The process of woody encroachment into graminoid-dominated ecosystems has inspired more than a century of ecological investigation, yet we do not fully understand the rapid shrubification currently occurring in grasslands from the Arctic to Australia. Neither bottom-up control, competition, nor top-down control has emerged globally as the most important factor in woody encroachment. This research attempts to understand the relative importance of these factors in meadows in the temperate rainforest of the Oregon Coast Range. I exposed 200 Pseudotsuga menziesii seedlings to fully-crossed treatments of site, competition, and herbivory. I manipulated site, representing bottom-up controls, by planting 20 seedlings in each of 10 meadows. Five of these meadows were recently created, and five have persisted for decades with little to no human intervention. I manipulated competition by removing all aboveground vegetation within 1m of 10 random seedlings at each site. I manipulated herbivory by protecting 10 random seedlings at each site from browse with cages. Proportional change in height over one year was measured as the response. Results were analyzed with a generalized linear mixed-effects model, considering site as a random effect, competition and herbivory as fixed effects, and dropping insignificant interactions.
Results/Conclusions
Preliminary results indicate that site was the only significant determinant of change in height across all seedlings, suggesting that bottom-up controls outweigh competition and top-down controls in this ecosystem. Top-down effects were difficult to isolate from bottom-up effects, however: the level of browse on unprotected seedlings differed between sites, and multiple seedlings disappeared out from under intact cages, suggesting prevalent gopher herbivory as observed in meadow studies in the Oregon Cascades.