Tuesday, October 12, 2010
Using Aerial Photography to Locate Atlantic White Cedar Stands in the Hockomock Swamp, Easton Massachusetts
By Douglas Watts
The Hockomock Swamp in Easton, Raynham, West Bridgewater, Bridgewater and Taunton, Massachusetts is still unexplored and unknown, except by occasional deer hunters. No scientific studies have ever been done to determine how much of the 6,000 acre swamp was logged in the 1700s and 1800s and how much of it was not.
For this reason, there are no published reports which establish the locations of Atlantic White Cedar stands in the Hockomock Swamp. I'm not certain if such maps even exist.
By correlating aerial photo surveys taken at different seasons (summer, fall, early spring) during the past 40 years these white cedar stands can be accurately located by vegetation coloration. These sources include Massachusetts GIS, satellite data from Google Maps and private aerial surveys done in the 1970s. 
These map data show three large cedar stands in the Hockomock. By far the largest is west of Route 138 in Taunton and Raynham behind the Raynham dog track, a second smaller stand is east of Maple St. at the extreme southeast corner of Easton, and a third stand is just west of Route 24 in Bridgewater. Smaller, isolated stands undoubtedly exist and can be seen on the maps.
Google Map aerial imagery of the Hockomock was all taken in full summer, making identification of tree stands by species problematic. However, Mass GIS imagery was taken in early spring, before deciduous trees had leafed out. This makes the Mass GIS imagery extremely useful to differentiate swamp red maple stands from Atlantic white cedar stands in the swamp.
Since large stands of white pine (Pinus strobus) are common on islands of higher ground in the swamp, the difference in coloration between known white pine stands and known white cedar stands in the same photographs is easily seen. The white pine stands are distinctly lighter and yellower green than the cedar stands, which are much more dark and bluish green. The white pine stands, which are only found on islands of higher ground in the swamp are easily identified by correlation to USGS topo maps of the swamp, for example, adjacent to the Easton Rod & Gun Club on Howard Street in Easton. Known white pine stands on the margins of the Hockomock Swamp provide additional color correlation between white pine islands and white cedar stands within the swamp in the same photograph.
As anyone familiar with the Hockomock knows, most of it is absolutely inpenetrable by foot or canoe or kayak. The entrance to white cedar stands are particularly well guarded by perpetually wetted puddles and pools and are impossibly obstructed by thickets of downed trees and live and dead shrubs that require you to climb over them repeatedly every few feet. And 100 billion mosquitoes. For these reasons it is not surprising nobody has tried to field map the swamp. Nobody has paid them enough.
The two principal tree species of the truly wetted portion of the Hockomock are the swamp red maple (Acer rubrum) and the Atlantic white cedar (Chamaecyparis thyoides). In general, Acer rubrum dominates the swamp, forming uniform stands of thousands of acres. Atlantic white cedar dominates the swamp in only three distinct and disparate locations. In terms of acreage, red maple is by far the dominant of the two in the swamp as a whole.
It further appears that human intervention has favored the red maple at the expense of the Atlantic white cedar. First, Atlantic white cedar were aggressively sought after and logged in the 1700s and 1800s in the swamp, far more than red maple. Second, road and railroad building through the swamp in the late 1800s appears to have altered water levels in ways that favor red maple over Atlantic white cedar.
This can best be seen in aerial imagery of the Swamp just west of Route 138 in Easton, where it is bisected southwest to northeast by the straight, raised berm of the Old Colony Railway Line. Behind the Raynham dog track, Atlantic white cedar are the dominant species to the west of the berm. But on the east of the berm, just 50-100 feet away, red maples are the only species. Cedars are completely absent. As viewed in the Mass GIS aerial images, the rail line appears to create a total boundary. To the west are cedars, to the east, only red maples.
Walking the rail bed it is easy to see why. The rail bed stands on a bed of imported gravel fill which raises it 10 feet above the level of the swamp. Water in the swamp flows from west to east, causing the berm to function as a giant earthen dam. To the east of the berm, "below" the dam, seasonal water levels are much lower. To the west, "above" the dam, they are much higher. Water flowing from the west must pass through a scattered number of old, narrow granite culverts or tunnels to continue moving eastward through the Swamp.
Because these culverts are few and far between (and are often clogged with debris), the water level on the west side of the Swamp is much higher, seasonally, than on the east side of the berm. It is proposed here that the alteration of the swamp's seasonal elevation by the construction of the berm in the late 1800s caused water conditions to the east of the berm to be unsuitable for cedar and favorable for red maple but kept water conditions on the west side of the berm suitable wet enough for cedar.
The best way to understand the Hockomock is to think of it as the Everglades. The Everglades is known as a 'river of grass', the Hockomock, though much smaller, is a 'river of trees.' The Hockomock is a glacial lake bottom, long filled in by vegetation and sediment and utterly flat. Water from the higher ground around its basin, from Lake Nippinicket to the south and all of the brooks and swamps from Easton and Norton and Sharon and Mansfield to the west and north all feed into it. Once all of these small brooks and rivulets hit the 'lake bottom' of the Hockomock, they spread out, creating the swamp and like, Moe of the Three Stooges said, "Spread out."
The spreading of the water from these dozens and dozens of brooks in a vast, flat sheet creates the Hockomock. Only at small pinch points, like at granite culverts put in at old road crossings in Easton and Raynham do the waters of the Hockomock briefly funnel and come together. Without these artificial pinch points, the sheet of water would stay wide. Eventually, all of the water in the Hockomock pours through a tiny indented notch in its soup bowl, called Nunketetest, the Town River, which becomes a 'regular' river that moves quickly down through West Bridgewater and Bridgewater to make the Titicut River (Taunton) until it, with the addition many other rivers, finally meet the ocean at Dighton and Segregansett to make Montaup (Mount Hope) Bay.
You might be wondering why all of this interest in Atlantic white cedar stands in the Hockomock. One small answer is that a very beautiful butterfly, the Hessel's Hairstreak, exists only in stands of Atlantic white cedar. It is a very rare butterfly and seldom seen. It is a species of 'special concern' in Massachusetts, primarily because its home, Atlantic white cedar swamps, have been almost totally destroyed in Easton and the U.S. by logging, draining and other abuses. The caterpillar of the Hessel's Hairstreak only eats the foliage of Atlantic white cedar trees.
The aerial photographs further show there is a large raised peat bog along Black Brook between Route 138 and Maple Streets in one of the most inaccessible parts of the Hockomock Swamp. This bog has never been documented.
View Larger Map
 In 1971 my dad, Allan E. Watts, asked a local pilot, Ed Chassis, to take him over Easton to get aerial photos of the town, especially its undeveloped places, in order to generate public support to preserve them. An aerial photo he took in Nov. 1971 from Route 24 in West Bridgewater toward the west and across the Hockomock clearly shows a large cedar swamp near the old Maple Street right of way between Raynham and West Bridgewater. Mass GIS aerial photos from the 1990s confirm the presence of this stand, even though it is almost totally impenetrable to access by foot. Finding this lost cedar swamp found by my dad 40 years ago in an airplane is the impetus for this investigation.