A Natural History of Millbrook Marsh,
A Wetland In An Urbanizing Setting

Non-Native and Invasive Vegetation

As well as the desirable species present in Millbrook Marsh, so too are several undesirable plant species inhabiting the marsh. There are 30 introduced plant species identified so far in Millbrook Marsh, and a possibility of others (Table 5-2). Of the genus Bromus, there are 14 non-native species and 3 native. Identification of the bromes in Millbrook is needed to confirm whether they are the native or not. This is also the case with other plants identified only to genus in Millbrook Marsh. Of those, the number of introduced species is Cirsium (5 spp.), Erigeron (2 spp.), Rubus (5 spp.), Solidago (1 sp.), Viola (4 spp.), Galium (7 spp.), Iris (5 spp.), Oxalis (1 sp.), and Potamogeton (1 sp.). 

Table 5-2
Non-Native Vegetation in Millbrook Marsh, 1954 to 1997

(Pursell 1954, PennDOT 1981, Western Pennsylvania Conservancy 1995, Brooks et al. 1998) 

NW = Noxious Weed status

Scientific Name

Common Name

1954

1980

1994

1994-7

Agrostis alba Redtop      

*

Alliaria officinalis Garlic mustard      

*

Arctium minus Common burdock      

*

Barbarea vulgaris Yellow cress      

*

Brassica rapa Field mustard  

*

   
Carex leporina Hare’s foot sedge      

*

Cirsium arvense (NW) Canada thistle  

*

 

*

Coronilla varia Crown vetch      

*

Daucus carota Queen Anne’s lace      

*

Dipsacus laciniatus Cut-leaved teasel  

*

 

*

Dipsacus sylvestris Teasel      

*

Eleagnus umbellata Autumn olive      

*

Festuca pratensis Meadow fescue      

*

Fragaria chiloensis Garden strawberry      

*

Hesperis matronalis Dame’s rocket      

*

Hieracium caespitosum King devil      

*

Lepidium campestre Peppergrass      

*

Ligustrum vulgare Common privet      

*

Lonicera tatarica Tartarian honeysuckle      

*

Lysimachia nummularia Moneywort  

*

 

*

Mentha spicata Spearmint  

*

   
Nasturtium officinale Watercress  

*

   
Phleum pratense Timothy grass      

*

Plantago major Broad leaf plantain      

*

Poa pratensis Kentucky bluegrass    

*

 
Polygonum lapathifolium Willow-weed      

*

Rhamnus cathartica Common buckthorn      

*

Ranunculus repens Creeping buttercup      

*

Rorippa amphibia Yellow cress      

*

Rosa multiflora (NW) Multiflora rose      

*

Rumex crispus Curly dock      

*

Rumex obtusifolius Bitter dock      

*

Solanum dulcamara Bittersweet  

*

 

*

Taraxacum officinalis Dandelion      

*

Trifolium hybridum Alsike clover      

*

Trifolium repens White clover      

*

Urtica dioca Stinging nettle  

*

 

*

Total non-native species reported 37

0

8

1

33

Of these non-natives at Millbrook, Cirsium arvense and Rosa multiflora are the two that have earned Noxious Weed status in Pennsylvania. Non-native vegetation becomes invasive at times when the species in question has no natural enemy, a high growth rate, abundant seed production with easy dispersal or some other prolific regeneration process, and little or no competition from other species. One or more of these factors contribute to how threatening the non-native species is to native species. The function of the entire ecosystem, especially its long term viability, comes into jeopardy in some cases when a native is displaced by a non-native that is less useful or even harmful to other flora or fauna in that system. The probable effects of invasive species on ecosystem composition and structure are an increased dominance of generalist species with a corresponding decrease in importance of specialists, decreased species richness, decrease in structural diversity, decrease in spatial heterogeneity, and replacement of native species functional interactions (Norton 1992). In general, the functional interactions are disrupted to various degrees depending on the characteristics of the invasive species.

Many non-natives flourish on disturbed sites, though undisturbed areas are not immune to invasion of some species. Millbrook Marsh is vulnerable to many of the invasives due to the past and continuing disturbances typical of a semi-urban location. The most conspicuously problematic plant species at Millbrook are Rosa multiflora and Lonicera tatarica (Map 9). These deciduous shrubs have caused parts of Millbrook Marsh to become almost impenetrable. They are extremely invasive and spread copiously. Multiflora rose, native to Asia, has been planted in the United States for use in erosion control, as food for wildlife, and horticulturally. It spreads outward from established clumps and the seeds are dispersed by birds. Tartarian honeysuckle is also found throughout the site, presently in even greater abundance than multiflora rose. It too has been used for wildlife and planted horticulturally. A native to Russian swamps and floodplains, it is extremely invasive, outcompeting most other vegetation. Seed dispersal is also through birds. Garlic mustard is a European introduction, brought for its use as an herb. It spreads quickly on disturbed ground due to prolific seed production. Garlic mustard also has no natural enemies here and therefore quickly becomes a monoculture groundcover. The preferred habitat is moist shade, but it will invade areas with varying light and moisture. Autumn olive, native to China, Korea and Japan, has also been planted horticulturally and as a food source for wildlife. It is extremely invasive and difficult to eradicate. Resprouting occurs from the roots after complete cutting to ground level. Several shrubs are at higher elevations at the edge of the grassy area in Millbrook Marsh along Route 322. Two plant species that must be constantly monitored for are Japanese knotweed and Purple loosestrife, both located not far from Millbrook Marsh. Though it has not been sighted in Millbrook Marsh yet, Japanese knotweed, Polygonum cuspidatum, has become established along Slab Cabin Run upstream of College Avenue. Dispersal is through rhizome fragments, which break off, are carried downstream, and get deposited along banks and floodplains. Once established, Japanese knotweed becomes a monoculture, replacing all other vegetation. At this point in time, it is essentially impossible to eradicate large established stands. The other extremely dangerous non-native invasive is Purple loosestrife, Lythrum salicaria. Purple loosestrife is a European introduction that thrives in emergent wetlands and fen areas. It quickly becomes a monoculture, threatening the existence of the native wetland ecosystem. It has become one of the major problems facing wetlands primarily in the north. Millbrook Marsh is extremely vulnerable to both of these invasive introductions. Monitoring and removing any initial plants is the only effective means of control. The common buckthorn, present in Millbrook Marsh along the southern edge near Slab Cabin Run, is also sometimes an aggressive invasive. Seeds are dispersed by birds. Canada thistle, despite it’s name, is a Eurasian introduction (Figure 5-8). It needs light for germination and is, therefore, only problematic in some areas but it is extremely invasive, especially in disturbed areas. It had already been determined to be a Noxious Weed in 25 states by 1918.

The minor invasive species that pose potential problems include Ranunculus acris, Hesperis matronalis, Poa pratensis, and Bromus inermis. In addition, some native vegetation periodically becomes invasive and forms a monoculture on disturbed sites. The possible problem species at Millbrook include Acer negundo and Vitis riparia (Brooks et al. 1998).

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