, or to account for pre-treatment variation. Our research has important implications for monitoring the ecological effectiveness of fuel treatments. It underscores the value – but rarity – of long-term multi-scale assessments. Pre-treatment data are critical for interpreting responses to treatments, documenting the range of conditions that are responsive to treatment, and quantifying the magnitude of response. Conclusions about treatment effectiveness can be sensitive to the temporal and spatial scales at which responses are measured. Short-term studies cannot detect effects that emerge slowly, nor can they document the longevity of responses to treatment. As a consequence, they can lead to false conclusions about treatment effectiveness and could trigger interventions that would not be suggested by results of longer-term studies. Multi-scale designs allow characterization of spatial variation and the scale-dependence of ecological responses. Long-term multi-scale assessments are necessary to ensure that fuel treatments provide desired ecosystem services and are resilient to changes in climate and disturbance regime.

Citation

Bakker JD, Halpern CB, Harrod RJ, Urgenson LS, Rossman AK, Peterson DW. 2018. Monitoring Effectiveness of Forest Restoration Treatments: The Importance of Time and Space. Joint Fire Science Project 13-1-04-4. Seattle, WA: University of Washington. 38 p.

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