Removal of invasive Scotch broom increases its negative effects on soil and plant communities

Abstract

Scotch broom is an aggressive invasive species of major concern in coast Douglas-fir forests of the Pacific Northwest USA. Control efforts are common, but potential for ecosystem recovery following Scotch broom removal is unclear. We assessed the potential for ecosystem recovery following broom removal at two sites that contrasted strongly in soil quality (i.e., texture and nutrient pool size) in western Washington and Oregon. Comparisons were made among replicated plots where Scotch broom was never present (uninvaded), retained, or removed. Microclimate (photosynthetically active radiation (PAR), soil temperature and moisture), soil properties, and vegetation were monitored during 2013 to 2017. Scotch broom removal increased PAR and soil temperature at both sites but had limited effects on soil moisture. Concentrations of Ca, Mg, K, and P were significantly lower with Scotch broom removal compared to the uninvaded and retained treatments, with the effect being most pronounced at the low-quality site. NMS ordinations indicated that the treatments differed in vegetation composition, with limited evidence for recovery in the removal treatment. Nonnative and native species varied inversely in their abundance responses, where nonnative species abundance was greatest in the removal treatment, intermediate in the retained treatment, and lowest in the uninvaded treatment, indicating occurrence of a secondary invasion following removal. As with the soil response, effects were more pronounced at the low-quality site. Our findings indicate that Scotch broom removal exacerbates negative effects on soil and plant communities, with little evidence of ecosystem recovery over our study period. These findings highlight the importance of controlling Scotch broom invasions immediately after the species establishes, especially at low-quality sites that are more susceptible to Scotch broom invasion and negative legacy effects.