Cytisus scoparius

In the recently invaded stands, there were no significant differences (P < 0.05) in species richness between invaded and uninvaded areas of both sites (Fig. 1a, b). However, native species cover declined significantly within the invaded area at R2 (P < 0.05)

Several environmental variables were correlated with vegetation changes including percent cover of C. scoparius, percent light availability, percent cover of litter, litter quantity, and percent bare ground.

Native species richness and cover dramatically declined as the duration of invasion increased with the most significant losses occurring in the long-term invaded areas (Fig. 1). In the mature invasive stands, there was a significantly lower species richness and lower vegetation cover in the invaded areas in comparison to the edge and native vegetation (P < 0.05; Fig. 1c). Changes due to invasion were associated with a dramatic loss of native species or a reduction in their abundance. No ‘new species’ were evident under invaded stands. Forbs were most affected by the establishment of C. scoparius, although all growth forms responded negatively.

Dense canopy shading and an increasingly dense, homogeneous litter layer in the understorey as a result of C. scoparius were strong environmental drivers of vegetation change

[Euphydryas editha taylori] Taylor’s checkerspot butterflies were documented at the Bald Hills through 2007, but there have been no detections since, despite regular and thorough surveying from 2001 through 2011 (Potter 2011, p. 3). This number has declined substantially in recent years as habitat has become increasingly shaded and modified by encroaching trees, nonnative grasses, and the invasive, nonnative shrub Scot’s broom (Cytisus scoparius). Potter (2010, p. 1) reported multiple site visits to conduct redundant surveys in formerly occupied bald habitat during the 2008–2010 flight period with no Taylor’s checkerspot butterflies observed. The subspecies is presumed to be extirpated from this location. The spread of non-native plants has reduced, degraded, or eliminated habitat for the Oregon silverspot butterfly [Speyeria zerene hippolyta] at many sites. Most notable of the non-native shrubs is Scotch or Scots broom (Cytisus scoparius).