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A non-native or exotic species of plant or animal is one that was introduced, either intentionally or unintentionally, by human endeavor into a locality where it previously did not occur (SER 1994). Most introduced plant species form an important part of our environment, contributing immensely to agriculture, horticulture, landscaping, and soil stabilization. But among the thousands of plant species introduced to our area, approximately 10 percent have displayed unexpected aggressive growth tendencies, resulting in real threats to native ecosystems (Blossey 1997). These invasive non-indigenous plants typically exhibit the following characteristics (Miller 1994):

• highly successful seed dispersal, germination, and colonization;
• rapid growth and maturity;
• prolific seed production;
• rampant spread;
• ability to out-compete native species; and
• high cost to remove and control.

Invasive plants thrive on disturbed sites such as Millbrook Marsh, where the floodplain is repeatedly scoured and eroded. These species have left behind the natural controls (usually insects) that kept them in check in their native habitats in favor of the somewhat compromised wetland and riparian environments evident in Millbrook Marsh. Some non-native plants, such as the common dandelion (Taraxacum officinale), have over time become integrated into the natural floristic pattern of the area. These can be considered "naturalized" and, although newcomers to an ecosystem with a long natural history, can be tolerated and even appreciated in an urbanizing watershed such as Spring Creek. Other plant species are of greater concern, for they’ve proven that they can out-compete and displace indigenous vegetation. None is more evident on site than honeysuckle (Lonicera spp4. As with most other species successful in invading disturbed ecosystems, dense thickets of Lonicera may modify ecosystem structure and functions to their exclusive advantage (Luken et al., 1997). Following is an overview of existing and potential invasive species of primary concern.

Existing Invasive Species. Honeysuckle (Lonicera spp.), was introduced to the eastern U.S. in the late 1 890s as a horticultural shrub and for wildlife habitat improvement. It is present in an intermittent ring around Millbrook Marsh, generally extending back from the outer edge of the floodplain (Map 6). While it has clearly associated with areas of disturbed soil and fill situations, a comparison of 1948 vs. 1994 aerial photographs (Photo 2) shows a marked advancement of thicket formation along the riparian corridors, particularly of Slab Cabin Run. It is clear that large areas upland areas, especially those affected by fill, are dominated by honeysuckle thickets. Although tentatively identified as Amur honeysuckle (L. macku), it is likely that hybridization has been occurring with other Lonicera species, particularly Tartarian honeysuckle (L. tatarica) and Morrow’s honeysuckle (L. morrowi). Williams (1995) has noted that Lonicera species freely hybridize. This presents a concern, because L. morrowi is able to invade fens and wetter riparian areas. Lonicera often out-competes native plants because of its earlier leaf expansion and later fall leaf retention. Large thickets interfere with the life cycles of many native woody and herbaceous plants. These stands can alter habitats they invade by decreasing light availability and depleting soil moisture and nutrients. It is suspected that Lonicera is allelopathic, releasing chemicals into the soil that inhibit the growth of other plant species (Converse 1995). The fruit is consumed by many birds, which make effective control difficult (Williams 1995). Occasionally, undisturbed native plant communities are invaded. Barnes (cited in Converse 1995) found that gray dogwood (Cornus racemosa) thickets, unless very dense, could not resist honeysuckle invasion, but that honeysuckle thickets of various densities could resist dogwood invasion.

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Garlic mustard (Alliaria petiolata), a biennial herb of European origin, is unusual among invasive species in that it is also shade tolerant and spreads readily under forest canopy, especially in moist sites such as riparian woodlands. It is extant in several large patches on site, most notably as the chief groundcover around the several large bur oaks (Quercus macrocarpa) along Slab Cabin Run. This resilient species is very aggressive in disturbed soil, forming a dense monoculture and displacing native ground flora. Spring seedlings have been known to attain densities of 20,000 seedlings/in² (Blossey 1997). The mode of dispersal is unknown, but is believed to be influenced by white tailed deer (Odocoileus virginicus) populations, where trampling exposes soil and allows seeds to germinate. Human trampling and alteration of light conditions can achieve the same affect. A prime local example is the nearby Lederer Park woodlot, in which recent manual grubbing of shrubby understory by prison work crews was immediately followed by a flush of garlic mustard, to the exclusion of virtually all other ground cover. Once a foothold is gained, a lack of natural competitors and predators allow this plant to further dominate ground cover.

Multi-flora rose (Rosa mu1t~1ora) is an adaptable, thorny shrub attaining a height of between 2 - 3 m (6-10 if). This Asian rose has arching stems that inter-tangle to form impenetrable clumps up to 6 m in width, choking out native plants. Long planted for wildlife food and erosion control, it has become a major pest in fallow field and bottomland clearings throughout the United States. It has been declared a noxious weed in Pennsylvania. Studies have shown that it is a highly competitive for soil nutrients, and can lower yields if crops are planted following its eradication (Evans and Eckardt 1995). Although decidedly less ubiquitous than honeysuckle, multiflora rose is found scattered throughout the middle elevations of Millbrook Marsh. It can spread clonally outward from established clumps, but seems to be dispersed largely through consumption and subsequent deposition of seeds by songbirds.

Autumn-olive (Elea gnus wnbellata) has, until recently, been planted as habitat for upland game birds. It has been noted as a problem in both Shaver’s Creek Environmental Center and at Bald Eagle State Park. There are several specimens along the east side of the site, fringing the grassy slope down from the bike path. Autumn olive has the potential of becoming one of the most troublesome shrubs in the central and eastern U.S. It exhibits prolific fruiting, rapid growth, is widely disseminated by birds, and can easily adapt to many sites. It is vigorous and competitive against native species, and resprouts after cutting. Due to its nitrogen-fixing capabilities, it has the capacity to adversely affect the nitrogen cycle of native communities that may depend on infertile soils (Eckerdt 1995).

Common buckthorn (Rhamnus cathartica) is a small European tree that can be an aggressive invader of both upland and slightly wetter sites, favoring abandoned pastures and fields (Haber 1996). Din (1977) remarked on its tendency to become a weed, since the fruit are eaten by birds and the seeds are deposited along fences rows, shrub borders and other edge habitats. In Millbrook Marsh scattered individuals can be found in the old field just down-gradient from the bike path and near the College Township Municipal Building along Slab Cabin Run.

Japanese knotweed (Polygonum cuspidatum) is a tuberous perennial herb which is spreading quickly in the eastern United States and Great Lakes area. In the central reaches of the Susquehanna basin it has begun to form large monospecific stands along forested riverine floodplains. A small stand has recently established along Slab Cabin Run and Thompson Run, both just upstream of College Avenue. This suggests that immigration into Millbrook Marsh is imminent. Dispersed mostly through the spread of rhizome fragments, it forms large, monospecific stands which displace virtually all other vegetation.

Other minor invasive species in Millbrook Marsh should also be monitored for range expansion. Common buttercup (Ranunculus acrus) is an introduced perennial forb that flowers briefly in spring along gravel bars and stream banks in Millbrook Marsh, especially along Thompson Run.

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Although its monocultural stands are substantial, it is unclear whether this forb is invasive in riparian areas. Dame’s rocket (Hesperis matronalis), a member of the mustard family, is aggressively true to form. Considered a potentially troublesome species, it occurs side-by-side at many sites with garlic mustard. In Millbrook Marsh it is found along stream margins throughout the site. Several grasses also pose a concern, but positive field identification has yet to be made. These typically include smooth brome grass (Bromus inermis) and the cool-season Kentucky bluegrass (Poa pratensis). Further field work is required to determine the extent and impact of invasive forbs, grasses, and legumes in Millbrook Marsh. Crown vetch (Coronillia varta), a legume ground cover often used for erosion control can aggressively invade fields and forest edges. Its is a dominant species along the bike path on the eastern border of Millbrook.

Potential Invasive Species. Purple loosestrife (Lythrum salicaria) has yet to make its appearance in Millbrook Marsh. However, this European native has become a well-established resident of Bald Eagle Valley and Penn’s Creek since at least 1993, and has recently been found downstream of Millbrook Marsh in Spring Creek Park. This broadleaf persistent emergent thrives in fen areas and

— unlike most wetland herbs which reproduce largely through rhizomes — spreads exceedingly fast due to prolific seed production. It has been suggested that the tiny seeds (up to 2 million per plant) are spread to headwater areas when they mix with mud and adhere to birds, livestock and people. L. salicaria tends to establish in a zone between several inches and several feet above the water table, forming colonies whose thick herbaceous canopy easily out-compete most native wetland herbs. In low-lying glaciated areas to the northeast and northwest, it has become the predominant wetland plant over vast areas, out competing even cattail (Typha spp.). Invasion of purple loosestrife into a wetland can result in the suppression of the resident plant community and the eventual alteration of the wetland’s structure and function (Blossey 1997). Large monotypic stands of purple loosestrife jeopardize various threatened and endangered native wetland plants and wildlife by eliminating natural foods and cover (Eastman 1995, Haber 1996)

Common reed (Phragmites australis) is also worth watching for. Normally a stable and benign native species also found in temperate zones around the world, researchers suspect that a newly introduced genotype is responsible for the recent vast territorial expansion along the marsh-upland interface of many brackish and freshwater emergent wetlands.

Norway maple (Acer platanoides), a widely planted shade tree that favors urban riparian slopes, has shown up along Slab Cabin Run just downstream of the Military Museum in Boalsburg. Able to establish in shaded conditions, it rapidly establishes complete canopy closure. The resulting dense shade and aggressive uptake of soil moisture can result in complete exclusion of sub-canopy and ground flora.

The list of aggressive invasive plants would not be complete without mention of several indigenous plants. Box elder (Acer negundo), wild grapes (Vitis spp.), and a few other native species possess the ability to form fairly exclusive monocultures that thrive in disturbed environments. Whether population management of invasive, but native species should occur in Millbrook Marsh is a topic worthy of further discussion among experts and citizens in the community.