Habitat Characteristics of Spiraea virginiana Britton, a Federally Threatened Riparian Shrub, in North Carolina

We studied the habitat characteristics of Spiraea virginiana Britton (Virginia spiraea), a federally threatened riparian shrub, along eight rivers in western North Carolina. Comparisons between plots with and without S. virginiana revealed that S. virginiana plots were on steep, south-facing slopes and had a higher percentage of large substrate, lower herbaceous and vine cover, higher non–S. virginiana shrub density, lower tree influence, and higher visible sky than control plots. When relating these habitat attributes to the presence/absence of S. virginiana using conditional logistic regression, only substrate size and non–S. virginiana shrub density had significant effects on S. virginiana presence. Principal components analysis (PCA) of all plots (S. virginiana and control) found S. virginiana plots separating from controls that had higher vine and herb cover and lower non–S. virginiana shrub density and slope. Spiraea virginiana plots at the Cheoah River separated from other sites by having greater substrate size and tree influence, and lower visible sky and herb cover. Regression analysis between S. virginiana volume and PCA axes (of plots with S. virginiana only) indicated that volume increased with increasing small substrate, visible sky and non–S. virginiana shrub density, but decreased with increasing tree influence, herb and vine cover, and decreasing slope. Spiraea virginiana restoration efforts should focus on habitats with the attributes described in this study to favor its presence. To ensure S. virginiana vigor, competition from trees, non–S. virginiana shrubs, vines and herbs, especially aggressive nonnative species, should be reduced.