The vegetation on Martha’s Vineyard has changed dramatically over time, yet it has also shown remarkable consistency. How can both of these statements be true?
Ice was one mile thick over much of what was to become Martha’s Vineyard. This was about 20,000 years ago, when there was so much water locked up in the glaciers that the ocean shoreline was at the edge of the continental shelf. No vegetation would have existed near this big block of ice, though there may have been tundra-like vegetation closer to what was then the ocean.
What happened since then has been documented by numerous scientists studying the layers of pollen deposited in wetlands. The pollen grains are layered, with the oldest at the bottom and newer deposits occurring as the muck builds up from the yearly addition of dead vegetation. The vegetation rots but the sturdier pollen survives. This process continues today; you may have noticed pine and oak pollen floating on our pond surfaces in the spring.
Vegetation became established as the ice retreated and our climate warmed. Our historic vegetation would likely match the progression that is seen if we were to travel southward from the Arctic. First would have been arctic (tundra) vegetation, then coniferous trees like spruces and firs. Next deciduous trees — probably maples and birches — would have arrived as our climate continued to warm and the ice retreated further northward. Finally, at some point in time, the oaks and pines would have arrived from the south.
Five thousand years ago sea levels had finally gotten high enough to flood Vineyard and Nantucket Sounds, thereby separating Martha’s Vineyard from the mainland.
By the time Europeans colonized the Vineyard, it was mostly woodland and likely there were no large areas of grassland, shrubland or heathland. Moist (mesic) deciduous woodlands dominated from Aquinnah to West Chop — the hilly terminal moraine deposited by the glacier. Oak and beech dominated, while red maple, black gum (beetlebung), birch, hickory and hop horn-beam were also present. Drier woodlands were found across the rest of the Island. Pines and oaks dominated, while both hickory and beech were present but not abundant.
Vegetation then changed dramatically as colonists cleared the woodlands for agriculture and cut down other trees for building and heating houses or other activities. Pollen from grasses and various agricultural weeds (including ragweed and sorrel) became common in the pollen record. Around 1800 there were perhaps 20,000 sheep on the Vineyard, and about 40 per cent of the Vineyard was pasture. Most of the woodlands that remained were on the central portion of the Island, part of which is now the Manuel Correllus State Forest.
After 1800, sheep farming declined and by 1880 there were perhaps only 5,000 sheep left. Woody vegetation started to establish itself in abandoned pastures, and by 1880 this natural succession had expanded woodlands to cover about half of the Vineyard. Many of these new woodlands were in Oak Bluffs, Vineyard Haven and northern West Tisbury.
Up-Island we can find large oaks with branches that spread outward parallel to the ground; the most conspicuous of these showy trees is in North Tisbury, at the intersection of North and State Roads. This tree and many others like it started growing in the abandoned pastures. Their unusual shape is because they grew in pastures where there were no nearby trees to shade them so they did not need to grow upward to reach for the sun. Dense woodlands of much younger trees now surround such stately trees.
By 1950 most of the sheep were gone, and woodlands covered about 70 per cent of the Island. Woodlands along the north shore and on the western portion of the Island were mostly hardwoods, but oaks may be slightly more prevalent than in the pre-colonial woodlands. The rest of the Island closely resembled the oak and pine woodlands of pre-colonial times, although pine may be slightly more abundant.
It seems rather remarkable that the post-sheep woodlands closely resemble the historic woodlands that were cut down to create the pastures. This similarity is not present in much of the rest of eastern North America because the past 100 years has seen fungal diseases wipe out two of the historically abundant forest trees — American elm and American chestnut. Apparently neither of these species was common on Martha’s Vineyard.
Woodlands now cover about 55 per cent of Martha’s Vineyard. This decrease is primarily due to increased residential development over the entire Island. Although woodlands are not as abundant now, they closely resemble the woodlands of the pre-colonial times.
Robert Culbert leads guided birding tours and is an ecological consultant living in Vineyard Haven.