NATIONAL PARK SERVICE The Impact of Three Exotic Plant Species on a Potomac Island

Three exotic plant species were studied. Lonicera japonica. Thunb. (Japanese honeysuckle) and Hedera helix, L. (English ivy) are destroying the forests (except the swamp) of this low lying island in Washington, D.C. The marsh on this Potomac River island is being changed by Iris pseudacorus, L. (European yellow iris). Besides studying impact, limiting factors were also determined.

On the island upland L. japonica growth increases with light intensity. When shade is not a factor that limits this evergreen vine, it overwhelms and kills small trees and shrubs, and it inhibits reproduction, especially of the following trees which are among the overstory dominants on the upland of the island: Ulmus americana, L., Prunus serotina, Ehrh., and Liriodendron tulipifera, L.

On the upland there is no other factor that is stronger than light for limiting H. helix growth, but it is not as strong a limiting factor for this species as it is for Lonicera. On the flood plain, growth and distribution are limited by water table height. In both habitats, Hedera suppresses the growth of native herbs. This evergreen tendril liana climbs overstory and understory trees as well as small trees and is able to shade and kill them.

Marsh that would otherwise be occupied by Peltandra virginica, (L.) Schott & Endl. is taken over by I. pseudacorus. In the swamp—marsh transition Acorus calamus, L. takes over Iris areas. The length of time Iris is inundated by water comes closest to a factor limiting its growth. The shorter the inundation, the greater is the growth rate.

Marsh and adjacent forest on Theodore Roosevelt Island.