Initial Response of Understory Vegetation to Three Alternative Thinning Treatments

This study compares initial understory vegetation response among three thinning treatments and a control in 30- to 50-year-old even-aged Pseudotsuga menziesii (Mirbel) Franco (Douglas-fir) stands. It was conducted on four sites on the western slope of the central Oregon Cascades. Treatments included a control (no thinning), a light thinning, and two treatments designed to encourage development of understory vegetation: a light thinning with gap creation and a heavy thinning. Vegetation response was measured during the first post-treatment growing season and 5–7 years later. At a treatment-scale, vegetation structure and composition differed between thinned and unthinned stands but varied little among thinning treatments. Thinnings resulted in initial declines of bryophytes, tall shrubs, and low shrubs followed by subsequent recovery and growth. Herbs displayed little initial response, but a release of early-seral species was evident in thinned stands by 5–7 years posttreatment. The addition of gaps resulted in differentiation of plant composition across the gradient from gap center to the thinned forest matrix, but this was only statistically detectable at a within-treatment scale. The causal mechanisms driving initial post-thinning response are discussed to better understand long-term implications and potential roles of thinning in managing for understory vegetation.