Invasive plants are often symptoms of ecosystem change or stress. At low densities, invasive plants may not pose a significant threat to the ecosystem. However, when they crowd out native plants and cause a significant change in the plant community, wildlife habitat is lost and the basic nature of the ecosystem is changed.

Invasive plants become established in two basic ways. First, exotic (non-native) plants sometimes are able to outcompete native plants due to faster growth and/or an ability to tolerate a wide range of site conditions. Purple Loosestrife (Lythrum salicaria), a native of Europe, has invaded many freshwater and slightly brackish marshes in southern and central New England. It is spreading northward, and in midsummer its striking spikes of purple flowers now decorate many wetlands in southern Maine. Purple Loosestrife is very aggressive, and will invade open water habitat and choke out native sedges and grasses, displacing waterfowl and other open-wetland animals. Native plants may also become invasive when human alterations of an ecosystem create conditions favorable for a species to invade new habitat. When tidal restrictions limit the amount of salt water entering a tidal marsh, native species such as Narrow-leaf Cattail that are typically limited to the upper reaches of the marsh where salinity levels are low often spread seaward and eliminate typical salt mash plants and the animals that depend on them.

Common Reed (Phragmites australis).³ Phragmites australis, or Common Reed, is an invasive plant that is a native of the Americas and Eurasia. However, in recent decades it has become a widespread invasive species on this continent. Observers of Scarborough Marsh have witnessed the rapid spread of Phragmites over the last 20 years along Route 1 where it crosses the marsh. Phragmites is easily recognized by its height (5-15 feet), plume-like inflorescence, and habit of growing in dense, single-species stands.

Although native to North America, scientists now believe that the invasive form of Phragmites now overtaking wetlands throughout the US was introduced from Europe. Phragmites is especially common in brackish wetlands, where it is able to outcompete native tidal marsh species, and on disturbed sites. Tidal restrictions caused by roads, railroads, and other forms of development have encouraged the spread of Phragmites in tidal marshes throughout the East Coast. High nitrate concentrations from farms and urban areas may also stimulate the growth of Phragmites in wetlands, while highway construction and other development create exposed soils that facilitate colonization of new areas. Phragmites may become established by wind-blown or bird-deposited seeds, or by movement of rhizomes (underground stems). Construction equipment is probably responsible for moving rhizomes to many new sites, especially excavators that are responsible for the maintenance of highway ditches where Phragmites frequently grows. Tidal ice may also move rhizomes within a marsh system. Once it becomes established, Phragmites typically spreads through growth of rhizomes. Lateral expansion of colonies is typically about 1 meter per year, but new colonies may spread up to 10 meters per year in nutrient-rich areas.

An undergraduate research project at the University of Southern Maine found that the area of Phragmites in the vicinity of Route 1 increased by approximately 50% in the nine-year period from 1986 to 1995 (Moore 1996). This is confirmed by long-time observers of the marsh who report that Phragmites was virtually non-existent in the Route 1 area 25 years ago.

The invasive plant search primarily focused on locating and mapping Phragmites stands within Scarborough Marsh. Although observations show that Narrow-leaf Cattail appears to be expanding on the inland side of Route 1, the search concentrated on Phragmites because it is more aggressive than Narrow-leaf Cattail. Phragmites is able to grow in more saline areas than Narrow-leaf Cattail, spreads more rapidly, and usually eliminates all other native marsh plants once established.

Two data sources were used. The first was from an unpublished study conducted by Bates College (Bohlen et al. 1998). Bates students under the direction of Dr. Curtis Bohlen mapped the perimeter of Phragmites stands and other plant communities by locating the stand boundaries in the field with an accurate (< 2 meters) Global Positioning System (GPS) unit. The Bates study was limited to the marsh area between Route 1 and Payne Road and that area of the marsh located with 200 meters of the seaward side of Route 1.

Volunteer Tidal Marsh Stewards identified invasive plants on the remainder (>95%) of the 3,000-acre marsh system. The volunteers were asked to survey the marsh from vantage points (bridges, roadsides and other points offering good views of the marsh) and to sketch the location and extent of stands of Phragmites and other potentially invasive species on a base map. They were then asked to estimate the size of the stand (as defined by three broad size classes), identify the dominant species, and estimate the relative dominance of each species if possible. The invasive plant survey protocol is included in Appendix C.

Once the data were collected they were compiled on Maine Audubon’s Geographic Information System (GIS). National Wetland Inventory (NWI) maps were used as the base layer for the GIS mapping. The Bates GPS Phragmites data and the estimated locations identified by volunteers were added to the base map.

The combined extent of Phragmites mapped by Bates and Maine Audubon is shown in Figure 1. Approximately 55 acres of Phragmites were found in the Scarborough Marsh estuary (Table 2). Of this, 25 acres (over 40%) was found on the inland side of Route 1. After the Route 1 area, the largest concentrations of Phragmites are found between Blue Point Road and Pine Point Road and in the Jones Creek area near Pine Point. Scattered small stands are found throughout the remainder of the estuary. Most stands are located in the transition zone at the marsh edge, although a few stands are surrounded by comparatively healthy high marsh.

Table 2. Approximate area of Phragmites australis in the Scarborough Marsh estuary, 1998.

1. Source: Bohlen et. al 1998. Stand boundaries field-located with accurate GPS.
2. Source: Volunteer surveys. Estimate based on hand sketch maps. Total area may be +/- 5-10 acres.

Several possible factors may explain the abundance of Phragmites in the vicinity of Route 1 and Dunstan Landing. First, these areas are quite far from the ocean and have several small streams draining into them that result in lower salinity. Second, there are three tidal restrictions between each of these areas and unrestricted tidal waters. The most seaward of these is the Boston and Maine Railroad (Restriction #14, Figure 3), which forms a barrier approximately 2 miles in length across the marsh. Although an opening 400 ft. wide exists over the main channel, the remainder of the railroad bed forms a dam that prevents water from flowing over the high marsh during the "spring" tides that occur near the times of full moon and new moon. Thus, only the water that can pass under the railroad bridge is available to reach the inner marsh.

The second major tidal restriction affecting the Route 1/Dunstan Landing area is the Eastern Road, located about one mile seaward of Route 1 (Restriction 15, Figure 3). The Eastern Road forms a nearly continuous barrier over 0.8 miles of marsh. There is a 100-ft. bridge over the main channel. However, because the channel averages approximately 125 ft. in width in this vicinity, this opening constricts tidal flow during normal tides. More significant may be the roadbed itself, which prevents flow across the marsh during the peak tides that occur several days each month. Approximately 7 acres of Phragmites are located between the Eastern Road and Route 1. Two notable stands of Phragmites are located just seaward of Route 1 opposite Anjon’s Restaurant near the Maine Inland Fisheries and Wildlife pullout. Unlike many stands that are located at the upland edge of the marsh where fresh water influence is strong, these stands are surrounded by comparatively healthy salt marsh. While smaller in size than stands on the inland size of Route 1, these stands are robust and are spreading.

Route 1. Approximately 25 acres of Phragmites are located between Route 1 and Payne Road on the inland side of Route 1. Two relatively large (14 ft. diameter) culverts located under Route 1 constitute the third restriction between the inland side of Route 1 and unrestricted tidal flow (Restriction #4, Figure 3). These culverts account for 53% of the stream channel width and thus may be a moderate, but not severe, restriction to tidal flow. Other factors may also affect the amount of Phragmites inland of Route 1. In particular, most of the Phragmites is located adjacent to a large freshwater wetland on the northwest side of the marsh. It is likely that this wetland reduces the salinity on the side of the marsh where most of the Phragmites is found. In contrast, ditches on the south side of the marsh may result in better drainage of fresh water and allow more tidal flooding. In addition, upstream land uses may be contributing to high nitrate levels which could stimulate more robust Phragmites growth. Residential and commercial development in the area, as well as a farm located on a marsh tributary on Payne Road, are potential sources of nitrogen.

Dunstan Landing Marsh. A full-channel bridge on Pine Point Road crosses the Dunstan canal (Restriction #2), which may explain in part why there is less Phragmites here than near Route 1. This area is also located closer to unrestricted tidal flows than Route 1, and has fewer freshwater sources. However, there are eight stands of Phragmites totaling about 7 acres in this area.

Jones Creek Marsh. Two large stands of Phragmites totaling about 8 acres in area are located just inland of Pine Point and west of Pine Point Road. As discussed in Section III, a tide gate was located in the Pine Point Road culvert (Figure 3, Restriction #1) for many years. Salt marsh vegetation has since returned to part of the marsh, but two large stands of Phragmites remain in areas where freshwater vegetation is still dominant. It is not known whether these stands are spreading.

In order to evaluate the potential impact of tidal restrictions on invasive species and other aspects of marsh ecology, more detailed studies were undertaken in the vicinity of the Eastern Road and Route 1 (see Section V).

³ The information on Phragmites in this section is primarily based on reports by The Nature Conservancy (1995) and Niedowsky (1999).