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

The introduction, establishment, and spread of all three exotic species is associated with a habitat disturbed by man, and each of these species is in turn bringing about major changes in the island ecosystem.

Replicated experiments verified the surveys which show that light is a strong limiting factor for Lonicera japonica. Depending upon circumstances, both herbs and woody plant reproduction are suppressed. However, even in the mildly disturbed forest, tree reproduction is suppressed. Ulmus americana, Prunus serotina, Liriodendron tulipifera, which are among the important dominant trees of the island, are more sensitive to Lonicera japonica presence than other trees. Lonicera, once it is established, creates its own sunny habitat by changing the vertical structure of the forest both through suppression of reproduction and strangling and shading out vegetation up to the size of small trees. The cutting down of trees further promotes this exotic vine as does the fungus Ceratostomella ulmi when it produces dying Ulmus americana trees.

Habitat of Lonicera japonica (Japanese honeysuckle) in a natural understory. Vine climbing a small tree.

Replicated experiments and surveys also show that on the island upland no single factor is more important than light for mature Hedera helix growth. Hedera, however, does not respond as quickly in growth to increased light as L. japonica does. This may be because H. helix is more of a climax species, which in turn may be related to its forming chlorophyll A slightly faster than leaf biomass as a whole. A high water table appears to be a limiting factor for Hedera on the flood plain. Regardless of habitat, H. helix prevents the reproduction of herbs, especially on the island upland. On the flood plain, the destruction of herbs by this exotic vine is about the same as that of a hurricane-force flood. The vine also kills trees in the overstory and understory by shading them out. Although the opening left by falling overstory trees increases the growth of H. helix, a significantly greater increase is brought about by cutting elm (Ulmus americana) trees rather than by cutting other canopy trees.

Apparently, Lonicera japonica has produced a peculiar structural change (high correlation of one vegetational layer with another) that is conducive to invasion by Hedera helix, and between the two of these vines, all the forests on the island, except the swamp, are being killed. Hedera is outcompeting Lonicera, and it is projected that it will dominate the vine community.

It is in error to suppose that light relations are preserved in a forest as long as the canopy trees remain. The removal of vegetation in the lower layers disturbs the light relations sufficiently to encourage these exotic vines.

The factor that most nearly approaches a limiting factor for Iris pseudacorus in the marshes of the island is the time length of water inundation. The longer the inundation, the more poorly it does. A second important factor is the presence of Acorus calamus in the swamp-marsh transition. This species is able to take over Iris areas near the tree line. When the water level drops during the growing season and trash is deposited in the marsh, conditions are favorable to I. pseudacorus which begins replacing Peltandra virginica, an important food for the Wood Duck. As the Iris grows, it apparently creates habitat favorable to itself and in the swamp-marsh transition its presence speeds up succession from marsh to swamp in an abnormal manner, with the end result being little or no marsh. The present rising water level in the river during the growing season is slowing the succession down, but it will not entirely eliminate the Iris.