INVASIVE EXOTICS The following is an excerpt from Landscape Management & Restoration Manual, in progress, Š 1991 by Andropogon Associates, Ltd. One of the most visible aspects of environmental damage in relic natural areas is the spread of exotic, invasive vegetation. Native forests, oldfields, and meadows alike can be overwhelmed by exotics and end up looking like overgrown vacant lots, if adequate control is not undertaken. Beyond jeopardizing native habitats, invasive exotics often outcompete desirable exotics in horticultural landscapes as well as indigenous species, representing significant maintenance costs. The components of natural habitats found in a region have co-evolved over millennia and produced a natural system of checks and balances. While this does not mean that dramatic change will never occur, the overall vulnerability of a complex community to natural stresses is reduced. The introduction and often widespread dissemination of an alien species, such as Norway maple or Japanese honeysuckle, planted by man into an environment where there are no natural controls or defenses, has been devastating. While it is true that, over time, natural systems can adapt to the presence of a new plant, it is also true that these introductions can decimate extensive areas of native habitat and limit the capacity for recovery in a system already severely hampered by a wide range of other environmental stresses. The diversity and quality of protected natural areas are deteriorating everywhere in the developed corridor along the eastern coast of the United States. Though not all introduced exotic species have become invasive, the success of a few species is more than enough to jeopardize almost every native habitat. When kudzu was in vogue, for example, and considered a cure-all for erosion, over 34 million seedlings were distributed from a single government nursery in Georgia. Today, this plant is a menace, renowned for the unparalleled rate at which it swallows up forests and farms alike. Once thought to be confined to the South, kudzu has begun a slow but effective invasion of the North. Japanese honeysuckle, like kudzu, was once widely perceived as an excellent ground stabilizer. Again, like kudzu, this honeysuckle was planted on a massive scale and was used especially by the railroads to provide quick cover on the steeply sloped embankments. However, with both these vines, although growth is rapid and cover seems complete, the shallow, opportunistic root
Invasive Exotics -- 2
systems do not provide anything close to the level of stabilization of native forest systems. Slumping and soil slippage continue to occur and surface soils are still exposed to erosion beneath the heaping stems. As they mature, new stems heap higher and higher over the old stems and the number of rooting sites diminishes, decreasing the capacity to check erosion. Not only do both these vines fail to stabilize the ground, they have also proved to be largely free from the natural controls of their own native habitat and are now highly invasive. Many of these invasive exotic plants produce abundant berries. Because of these abundant berries, plants such as Russian and autumn olive, the bush and vine honeysuckles and multiflora rose are still frequently proposed as valuable plants for wildlife. This perspective, however, is a very short-sighted one, because with the continued loss of habitat diversity, there will also be the loss of a representative range of native birds and other wildlife. As wildlands throughout the larger region become increasingly more disturbed and fragmented, they become increasingly vulnerable to honeysuckle and other invasive exotics. Today, in the relic wildlands of our urban and suburban areas, it is difficult to find any fragment of habitat that is uncontaminated and in many places nearly all the hedgerows and woodland corridors are thoroughly infested. Equally invasive are the Norway and sycamore maples, which are gradually taking over many forests and forest edges in the Northeast. Because they are trees, and do not have the heaping vine form or dense shrubby form, but look to most people like any other tree in the forest, the evidence of disturbance is often less easily recognized. However, in late fall, when the leaves from native trees have fallen or turned brown, the butter-yellow foliage of the Norway maple, often reveals that a seemingly healthy forest of beech and oak has a continuous understory of Norway maple saplings. When these saplings mature, the woodland will be entirely Norway maple, displacing a host of native species. As with the invasive vines and shrubs, exotic tree species provide poor erosion control, in comparison with a healthy native community, or a well maintained horticultural landscape. These trees have very large, thick, dark leaves which emerge early in the spring and typically fall off the tree long after those of the native maples, creating a deep shade that severely inhibits ground layer vegetation. More problematic is the fact that Norway and sycamore maples are strongly allelopathic; that is, they suppress the growth of other plant species due to the release of toxic substances in the soil. As a result, once Norway or sycamore maple is established, there is no reproduction by other species. Because almost no other species can coexist with Norway and sycamore maple, beneath the tree canopy the ground is bare and often subject to erosion. Without the additional protection of an herbaceous layer and/ or small trees and shrubs, tree roots alone cannot provide adequate soil stabilization. Like many other disturbance species, government agencies, conservation groups and enthusiastic horticulturalists have been responsible for the widespread dissemination of the Norway maple - its vigor in our landscape is part of what has made it such a
Invasive Exotics -- 3
popular tree. Until recently, more than half of the trees recommended for use on city streets by Philadelphia's Fairmount Park Commission were varieties of Norway maple. Typically, a nursery will have more varieties of Norway maple than all other maples combined. Philadelphia has stopped recommending this tree and a growing number of townships discourage its use, but demand for the plant is still large and it continues to be widely cultivated and planted. There are, at present, no effective natural controls for these invasive exotics. Even though Norway maple, for example, is subject to numerous diseases and pests which create problems in horticultural settings, none of these has yet checked its spread in the wild. Over time, it is inevitable that some organism will take advantage of so widespread a host but, in the meantime, extensive areas of native habitat are being decimated and their recovery is severely hampered by a wide range of other environmental stresses. There are only a few other exotic species which appear to pose such a threat to native plant populations. Japanese knotweed, for example, is taking over our woodland streambanks, and purple loosestrife, escaped from garden cultivation, has overwhelmed wetlands from New York to Minnesota. With purple loosestrife, the spread is so rapid that a single plant is considered an infestation. Even today, new exotic plants are still being promoted as "cure-ails" exactly because they spread so rapidly. What makes a plant an exotic invader, instead of a simple garden escapee? It is all a question of degree and context; how fast, how widespread, and most importantly to what extent does it displace native habitats and change the structure of the ecosystem. Appropriate evaluation of exotics requires careful observation of their behavior throughout the region. It is critical to identify those plants which are not only reproducing rapidly in highly disturbed sites, such as vacant lots, but are also making significant inroads in less disturbed areas, such suburban parklands. The most conspicuous trait of a dangerous invasive is that it displaces whole communities, not just a few species. Looking at wildlands in the Northeastern corridor, for example, most people have observed the extensive areas of honeysuckle, knotweed and Norway maple. There is already more than enough evidence to alert anyone involved in the management of native ecosystems to the seriousness of the threat these plants pose. It is clear that a very large effort is required to repair the damage already done. The future of our native ecosystems may depend on our taking immediate action. In 1977 Jimmy Carter signed The Exotic Organisms Executive Order 11987, which give broad authority to federal and other agencies to control the introduction and spread of exotic invasive species.
It is strongly recommended that no species which has demonstrated itself to be a successful invader at the expense of native habitats in the region, or is even suspected of being a pest, be planted at all. This policy is a very conservative one, but the consequences of being too optimistic may be very costly to remnant native plant
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communities. Urban and suburban wildlands, already severely stressed by abuse and overuse, change of water regimen, soil disturbance, and disease and animal pests are deteriorating rapidly, and it is important not to compound these problems, but to take actions which contribute to recovery of health. While a ban may seem extreme, effective control means that as many existing individuals as possible must be eliminated, and no new ones planted. Many of these species are dispersed by birds and animals and spread very quickly over large distances. All of these plant species are extremely difficult to eradicate once established, and none are so critical to wildlife or to landscape character that they cannot be replaced by other, less destructive, species. An informative article on the real costs of invasives is entitled, "Exotics & their Ecological Ramifications" (1987) Harty, F.M., Natural Areas TournaI, Vol. 6, No. 4:20-26. It is strongly recommended that the following species be banned from being planted in the Northeast:
Trees: Norway maple Sycamore maple Russian olive Autumn olive White mulberry White cottonwood
(Acer platanoides) (Acer pseudoplatanus) (E1eagnus angustifolia) (E1eagnus umbellatus) (Morus alba) (Popul us alba)
Shrubs & Small Trees: Barberry Winged euonymous Amur honeysuckle Tartarian honeysuckle Blunt-leaved privet Smooth buckthorn Shining buckthorn Multiflora rose
(Berberis japonica) (Euonymous alatus) (Lonicera maackii) (Lonicera tatarica) (Ligustrum obtusifolium) (Rhamnus cathartica) (Rhamnus frangula) (Rosa multiflora)
Vines: Porcelain berry Oriental bittersweet Japanese honeysuckle Kudzu Japanese wisteria
(Ampelopsis brevipedunculata) (Celastrus orbiculata) (Lonicera japonica) (Pueraria lobata) (Wisteria floribunda)
Herbaceous Plants: Purple loosestrife Japanese knotweed
(Lythrum salicaria) (Polygonum cuspidatum)
I nvasive Exotics -- 2
It is clear that a very large effort is required to repair the damage already done.The future of our native ecosystems may depend on our taking immediate action. In 1977 Jimmy Carter signed The Exotic Organisms Executive Order 11987, which give broad authority to federal and other agencies to control the introduction and spread of exotic invasive species.
It is strongly recommended that no species which has demonstrated itself to be a successful invader at the expense of native habitats in the region, or is even suspected of being a pest, be planted at all. This policy is a very conservative one, but the consequences of being too optimistic may be very costly to remnant native plant communities. Urban and suburban wildlands, already severely stressed by abuse and overuse, change of water regimen, soil disturbance, and disease and animal pests are deteriorating rapidly, and it is important not to compound these problems, but to take actions which contribute to recovery of health. It is strongly recommended that the following species be banned from being planted in the Northeast: ~:
Norway maple Sycamore maple Russian olive Autumn olive White mulberry Amur cork tree White cottonwood
(Acer platanoides) (Acer pseudoplatanus) (Eleagnus angustifolia) (Eleagnus umbellatus) (Morus alba,) (Phellodendron amurense) (Populus alba)
Shrubs & Small Trees: Barberry Winged euonymous Amur honeysuckle Tartarian honeysuckle Multiflora rose
(Berberis ia,ponica) (Euonymous alatus) (Lonicera maackij) (Lonicera, ta,tarica,) (Rosa multiflora)
Vines: Porcelain berry Oriental bittersweet Japanese honeysuckle Kudzu Japanese wisteria
(Ampelopsis brevipedunculata) (Celastrus orbiculata) (Lonicera japonica,) (Pueraria, lobata,) (Wisteria floribunda)
Herbaceous Plants: Purple loosestrife Japanese knotweed
(Lythrum salicaria) (Polygonum cuspidatum)
INVASIVE EXOTICS The following is an excerpt from Landscape Management & Restoration Manual, in progress, Š 1991 by Andropogon Associates, Ltd. One of the most visible aspects of environmental damage in relic natural areas is the spread of exotic, invasive vegetation, which outcompetes and replaces native plant communities. Though not all introduced exotic species have become invasive, the success of a few species is more than enough to jeopardize almost every native habitat. Because many of these invasive exotic plants produce abundant berries, plants such as Russian and autumn olive, the bush and vine honeysuckles and multiflora rose are still frequently proposed as valuable plants for wildlife. This perspective, however, is a very short-sighted one, because with the continued loss of habitat diversity, there will also be the loss of a representative range of native birds and other wildlife. Equally invasive are the Norway and sycamore maples, which are gradually taking over many forests and forest edges in the Northeast. Because they are trees, and do not have the heaping vine form or dense shrubby form, but look to most people like any other tree in the forest, the evidence of disturbance is often less easily recognized. However, in late fall, when the leaves from native trees have fallen or turned brown, the butter-yellow foliage of the Norway maple, often reveals that a seemingly healthy forest of beech and oak has a continuous understory of Norway maple saplings. When these saplings mature, the woodland will be entirely Norway maple, replacing a host of native species. These trees have very large, thick, dark leaves which emerge early in the spring and typically fall off the tree long after those of the native maples, creating a deep shade that severely inhibits ground layer vegetation. More problematic is the fact that Norway and sycamore maples are strongly allelopathic; that is, they suppress the growth of other plant species due to the release of toxic substances in the soil. As a result, once Norway or sycamore maple is established, there is no reproduction by other species. Because almost no other species can coexist with Norway and sycamore maple, beneath the tree canopy the ground is bare and often subject to erosion. As is the case with many other disturbance species, government agencies, conservation groups and enthusiastic horticulturalists have been responsible for the widespread dissemination of the Norway maple. Until recently, for example, more than half of the trees recommended for use on city streets by Philadelphia's Fairmount Park Commission were varieties of Norway maple. Typically, a nursery will have more varieties of Norway maple than all other maples combined. Philadelphia has stopped recommending this tree and a growing number of townships discourage its use, but demand for the plant is still large and it continues to be widely cultivated and planted.