Species interactions during succession in the western Cascade Range of Oregon, 1990 to present

The factors that contribute to plant species establishment and decline following disturbance determine the rates and patterns of successional change of a system. In this long-term field experiment, we test the commonly held assumption that competition for space or resources by dominant species determines the outcome of succession. Specifically, we examine the population- and community-level consequences of removing one or more potentially dominant species from the post-disturbance community after clearcut logging and burning of a mature/old-growth Douglas-fir forest. Experimental treatments include: (1) removal of early-seral annual, Senecio sylvaticus, or perennial, Epilobium angustifolium—or both—to test the influences of these early-seral dominants on subsequent community development; (2) removal of all species except Senecio or Epilobium, to test whether the decline of these early-seral dominants is driven by competitive displacement; or (3) removal of shade-tolerant forest species that dominate subsequent stages of succession—Rubus ursinus or Berberis nervosa plus Gaultheria shallon—to test the influences of these long-lived perennials on understory development. The experiment is a randomized block design comprising eight removal treatments plus a control replicated in each of 25 blocks. Removal (reduction in competition) is achieved by removing seedlings or vegetative stems annually from a treatment area of 2.5 x 2.5 m. Sample plots (1 x 1 m) centered within these are used to estimate cover of all vascular plant species and, for the first 8 yr of the experiment, stem density and height, facilitating estimates of above-ground biomass. Pre-harvest data were collected in 1990, logging/burning occurred in 1991, and removal treatments and post-treatment sampling were initiated in 1992. Six of the nine experimental treatments were terminated between 1996 and 1998, with loss of early-seral Senecio and Epilobium from the system. The remaining three treatments (removal of Rubus, removal of Berberis plus Gaultheria, and the control) have been sampled annually since 1992. All regeneration of trees within the experimental area has occurred naturally. In September 2018, the study site was subjected to a controlled burn to limit spread of the Terwilliger Fire from the north. In 2019, plots were assessed for burn severity. Species' removals were terminated, but cover measurements continue in the plots.