PS 29-152 - Fire and Mechanical Treatments Differentially Affect Aspen Recruitment Under High Herbivory Pressure

Abstract: Disturbances such as fire and herbivory strongly influence plant community composition and succession; however, modified disturbance regimes related to human activities change the timing, extent, and severity of these disturbances. Here we empirically test the effect of large-scale prescribed fire and mechanical thinning treatments at a landscape scale on the competitive balance of mixed aspen-conifer forests that have experienced fire suppression and excessive ungulate herbivory pressure that has resulted in forest regeneration failure. A total of thirty-nine randomly placed 100 x 2 meter transects were placed in the study landscape including fifteen transects in fire treated forests, sixteen transects in mechanically treated forests, and eight transects in untreated forests. All measurements were taken two- and three-years post treatment. Each aspen sucker along a transect was measured for height and inspected for number of meristems browsed. Ungulate fecal groupings within the transect were identified to species and counted to determine ungulate presence in the area. Overstory tree density was characterized using the point quarter method every 10 meters. 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 slower 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 ( >40% browse). 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.