Negative effects on native plant populations are often attributed to invasions by exotic plants, but experimental evidence is lacking to support many of these claims. Lonicera maackii, an exotic shrub with long leaf phenology, has become naturalized throughout the eastern United States. This study investigated the effects of L. maackii on demography of Galium aparine, Impatiens pallida and Pilea pumila, native annual herbs in differing phenological categories. These interactions were examined in two Ohio forest stands. One stand has a history of logging, burning and grazing and a higher L. maackii density, whereas the other stand has little anthropogenic disturbance and a lower L. maackii density. Three types of experimental plots were established: L. maackii removal, L. maackii present and, at the less disturbed stand, L. maackii absent. Seedlings of the annuals were transplanted and monitored for 1 y for survival to reproductive age and fecundity.
In the more disturbed stand, survival of Galium aparine and Impatiens pallida and fecundity of all three species were significantly greater in the removal treatment than where Lonicera maackii was present. In the less disturbed stand there was no treatment effect on survival, but fecundity of all annuals was greater in the removal treatment than where L. maackii was present. Also, fecundity of I. pallida and Pilea pumila was greater where L. maackii was absent than where it was present. At both sites fitness (estimated as the product of survival and fecundity) was highest for each species in the removal treatment and lowest where L. maackii was present.
These results demonstrate direct effects of the invasive shrub Lonicera maackii on populations of annuals. They suggest that other annuals, particularly those that are shade-intolerant or photosynthesize only in the early spring, will decline in the presence of shrubs with early leaf expansion.