Regeneration Strategies of Japanese Barberry (Berberis thunbergii DC.) in Coastal Forests of Maine

Japanese barberry (Berberis thunbergii DC.) has become invasive in forests of the northeast since its U.S. introduction as an ornamental shrub in 1875. This non-native invasive species can occupy a wide range of environmental conditions, has a longer growing season than most native species, multiple methods of reproduction, and forms thickets under which few other plants can persist. Effective control strategies and management of invaded forest stands would be improved by knowledge of how Japanese barberry regenerates in the forest, whether it forms a seed bank, and to what extent it impacts other plant species. This study focused on the following questions: 1) Which species successfully regenerate under a Japanese barberry overstory? 2) How does forest canopy cover affect the regeneration of Japanese barberry and other species? 3) Does a portion of Japanese barberry seeds from previous years remain viable in the soil for more than one growing season? Data were collected from two coastal sites in Maine (Monhegan Island and Wells Research Reserve) that had Japanese barberry thickets under a closed tree canopy. The Japanese barberry overstory was clipped in 1 m radius plots in fall 2004 and spring 2005. At these times and in fall 2005, data were collected in the field, soil samples were taken from the plots for soil incubation studies, and a seedling emergence test was conducted on seed from the study sites which was compared to a commercial source. Japanese barberry seedlings were the most abundant plant group to regenerate under a Japanese barberry canopy with a maximum average 29.3 stems/m2 at Monhegan Island, after which the next most abundant plant group, understory herbs, were 21.4 stems/m2. At Wells Research Reserve, Japanese barberry seedlings had a maximum average of 0.4 stems/m2 and understory herbs had a maximum of 6.8 stems/m2. Only the Monhegan Island plots had sufficient non-barberry species regeneration to perform statistical analyses. Tree and shrub regeneration were too sparse to analyze statistically at either site; 50-80% of plots lacked seedlings. Density of understory herbs was reduced by forest canopy cover (P = 0.003) as was Japanese barberry sexual regeneration (P = 0.003) but not necessarily vegetative sprout density (P = 0.058). Soil incubated in a greenhouse yielded few Japanese barberry seedlings beyond those observed in the field, while a large seed bank existed for other species. A seedling emergence test showed no significant difference between 2004 commercial seed and seed from the study sites (P = 0.218), while mean seedling emergence of both were significantly higher than 2003 commercial seed (P \u3c 0.001). Japanese barberry seed viability declined significantly in the second growing season after seed drop, indicating that Japanese barberry generally germinates the growing season following seed maturation and may not have a viable seed bank beyond that time. The lack of a seed bank will aid management of this species after removal at invaded site