Department of Ecology and Evolutionary Biology

The pest plant Scotch broom (Cytisus scoparius) is hindering effective reforestation at Fort Lewis, resulting in both a loss of land available for military training as well as a loss of native forest habitat for native plants and animals. Our primary objective was to examine the relative effectiveness of different broom control strategies that are appropriate for the forestry context, where the use of fire is not permitted. This document reports on the first year’s activities of a large research collaboration between personnel at the University of California Santa Cruz and at Fort Lewis. Key research questions include: What are the costs and benefits of an extended preliminary treatment phase, including two or even three years of soil scarification and control of small broom plants before planting? What is the relative effectiveness of chemical and mechanical treatment?

We set up a large-scale experiment (6.3ha total treatment area) at 5 sites, with a blocked design of 4 blocks per site with 6 plots per block. The plots were 56’x56’, designed to accommodate 49 tree seedlings (7x7) at 8’ spacing.  Scotch broom plants were removed from the entire site in fall of 2007 before the start of the experiment. We quantified Scotch broom stump density, mean diameter and height in each plot, as well as stump resprout rate and number of seedlings in spring of 2008. Initial stump density and size varied across sites. The resprout rate was low in four of the five sites, averaging 4 - 13% of stumps, but one site showed a very high resprout rate of 36%. Average seedling density, representing germination from the seedbank, varied across sites from <5 seedlings/m²to >100 seedlings/m². Most of the variation in seedling number was among sites (56%), followed by small-scale variation from quadrat to quadrat (30%), and plot to plot (12%). Almost no variation was seen across blocks within sites. We planted 7,448 two-year-old Douglas fir seedlings in March 2008. Over half of all Douglas fir seedlings died between March and September in almost all blocks, with close to 100% of the trees dying in one site, and over 80% of trees dying in some of the blocks of all sites. This was an unexpectedly high mortality rate, substantially higher mortality in these invaded clearcuts than mortality seen in other reforestation projects in smaller forest clearings on Fort Lewis.