Experimental Removal of the Non-indigenous Shrub Rhamnus frangula (Glossy Buckthorn): Effects on Native Herbs and Woody Seedlings

Northeastern Naturalist, 2004 by Frappier, Brian, Eckert, Robert T, Lee, Thomas D

Abstract Effects of the non-indigenous shrub Rhamnus frangula L. (glossy buckthorn) on tree recruitment, herb cover, forest floor plant species richness, and R. frangula recruitment were tested in two southeastern New Hampshire Pinus forests using a randomized complete-block field experiment. The treatment, applied in January of 2000, was the presence of well-established R. frangula populations with three levels: R. frangula absent prior to experiment ("uninvaded"), > 90% R. frangula cover ("Rhamnus present"), and removal of > 90% R. frangula cover ("'Rhamnus removed"). After 2 years of measurements, Rhamnus present had significantly lower first-year native tree seedling density than Rhamnus removed and uninvaded plots (0.11, 0.40, and 0.40 seedlings/m^sup 2^ respectively). First-year native tree seedling density in the Rhamnus removed and uninvaded treatments were similar. Neither percent herb cover nor plant species richness were significantly affected by the removal of R. frangula in the two years following treatment. We believe these results indicate that the presence of dense R. frangula inhibits the establishment of tree seedlings. Rhamnus removed plots sampled one year after removal had five-fold greater first-year R. frangula seedling density than the other treatments. However, after two years first-year R. frangula seedling density was similarly low in all treatments (

Introduction

Rhamnus frangula L. (Rhamnaceae), a shrub native to Europe (Gleason and Cronquist 1991), has become a widespread invasive plant in the northeastern United States and adjacent Canada (Catling and Porebski 1994, Converse 1984). Its range will likely continue to expand in North America, becoming abundant in open and semi-open wetlands and some upland woodlands (Catling and Porebski 1994). Rhamnus frangula inhabits a wide range of soil and soil moisture conditions (Converse 1984). The fruit is a small, black berry with 2-3 seeds; production is abundant, ranging between 430 and 1804 fruit per genet in one study (Medan 1994). Besides humans who plant for landscaping purposes, birds are the most likely long-range dispersal vectors, possibly including European Starlings, Grosbeaks, Cedar Waxwings, and American Robins (Catling and Porebski 1994).

Rhamnus frangulu cover was negatively associated with herbaceous cover in Pennsylvania savanna areas of the Allegheny Plateau (Possessky et al. 2000). Frappier et al. (2003) found R. frangula basal area was inversely associated with tree seedling number, percent total herb cover, and ground-level species richness in several southeastern New Hampshire forests. Only a direct experimental manipulation, however, can establish the causality of the observed negative associations between basal area of R. frangula and woody seedling density, herb cover, and forest-floor plant species richness.

Sinclair and Catling (1999) experimentally removed R. frangula cover from Ontario wetland plots and found increased native plant species cover in plots in which the R. frangula had been removed. They also found R. frangula reduced plant species richness (Sinclair and Catling 1999). This contrasts with the findings of Possessky et al. (2000) who observed higher plant species richness under R. frangula cover in savanna areas of Pennsylvania. Different ecosystems appear to respond differently to R. frangula invasion.

In cases where a non-indigenous species occurs in substantial numbers or biomass, the additional effort of performing field experiments to confirm that a non-indigenous species is causing an alteration in community structure or function is fully justified and should be more widely practiced for three reasons. First, a non-indigenous species may be an "add-on" in a community, accruing biomass but hardly altering native species composition and relative abundances (e.g., Anderson 1995). second, control efforts are very costly and may adversely impact native species or encourage the spread of another non-indigenous species. Third, acceptance of the individualistic paradigm of species distributions necessitates that even closely related species may have very different effects on native species, requiring independent study (Westman 1990). However, where the non-indigenous species in question is new to an area or exists in small isolated populations, that species should be eradicated as a precautionary measure against future impacts because small populations are much easier to control than larger populations (Simberloff 1997).

Some managers have tested the efficacy of several control methods on R. frangula, with good immediate results (Post et al. 1989, Reinartz 1997). However, the response of R. frangula seeds in the seed bank to the eradication of overstory R. frangula was not measured in these studies. Mature stands of R. frangula are usually associated with very high fruit production and high numbers of conspecific seedlings (Converse 1984, Godwin 1943). Thus, the potential establishment of high densities of new R. frangula seedlings might require repeated follow-up efforts to assure lasting control. Furthermore, there has been no indication whether the plant community would attain pre-invasion structure after control efforts.

This paper reports the results of a two-year field experiment manipulating the presence of R. frangula in two Pinus strobus L.-P. resinosa Soland. stands in southern New Hampshire. The experiment was designed to test the following null hypotheses: > 90% R. frangula cover does not suppress first-year native tree seedling density; > 90% R. frangula cover does not reduce herb cover; > 90% R. frangula cover does not affect ground level plant species richness; and removal of R. frangula cover does not increase the number of germinating R. frangula seeds.

Methods

A randomized complete-block field experiment with six replicates was performed. The treatment was the presence of well-established R. frangula populations with three levels: uninvaded by R. frangula ("uninvaded"), > 90% cover ("Rhamnus present"), and removal of > 90% R. frangula cover ("Rhamnus removed"). The experiment was performed in and blocked by two forest stands, College Woods (43.1334°N, 70.9425°W) and MacDonald Lot (43.1248°N, 70.9275°W), both woodlands on University of New Hampshire property. College Woods is a 2.5 ha Pinus strobus - P. resinosa stand planted in the 1930s. MacDonald Lot is a 3 ha P. strobus stand approximately 70 years old, that originated in an old field. Except for R. frangula, there was no apparent shrub-sapling layer in the forests studied, perhaps due to forest management activities or canopy suppression.