Alien Plant Intrusion:
Management Implications & A field guide of invasive exotic plants that threaten native species and natural habitats in Maryland.

Introduction
Among the threats to Marylandıs vegetative biodiversity is the spread of invasive alien plants. Since European settlement, the landscape of Maryland has been modified by exotic plants which invade, displace and disrupt the balance of native plant ecosystems, negatively impacting their function and stability. Although the number of introduced species that have become significant ecological problems is relatively small, single aggressive species have been known to transform entire ecosystems. Most major ecosystem types in Maryland have been extensively altered and continue to be degraded as aggressive alien species expand their range.

In all environments, the habitats most at risk are generally watercourses. Watercourses provide moist conditions, act as nutrient traps and natural transport mechanisms for seeds and propagules, and therefore tend to support greater varieties and densities of exotic species than the surrounding landscape. Watercourses tend to be sites of higher native species diversity too, but invasive aliens are displacing this native diversity at a rapid rate.

Definition of Terms
Native species are defined as plants occurring in North America prior to European settlement. Indigenous and endemic refers to native plants within a specific region. Local plants may be confined, having a comparatively restricted distribution, such as to a specific watershed. The terms introduced, cultivated, exotic and alien refer to plants that were brought into an area that were not known to have previously occurred there. Introduced plants which are naturally reproducing and commonly found in the landscape but are not displacing natives are considered to be naturalized. Naturalized aliens such as queen anne's lace, chicory, and many species of grasses have been present for such a long period of time that they are often mistaken for natives. Also entering the environment due to availability in the trade, are horticultural selected species known as cultivars. Cultivars are especially common in grasses and forbs and can be derived from both native and alien species.

Alien Plant Intrusion
Shelter and Weiser (1987) of the Smithsonian Institution performed a 13 year phenological study of plants within a 35 mile radius of Washington, DC and found that exotic species accounted for 25% of all plants observed within natural plant communities. In the more urbanized areas of Maryland, when a piece of land experiences a light gap opening or soil disturbance, the plants most likely to dominate will be aggressive Eurasian species.

The establishment and spread of aliens is often related to human activities, such as roadwork, utility rights-of-way, and grazing. These activities typically cause fragmentation of vegetation and soil disturbance. The link between alien plant intrusion and land use has important implications for management.

Fragments of native vegetation in an alien matrix of urban or agricultural land have much lower resistance to invasion by alien species than intact native ecosystems. This is a result of ecological changes that occur once vegetation is fragmented. Cleared land is often a repository of exotic species, offering favorable conditions such as increased nutrients, changed hydrological and light regimes, and soil disturbance.

Livestock grazing induces drastic changes to plant species composition and relative abundance, ground cover characteristics, soil structure, and nutrient cycling. Many exotic species tend to be favored by grazing and consequently are spreading relentlessly in many agricultural areas of Maryland.

The ecological impact of most exotic species is poorly understood. Some literature on the subject exists but, for the most part, this information is highly generalized or focused on species control with herbicides. In ecological restoration we believe that, where possible, all alien, naturalized species, and cultivars should be avoided. Cultivars may seem more innocent since they do not exhibit the aggressive tendencies of the invasives, however they to, should be avoided in natural areas since they contaminate native gene pools.

A Field List of Common Invasive Non-natives Occurring in Central Maryland
The following table identifies a partial listing of invasive plant species which occur in central Maryland and are derived from numerous regulatory wetland and forest stand delineation reports as prepared by Environmental Systems Analysis, Inc. (ESA) within the piedmont and coastal plain of Maryland. Most of these species are well established in the State. Others are adventive species which are not fully established or only locally established. The list of aliens in Maryland is rather extensive, however this list contains only those species that are particularly insidious. Management of these species should be looked upon favorably by regulators and incentives given for control measures employed.

To determine if a species was indigenous or introduced, the following literature sources were consulted for range and distribution: Brown and Brown (1972,1984), Gleason (1952) Hitchock and Standley (1919), McAtee (1918,1930,1940), Shreve (1910), and Gleason and Cronquist (1991).

Priority invasive plants typically exhibit the following characteristics:

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

Priority Invasive Exotic Plants
Plants considered to be particularly aggressive, noxious and invasive, often found in monocultures displacing native community structure:

Acer plantanoides	Norway Maple
Ailanthus altissima	Tree-of-Heaven
Alliaria petiolata	Garlic Mustard
Ampelopsis brevipedunculata	Porcelainberry
Arthraxon hispidus	a grass
Bidens polylepis	Long-bracted Beggar-ticks
Celastrus orbiculatus	Asiatic Bittersweet
Cirsium arvense	Canada Thistle
Cirsium vulgare	Bull Thistle
Eleagnus umbellatus	Autumn Olive
Festuca elatior	Tall Fescue
Glecoma hederacea	Ground Ivy
Hydrilla verticillata	Hydrilla
	Privet
Lonicera japonica	Japanese Honeysuckle
Lonicera spp.	Bush Honeysuckle
Lythrum salicaria	Purple Loosestrife
Microstegium vimineum	a grass
Morus alba	White Mulberry
Perilla frutescens	Beefsteak
Polygonum cuspidatum	Japanese Knotweed
Polygonum perfoliatum	Mile-a-Minute
Pueraria lobata	Kudzu
Ranunculus ficaria	Lesser Celandine
Rosa multiflora	Multiflora Rose
Sorghum halepense	Johnsongrass
Spiraea japonica	Japanese Spiraea

Management Implications for Ecological Restoration
When preparing wetland mitigation or reforestation plans, it is necessary to recognize that the invasiveness of exotic species challenges the basic goal of ecological restoration which is to recreate ecologically healthy environments. Ideally, a restoration project should consist entirely of indigenous species. In order to meet the goal of a forest conservation or wetland mitigation plan, the control of exotic species must be implemented and may require ongoing management, monitoring and evaluation. It is recommended that the following principles be considered during the planning, implementation and evaluation of forest conservation and wetland mitigation plans, as they pertain to alien plant management:

• the control of priority noxious aliens should be an integral component of all reforestation, forest retention and wetland mitigation projects,
• monitoring of exotics and periodic reassessment through the 2 or 5-year maintenance / monitoring / bonding period should be integrated into all plans,
• highest priority should be given to the control of exotics that pose the greatest threats, namely:

exotics that replace indigenous dominant or key species, exotics that substantially reduce indigenous species diversity, exotics that significantly alter ecosystems or community structure or function, exotics that persist indefinitely as reproducing or clonally spreading populations, exotics that are expanding locally,

• forestry and wetland programs should include contingencies for removing exotics as they first appear and for implementing ongoing control methods as necessary,
• control programs should cause the least possible disturbance to indigenous species and communities and, for this reason, may need to be phased,
• alien plant management for forestry or wetland plans must be strategic; protection of indigenous habitats, levels of infestation, appropriate resource allocation, and knowledge of control methods should be integrated into the monitoring and management program,
• exotic species should not be introduced to the site during the forest conservation or wetland mitigation plan process,
• native species should be evaluated for their potential threat to indigenous communities; as possible, weedy native species should be avoided in plans unless being used to prescribe a succession seral stage.

Sources for management strategies, abstract characterizations of alien plants, studies, and labor expertise may be obtained through ESA, Inc., The Nature Conservancy, Maryland Natural Heritage Program, Natural Areas Association, Society for Ecological Restoration, Society for Conservation Biology and Virginia Native Plant Society.