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Recent work on exotic species in island ecosystems has revealed that many exotic woody plants are capable of dominating forests in which they occur, substantially altering forest structure and nutrient cycling. In mainland forests, however, few empirical examples of exotic dominance exist. The invasive shrub Rhamnus cathartica L. (common buckthorn) is reported to infest temperate forest understories in North America and displace native species, but its degree of dominance has been described only anecdotally. We investigated the extent to which common buckthorn can dominate forest ecosystems, and found strong evidence for monotypic dominance in several mesic and wet sites in southern Wisconsin. Among eight forest sites where common buckthorn was dominant, its mean relative density and basal area was 81% and 45%, respectively. Compared to eight native-dominated sites on similar soils, common buckthorn dominance fundamentally altered forest structure: total woody stem density at Rhamnus-dominated sites was more than twice that of native-dominated sites (two-way ANOVA; P < 0,05, n = 16), but total basal area did not differ significantly (P > 0.3). When considering dominance by size class within only the eight Rhamnus-dominated sites, common buckthorn genets were more abundant than native genets at 5-cm size classes up to and including 20-25 cm diameter at breast height, evidence that common buckthorn dominance can extend well beyond understory size classes. Within Rhamnus-dominated sites, mean relative density and basal area for common buckthorn exceed that reported for four other woody invaders found in the northeastern US, and thus we suggest that common buckthorn is a particularly successful invasive species in eastern temperate deciduous forests of America and is capable of acting as an ecosystem dominant. [ABSTRACT FROM AUTHOR]

Lianas (woody vines) are an important and dynamic component of many forests throughout the world, and increases in CO2, mean winter temperature, and forest fragmentation may promote their growth and proliferation in temperate forests. In this study, we used a 45-year data set to test the hypothesis that lianas have increased in abundance and basal area in the interiors of 14 deciduous temperate forests in Wisconsin (USA) since 1959. We also censused woody plants along a gradient from the forest edge to the interior in seven of these forests to test the hypothesis that the abundance of lianas declines significantly with increasing distance from the forest edge. We found that lianas did not increase in abundance within the interiors of temperate forests in Wisconsin over the last 45 years. However, relative and absolute liana abundance decreased sharply with increasing distance from forest edges. Our findings suggest that forest fragmentation, not climate change, may be increasing the abundance of lianas in northern deciduous temperate forests, and that lianas may further increase in abundance if the severity of forest fragmentation intensifies.