Many of Nantucket’s grasslands and heathlands have become overgrown by taller shrubs and trees to the extent that few, if any, of the plant species characteristic of these habitats remain dominant. The most common species to invade and take over these sites are scrub oak and pitch pine, with pitch pine being more common in areas that were once plowed for agriculture and scrub oak being more dominant at sites where the soil has not been disturbed. These overgrown heathland communities are collectively referred to as barrens. Examples of this type of habitat occur on many of the Foundation’s properties, including the Middle Moors and along the south shore of the island between Nantucket Memorial Airport and the former Tom Nevers Naval Facility.
At many of these sites, scrub oak forms dense, impenetrable thickets that shade and out-compete most of the grasses and herbaceous plants in the understory. Plant diversity in barrens communities is further limited by the drought-prone, acidic, nutrient poor nature of the soils. Other species that are well-adapted to these conditions and can occur amidst the scrub oaks include northern arrowwood, bayberry, huckleberry, dwarf chestnut oak, sweet fern, blueberry and eastern red cedar.
Scrub oak is an extremely tough and resilient shrub. The plant has a large root collar that lies just below the surface of the soil and bears hundreds of dormant buds. If the above ground branches are destroyed by fire, brushcutting, or some other type of disturbance, the plant quickly sends up numerous resprouts that grow up to 3-4 feet per year. Acorn production begins when the resprouts are 3 years old, reaches its maximum between years 5-7, and slowly declines thereafter. Therefore, this species benefits from periodic disturbance such as fire, which stimulates the production of acorns and thereby promotes the establishment of new seedlings.
Pitch pine is another barrens species that is well adapted to periodic fires, with dormant buds that are protected under thick layers of bark. This tree is native to the eastern United States, but it was not common on Nantucket prior to 1847 because of the island’s extensive sheep grazing history. At this time, Josiah Sturgis planted pitch pine seedlings along the Milestone Road to serve as a windbreak and the trees thrived in the sandy, disturbed soil along the road edge. This species has since become one of the most common trees on the island in many areas that formerly supported grasslands, heathlands, and scrub oak barrens.
Barrens communities are dependent on periodic disturbance to prevent them from becoming overgrown by taller hardwoods such as black oak, white oak, black cherry, and shadbush. Management tools such as prescribed fire, brushcutting and soil disturbance can be used to maintain these communities and prevent tree encroachment. Frost bottoms are interesting landscape features that often occur within scrub oak and pitch pine barren communities. They are topographical depressions that pool cold air at their lowest elevation, resulting in frequent frosts that can occur at almost any time of the year and a much shorter growing season than the surrounding upland. Most shrubs, with the exception of scrub oak, cannot tolerate these conditions. Therefore, recurrent frosts maintain the vegetative composition of these sites without the need for fire or other disturbances. Scrub oak and pitch pine barrens and their associated frost bottoms provide critical habitat for a suite of rare moth species, including the barrens buckmoth, pine barrens zale and barrens dagger moth. The larvae of these moths specialize on feeding on scrub oak and other shrubs associated with these habitats.
On Cape Cod, many former barrens communities have already reverted to upland forests because of the lack of periodic fire or other disturbances. Because of Nantucket’s recent history of sheep grazing, mature forests are still relatively rare on the island. However, given time and the absence of fire, many of the scrub oak barren communities here will eventually become similar to the forests on Cape Cod today.