Thompson Island Plants

In a two-year project funded by the Island Alliance to study the vegetation of the Boston Harbor Islands National Park Area, 32 islands were surveyed and inventoried for vascular plant species. Field surveys began on 9 June 2001 and ended on 30 September 2002.

Below is the data collected for Thompson Island.

Island Acreage: 169.9

Owner: TIOBEC, Willaeur School

Rare Plants: Polygonum puritanorum (SC-tentative); Rumex pallidus (T-1984)

Quadrangle: Boston South, MA

Survey Dates: 11 June, 2001; 18 July, 2001; 21 August, 2001; 19 June, 2002.

Data Notes:

• = introduced species

(v) = voucher specimen

(p) = photograph

Due to formatting restrictions, species scientific names are not italicized in the data table.

Download This Dataset

Notes on Habitats and Flora

Thompson Island has a variety of upland and wetland habitats, featuring the largest salt marsh in the Harbor Islands. Upland communities include old fields, shrub thickets, woodlands, and disturbed open grounds near the school buildings and landing. All of the upland habitats have been altered by centuries of use, and much of the upland flora is weedy and invasive.

The largest woodland habitats are located along the island’s steep slopes west of the school buildings. Norway maple (Acer platanoides) and English oak (Quercus robur) are the dominant trees in this virtual forest of naturalized European species. Other non-native trees identified in these woodlands include European larch (Larix decidua), black pine (Pinus nigra), small-leaved linden (Tilia cordata), and Siberian elm (Ulmus pumila). Another woodland is located on a rise west of the salt marsh on the island’s southwest corner. Norway maple is the dominant tree here also, but there are some native trees, including red and black oaks (Quercus rubra and Q. velutina), gray birch (Betula populifolia), and aspens (Populus grandidentata and P. tremuloides).

The island has two major freshwater wetland habitats, a “skating pond” at the base of the slope south of the school buildings, and a Phragmites-dominated marsh behind the beach on the island’s northeastern side. The skating pond wetland, which is one of the Harbor Island’s most diverse (with respect to plant species) freshwater wetlands, had standing water throughout the field season. Phragmites communis is dominant here, but a number of other species also occur in abundance, including: Echinochloa walteri, Eleocharis acicularis, Lythrum salicaria, Panicum dichotomiflorum, Polygonum pensylvanicum, Scirpus americana, and Scirpus maritimus. This relatively high plant diversity notwithstanding, Karnauskas (2001) found few aquatic invertebrates in this wetland. The second “marsh” is a flat depression, slightly lower than the adjacent sandy field, supporting a virtual Phragmites monoculture. The soils here were barely moist when the habitat was visited on July 18. Besides these two wetlands, a small depresssion with Epilobium hirsutum, Lythrum salicaria, and Typha latifolia is located behind the beach at the base of the slopes east of the school buildings.

Thompson Island’s impressive salt marsh covers much of the southern quarter of the island. Plant species here are typical of Harbor Island salt marsh habitats in general, but occur in greater abundance: Atriplex patula, Carex hormathodes, Distichlis spicata, Elytrigia pungens, Chenopodium album, Juncus gerardii, Lepidium latifolium, Limonium nashii, Salicornia maritima, Solidago sempevirens, Spartina alterniflora, Spartina patens, Suaeda linearis, and Suaeda maritima. During the August visit, a marsh hawk was seen flying ovcer the salt marsh.

The island has extensive stretches of sandy and stony beach, with the species mix typical of beach strand habitats found elsewhere in the Harbor Islands. Two unusual plants (for the Islands) found at the upper edge of the beach on the island’s southwestern end include Lespedeza capitata (not found on any other island), and Strophostyles helvula.

In 1984, Bill Perkins documented two seabeach dock (Rumex pallidus) plants along the beach at the base of the steep bluffs on the northwest side of the island (one plant) and on the “cuspate spit” facing Spectacle Island. Apparently, these plants have not been seen since Perkins’ observation (Paul Somers, personal communication), and I did not find them in a thorough search of this area on July 18.

At the base of these same bluffs I did collect two Polygonum specimens that match the description of pondshore smartweed (Polygonum puritanorum) given in Fernald (1950). This species very closely resembles the common Polygonum persicaria, differing slightly in leaf features and achene size. In Massachusetts, where it typically occurs along sandy, coastal plain pond shores, pondshore smartweed is a species of special concern. According to Paul Somers, there is disagreement as to whether Polygonum puritanorum is truly distinct from P. persicaria, and Gleason and Cronquist (1991) do not recognize it as a separate species. This identification is tentative until more specimens from this population are examined.

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• Boston Harbor Islands National Recreation Area

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