Great Brewster in Sub-climax

I spent the summer of 1982 on Great Brewster Island in Boston Harbor. Great Brewster is a drumlin, a hill formed by an advancing continental icesheet, and it is one of several in Boston Harbor, including Lovells, Peddocks, and Gallops islands. These and other islands made up Boston Harbor Islands State Park. (It has since become a national recreation area overseen by the National Park Service.) In 1980 the state of Massachusetts Department of Environmental Management (DEM; now the Department of Conservation and Recreation, or DCR) began putting “island managers” in the state park. In 1982 there were seven pairs of us.

Military landscape of Great BrewsterGreat Brewster was the island furthest out from Boston – eight and half miles – and the only one that could be reached solely by private boat. The others were served by water taxis and had far more visitors who arrived on a schedule. Our job was part ranger and part interpreter. I usually summed up the ranger part as “We're there to keep people from burning the island to the waterline.”

The interpretive part of the job involved creating and carrying out programs that explained the human and natural history of the island to visitors. Some of the human history was obvious; there was a ruin of a World War Two era bunker that was used to monitor underwater mines that had floated in the navigation channels in the harbor. Great Brewster sat right in the middle of President Roads to the north and Nantasket Roads to the south.

The history before the mid 20th century was more informal. We had information from the resources at the DEM and through talking to visitors who were long-time residents in the towns around the harbor that suggested that the Great Brewster had been a temporary residence for fishermen before the military took it over. During the 1940s the island had been cleared of vegetation and reduced to a lawn.

I arrived on the island 37 years after the end of World War Two, which seemed like a long time when I was 21, but now at age 51, it doesn't. In any case the succession that I encountered had progressed to the seral stage that included tree species like staghorn sumac and white poplar (an exotic) on the upland regions of the island, but it had not gotten beyond the forb stage in the low saddle between the norther and southern uplands.

Great Brewster from the northeast (photo by Fish Cop)As you approach Great Brewster from the west by boat its outline resembles that of a cartoon whale with a large bump facing north and a smaller bump to the south. The northern end of the islands is defined by bluffs over a hundred feet high on the west, north and east sides. Drumlins are formed, in theory, when an icesheet moves forward relatively quickly beveling the landscape and compressing subglacial sediments into “cigar shaped” hills with the long axis pointing the direction of glacial movement. The sediments in these landforms are difficult to erode because they were formed by compressive forces. Even after being exposed to weathering forces for 14,000 to 16,000 years the top of Great Brewster island is still 105 feet above the high tide line.

The seral progression on Great Brewster is a variation of “old field” succession, but it has been retarded, probably by the exposure to the elements compared to an inland site, and because of the distance from sources of seeds. In inland New England an old field would be well on its way to being an oak-hickory forest after 37 years, not a mixture of early successional trees and perennial forbs.
Ostensibly as part of research for my interpretive program and also because we were, as island managers, expected to contribute to building a knowledge base for our successors, I mapped the vegetation of the island. I drew a map of the island roughly to scale and then walked every square foot of it to document the floral community.

Had I been a more enthusiastic student of ecology I should have made a meter-square quadrat, laid it on the ground at regular intervals in a grid and identified and counted every species in it. However, in my sophomore year of college I had been taken off guard by the quantitative aspects of ecology. I had gone into my biology major having been an avid consumer of field guides and descriptive texts since age 11. It took much too long for me to realize that my chief interest and talent was for natural history, not biology. My map of the 19-acre island's vegetation consisted of polygons that showed the limits of the distribution of common species like sumac and individual circles marked the position of rarer trees and shrubs. I simply eyeballed their positions; I didn't even bother to pace off the distance from a set of landmarks. The result was, I suppose, better than nothing.

Eroding bluffs on Great BrewsterThe benefit of hindsight allows me to criticize my lack of rigor and but my further education in succession allows me to see that the island would be an interesting place to study succession. There might be historical records that describe the climax community (what existed in the 18th or even 19th century). That would tell you what the floral community was “headed for.” The other Brewsters (Middle and Outer Brewster) were bedrock outcrops even further out than Great Brewster and yet in the early 1980s the trees on them were larger than those on Great Brewster. This suggests that Great Brewster was either disturbed more by World War Two era developments and/or succession was impeded by maintenance there for a longer period after the war.

Great Brewster may be an example of an edaphic climax in which some combination of factors prevents the community from reaching the climax stage more generally observed in the region, in this case an oak forest. Sumac and white poplar have wind blown seeds that could be transported to the island more easily than an acorn (or a hickory nut). Therefore generations of these subclimax trees may come and go on the island before the shade tolerant climax species become established, if they ever do.