Can mechanical thinning of forests provide the same ecosystem benefits as fire?

Jordan D Maxwell, Samuel B. St. Clair and Noelle V. Zenger, Plant and Wildlife Sciences, Brigham Young University, Provo, UT, Stanley G. Kitchen, Rocky Mountain Research Station, U.S. Forest Service, Provo, UT, Jason Kling, USDA Forest Service, Richfield, UT

Background/Question/Methods
Fire and herbivory are disturbances that strongly influence plant community composition and succession; however, modified disturbance regimes related to human activities are changing the timing, extent, and severity of these disturbances. Here we empirically test the effect of large-scale (700 ha) prescribed fire and mechanical thinning treatments on the competitive balance of mixed aspen-conifer forests that have experienced fire suppression and excessive ungulate herbivory pressure.
Results/Conclusions
We found that untreated aspen-conifer stands were not recruiting aspen for overstory replacement due to high conifer abundance and chronic ungulate herbivory. Prescribed fire released aspen from conifer competition and increased stand tolerance and resistance to ungulate herbivory by stimulating synchronous aspen regeneration, growth, and defense chemistry expression. Mechanical treatments also stimulated a strong regeneration response, but this benefit was counteracted by slow growth and increased ungulate activity. Specifically, deer and cattle were attracted to mechanically thinned stands which increased herbivory pressure and resulted in growth trends indicative of forest recruitment failure. These results suggest that: 1) where possible, fire be given high priority in late-succession aspen-conifer forest stands, 2) that treatments be sufficiently large for the aspen regeneration response to saturate ungulate herbivory; and 3) if mechanical treatments are used where ungulate herbivory may pose a threat to forest recruitment, then project plans must include resources for monitoring and mitigating herbivory impacts.