Effects of Invasive Scotch Broom on Soil Water Chemistry and the Soil Microenvironment.

Scotch broom is an N-fixing invasive species of major concern in Douglas-fir forests of the Pacific Northwest that has potential to alter soil properties and have lasting effects on related soil functions.  We evaluated the effects of Scotch broom on soil water chemistry, soil water content (SWC), and soil temperature at a recently harvested site on the Olympic Peninsula, Washington. Soil at the site developed from glacial outwash and is coarse textured. Experimental manipulations of broom (either present via planting or absent) and native vegetation (either present or absent via chemical control) were applied in a 2 x 2 factorial combination to four m² plots in the spring of 2013.  After two years, the broom-present/vegetation-absent treatment had significantly higher concentrations of calcium and magnesium in soil solution compared to the other treatments, but there was no effect on total nitrogen or potassium. SWC was significantly higher when native vegetation was absent throughout the study period. The effects of broom on SWC were limited until the third year when SWC in plots with broom-present / vegetation-present was similar to those with broom-present/vegetation-absent during a portion of the growing season. By the third year, the average maximum soil temperature was significantly lower in plots with vegetation present than plots with no vegetation present.  The results indicate that the presence of broom does modify nutrient cycling and SWC compared to native vegetation alone, with the effects of broom becoming more pronounced with time.