Effects of Invasive Shrubs on the Herbaceous Community in a Mixed Forest

Non-native, invasive species have the potential to overtake natural communities by directly competing with native species for resources such as light, space, and nutrients. Consequently, invasive species dominance may lead to changes in ecosystem structure and function (Heneghan et al. 2004). Invasive species have also been shown to adversely affect soil variables such as moisture, soil pH, and carbon and nitrogen cycling (Collier et al. 2002). Such invasions can lead to biotic homogenization (Rooney et al. 2002) and potential economic losses (Pimentel et al. 2005). Studies have indicated a number of associated effects, including altered community structure, hydrology, and fire regimes (see Levine et al. 2003 for a review).

In forests of eastern North America, shrubs that have become invasive after introduction have demonstrated capacity to alter ecosystem dynamics (Ehrenfeld et al. 2001, Henegan et al. 2004, 2006). Among the studied effects of invasive shrubs such as Lonicera species (honeysuckles) are altered patterns of litter decomposition and nitrogen loss (Ashton et al. 2005) and effects on soil chemistry and water availability (McEwan et al. 2012). Similar effects have been seen in soils of sites with Berberis thunbergii (Japanese barberry) invasions (Silander and Klepeis 1999, Ehrenfeld et al. 2001).

In addition, the effects of invasive shrubs on forest community composition is apparent upon visual assessment of a heavily-invaded site. Invasive shrubs tend to be denser, develop foliage earlier, and retain foliage longer than their native counterparts, shading out low-growing herbaceous plants (Gould and Gorchov 1999). Notably, the diversity of the native herbaceous community (richness and abundance) can decline (Collier et al. 2002), and fecundity of native herbs can decrease (Miller and Gorchov 2004). While it is known that invasion of non-native shrubs alters the understory herbaceous community, the extent and long-term effects of such changes are understudied and unclear.

Management of invasive shrubs frequently involves mechanical removal of woody stems and chemical treatment with herbicides to prevent regrowth. However, it is not clear that removal is enough to restore ecosystem integrity. Preliminary data indicate that native herbaceous communities may not "bounce back" when the invasive shrubs are removed (Silander and Klepeis 1999). It is likely that effects of invasive shrubs on the herbaceous community may be long-term, but more information is needed. Here we use several metrics, including species richness, diversity, and mean coefficient of conservatism (or modified Floristic Quality Index), to assess effects of invasive shrubs and invasive shrub removal on the herbaceous community. When invasive shrubs are removed from a landscape we expect that the understory community will show an increase in diversity and re-establishment of native herbaceous perennials.

For this study, we chose sites in a mixed forest in Two Rivers, Wisconsin, USA under the management of Woodland Dunes Nature Center and Preserve. Three types of sites were identified in the preserve: a site that had not been invaded by non-native shrubs (non-invaded) which served as the control; one that had been invaded by the non-native shrubs B. thunbergii, Lonicera tatarica, and L. morrowii (invaded); and one that was previously invaded but had been managed to remove non-native shrubs within the past five years (treated; J. Knickelbine, Woodland Dunes Nature Center and Preserve, pers. comm.). All sites are part of a forest complex of dunes and swales from old lake beds and thus have minimal slope, similar hydrology, and are located in a similar forest environment with comparable dominant canopy species (Acer rubrum-Betula alleghaniensis-Thuja occidentalis). The sites have similar land-use history aside from invasive species management in treated sites. Five 20 m × 20 m plots were established in each type of site studied (non-invaded, invaded, and treated).

We surveyed the sites in the spring and summer 2014 and compiled a list of all herbaceous plants to calculate species richness (R), diversity (Shannon-Wiener H′), and the mean Wisconsin Coefficient of Conservatism (mean CC) as a proxy for floristic quality (Bourdaghs et al. 2006). We assigned a cover class for each species using methods described by Peet et al. (1998) in which...