Scotch broom -- an aggressive, leguminous N-fixing shrub – is an invasive species of major concern in the Pacific Northwest. Broom has the potential to alter soil properties and have lasting effects on related soil functions and co-occurring plant species. On a recently harvested Douglas-fir forest on the Olympic Peninsula in Washington, we evaluated the effects of Scotch broom on soil moisture, soil water chemistry (water chemistry from tension lysimeters), and soil temperature. We also recorded the abundances of associated plant species in the presence and absence on broom. Soil at the site is coarse-textured formed from glacial-outwash with low water holding capacity. We expected soil water content to decrease and soil water nutrient cations to increase in the presence of broom. Furthermore, we expected the effects of broom on soil to influence associated plant communities, favoring more drought resistant species. Experimental manipulations of broom (present or absent via planting of container grown broom) and native vegetation (present or absent via annual herbicide treatments) were randomly applied in a 2 x 2 factorial design to four m2plots with six replications per treatment combination. The experiment was established in the spring of 2013 and concluded in the fall of 2016.
The effect of treatments on soil water chemistry and microclimate was variable and inconsistent early on, but the effect of broom on the responses became more apparent with time. This was likely due to broom maturing and occupying more growing space as the study progressed. The results indicate that soil water concentrations of Ca and Mg increased in the presence of broom while N and K concentrations were unaffected. Soil water content under isolated broom plants was equivalent to fully vegetated plots, which was associated with the exclusion of some broadleaved species and increased presence of more drought-adapted exotic grasses. Soil temperature was significantly greater under broom and in the absence of vegetation. These findings imply changes to the historic hydrological and nutrient cycling processes of this forest ecosystem, as well as species abundances. The data suggest that broom is likely assimilating all N it fixes and N availability may not change until the broom is removed. The four-year duration of this study represents a typical duration in working forests before broom is chemically controlled. This study demonstrates the impacts this invasive species can have on a site and how these changes relate to the ontogeny of broom.