The Potential Impact of An Introduced Shrub on Native Plant Diversity and Forest Regeneration

Over a period of 20 years, Chinese privet, Ligustrum sinense, invaded a mixed hardwood forest in western North Carolina, USA. The invasion penetrated about 30m under the canopy trees, providing 100% cover of the forest floor. Under the privets and in a nearby privet-poor reference area we marked off twenty, one square meter plots with string. All plants in each square meter of both areas were tallied in the spring of 1999 and young trees less than 1m high were again counted in September. We removed privets of the invaded area in November and again tallied all plants in both areas the following spring of 2000. In the spring of 1999, the mean number of herb species per square meter under the privet was 41%, and stem counts 75% less than in the reference area. 42% of herb species found in the reference area were missing under the privet. After removing the privets in the fall of 1999, the number of both native species and stems increased in the privet area the following spring. Plots of density of two native plants against privet density showed both native plants decreasing under increasing privet cover. In the spring of 1999, there were 4 species and 274 stems of tree seedlings in the privet area. In September of that year, we found only one small American holly tree, a highly shade-tolerant species, remaining under the privet. Our data support the thesis that Chinese privet can severely reduce herbaceous species and almost completely suppress tree regeneration in a mixed hardwood forest.