Friday, December 03, 2010
Reconstructing the Hockomock Swamp: What Used to be There and How Do We Restore It?
By Douglas Watts
Commonwealth of Massachusetts aerial photos from 2001-2008 provide a good overall picture of the landscape and vegetation types which exist today in the Hockomock Swamp. These photographs show that today, only 15-20 percent of the Swamp is dominated by Atlantic white cedar and most of the Swamp is bereft of the species. These photographs suggest that road and railroad grades built through the swamp have changed drainage patterns in the Swamp in favor of regeneration of pure stands of swamp red maple and against regeneration of existing and former stands of Atlantic white cedar. This essay proposes that unless active efforts are made to eliminate the negative effects of these man-made drainage changes and to actively re-seed Atlantic white cedar where it once grew and has been eliminated, Atlantic white cedar will eventually disappear from the Hockomock and be replaced by pure stands of swamp red maple. Such an event will eliminate much of the biodiversity of the Swamp and allow it to become a man-made monoculture, rather than a natural place.
In the map above, I have arbitrarily broken the swamp into four sections or "lobes." Section 1 is that portion from Howard Street in Easton east to Route 138, south of Route 106 and north from the Snake River at the outlet of Winneconnet Pond and Route 495.
Section 2, the "Central Section" is south of Route 106, north of Route 495, and east to Route 24, including the Nip and Nunkets Ponds.
Section 3 is the long vertical lobe east of Route 24 where the waters of the Nunketetest (Town River) join with West Meadows Brook south of Prospect Street and Route 106 in West Bridgewater.
Section 4 is Dead Swamp in Raynham, and just to the east, Titicut Swamp, both cut on their northerly sides by Route 495 and its cloverleaves. While I could add Little Cedar Swamp in Easton, just above Route 106 and Pine Swamp in Raynham, this map gives the lay-out of what most people consider the Swamp proper.
Looking at the large scale map, it is easy to see the Hockomock's ultimate origin as a glacial lake bottom, with the swamp now forming the 'shoreline' of the lake. This glacial lake is variously called "Glacial Lake Taunton" or the "Leverett Sea" depending on which geological text and author you look at. The higher grounds, easily seen because they are now the sites of roads and houses, are peninsulas in the original glacial lake where the land was higher. The smaller 'dots' of swamp and lowland on the peninsulas and around the main mass of the Hockomock correspond to 'deep' holes in the glacial lake.
Once you view the Hockomock as a glacial lake you can see how it formed: the lowest spots in the lake bottom, not having an outlet (due to various restrictive bottom conditions), slowly infilled over the past 8-9,000 years with vegetation which kept decaying into peat, creating a flat, level surface of rotted vegetation spanning 6,000 acres and five towns.
Mass GIS imagery from 2001-2008, taken in late fall and/or early spring, gives us a unique tool to gauge the existing vegetation types of the Hockomock because during these times of year, the deciduous trees in the swamp (mostly swamp red maple) have lost their leaves and look greyish purple at high altitude. White pine and Atlantic white cedar retain their evergreen color.
White pine only grow on islands and high ground in and along the swamp. Atlantic white cedar prefer much wetter areas that pine cannot grow in. By comparison to USGS topographic maps, comparative foliage color, and field checking, it is easy to distinguish stands of pine and cedar in the swamp. Through this process we can generate an accurate map of contiguous Atlantic white cedar stands in the swamp. By looking at such a map we can see there are surprisingly few stands of Atlantic white cedar in the Hockomock today.
The largest Atlantic white cedar stand is in Section 1, between Howard Street in Easton and the Raynham Dog Track on Route 138 in Raynham (click on image to see full size). I've drawn yellow lines to delineate the Swamp boundary from the 'high ground' around it. This helps to see the Swamp as the level remnant of a glacial lake bottom, with 'islands' as well as 'tributaries.' Route 495 and the Snake River, the outlet of Winneconnet Pond are at the bottom of the photo. The red line is the old railroad grade east of Route 138. The Atlantic white cedar stands can be easily seen as the green foliaged area in the middle of the photo. You can tell at least part of this aerial montage was taken in late fall, due to the cranberry bog in the left center being bright red. The Easton Rod and Gun Club and dug trout pond off Howard Street is just above and to the right of the bog.
In the above photo, I've narrowed down on the Atlantic white cedar stand in Section 1 to show it in greater detail. Here, the coloration difference between cedar and white pine is obvious (the white pine has foliage that is a 'yellower' green than the cedar); and I have been able to field check these differences in the photos by walking this section of the Swamp. The color differences are real. What is immediately obvious is the sharp line between the cedar stands on the left (west) side of the old railroad grade (the yellow line) and their complete absence east (right) of the railroad grade; and how dense cedar stands tend to hug the edge of the Swamp on its west side to the right of the cranberry bogs.
The yellow line is not an arbitrary marker: it is the railroad bed of the Old Colony Rail Line, built through the Swamp c. 1875-1880. It is raised above the Swamp by about 10 feet and is made of imported earthen fill. When it was built, a number of narrow 'box culverts' of roughly cut granite were placed at its base at scattered intervals to allow the water from the Swamp to continue moving from West to East (left to right). As these aerial photos show, the railroad grade has prevented the growth and regeneration of Atlantic white cedar on its east or 'downstream' side. As the photo shows, there are no white cedar stands east of the railroad grade, yet they are are very expansive on its west side right up to the grade. This is because the grade acts as a dam in the Swamp and makes the right side (the downstream side) drier than it used to be and dry enough to keep cedars from effectively competing with swamp red maple, which can tolerate drier conditions than cedar. From the railroad grade east to Route 138 and then to Maple Street/Hall Street, Atlantic white cedar are completely absent.
In these closer photos of the Swamp next to the old rail grade, you see darker 'lobes' moving left to right. These correspond to slightly wetter areas, which correspond to the location of box culverts on the railroad grade that let water through. In these aerial photos, denser stands of smaller swamp red maple are slightly darker in coloration and correspond to a wetter area.
In the second photo, I've drawn with yellow the 'plumes' of water flowing from the box culverts as they exit the railroad grade and travel due east. Three plumes are clearly seen in the photos. Note how they match well with dense stands of cedar on the left side of the railroad bed, but the cedar does not continue on the opposite side of the bed, which is pure swamp red maple.
To me these photos suggest that prior to construction of the railroad bed, the cedar stands continued to the east of the grade but were logged out and, for some reason, have not regenerated. This is odd if only because if we assume that the cedar stands were logged equally on both sides of the bed, both sides should have equally regenerated during the time it is presumed they were cut (150 years ago). Why the disparity?
Honestly, I don't know. But I think it has something to do with a complicated interaction with the fact that Atlantic white cedar trees propagate by dropping their cones on the floor of the swamp and the water levels of the swamp being changed by the erection of a 'dam' (the railroad grade) that made the east side of the grade not conducive to regeneration by cedar and also, by it being kept physically separate from extant stands of cedar on the west side of the grade.
Section II -- The Hockomock Central Portion
In Section 2, cedar are in a small cluster (I) just west of the now-abandoned Maple Street at the extreme southeast corner of Easton; around the shore of Nunkets Pond just west of Nippinicket Pond (II) and in a wide vertical swath just west of Route 24 (III) (click on image to see full size). The red line is Route 138 in South Easton and Raynham. The very high resolution photo below, of Nunkets Pond next to the Nip, shows how easy it is to discern white pine from cedar and high ground from low ground.
Hockomock Section III -- the Eastern Portion
Despite its size, Section 3 contains no contiguous stands of cedar, although scattered, isolated clumps appear to be present (click on image to see full size). Route 24 is on the left. The Nunketetest (Town River) is in the upper part of the photo, flowing left to right.
What Do the Images Tell Us?
Just as a ball park estimate, the Hockomock today appears to be comprised of less than 20 percent Atlantic white cedar; and those trees are in widely separated stands with vast, nearly pure swaths of red maple between them. Was it always this way?
Most likely not. Due to the durability of the wood and their straight trunks, Atlantic white cedar swamps were aggressively logged for their trees in the 18th and 19th centuries, including the Hockomock. Logging was most likely done in the dead of winter when the swamp was frozen and paths the width of oxen or horse team could be made to cut and remove the trees. In all likelihood most of what remains standing in the Hockomock today are the chest-high seedlings Atlantic white cedar left over from those cutting operations, now grown up over the past 100-200 years into mature trees. What is puzzling, and the real point, is why most of the swamp's Atlantic white cedars have never regenerated. I believe this is due to a complex folding of the cutting with the construction of roads and railroad beds; and when these factors conspired to prevent the logged out stands of Atlantic white cedar from regenerating, even after 150 years, and has allowed swamp red maple to become the apex and sole canopy tree species in the 'new' Hockomock Swamp.
Evidence for this is shown in that the wide and broad monotypes of swamp red maple east of the railroad bed and Route 138 show no sign of re-emergence of cedar (ie. no seedlings) and total dominance by swamp red maple. This represents a paradigm change in the forest pattern of the swamp; and one that appears to be permanent. By our understanding of the cedar/red maple interaction, the remaining Atlantic white cedar stands of the Hockomock are most likely headed toward extinction and eventual piecemeal replacement by swamp red maple. Should we care?
You Can't Prove A Negative
Was every pre-colonial Atlantic white cedar in the Hockomock chopped down? To show that you'd have to physically inspect every cedar tree in the Hockomock: a daunting task. Was it possible? Yes, given Yankee 'greed and ingenuity' from the 1800s and 1700s to cut down every tree in sight. Did it actually happen? Nobody knows because nobody today has done the type of exhaustive checking such a conclusion requires to be valid.
But from a landscape restoration and preservation perspective for the Hockomock, this question is secondary. The primary question is what are we going to do today? Because most of the Hockomock is protected as state wildlife management land, any further cutting is basically illegal. That's good.
But that 'steady state' of preservation alone does not provide insight into what used to be, what it could be, what is stopping it, and what can be done now to bring the Hockomock in a direction toward -- rather than exorably away from -- what it was.
The Dams of the Hockomock
This photo shows perhaps the best evidence of how the dams across the Hockomock Swamp have altered its natural character. Note how the large Atlantic White cedar swamp west of the old railroad bed abruptly ends at the bed and disappears on its 'downstream' side and does not re-emerge. It is hard to envision a scenario where 18th and 19th century loggers would selectively clear-cut only the cedars on the right hand side of the railroad grade and leave the left hand side uncut. This suggests that a change in hydrology due to the railroad bed must be the cause of the complete absence of cedars on the right hand side of the railbed and their continued abundance on the left hand side.
A cursory glimpse at aerial views of the Hockomock shows four long dams crossing its mid-section, three formed by roads and one formed by a railroad grade. These are from west to east, the N-S railroad grade east of Route 138, Route 138 itself, Maple Street from Raynham to West Bridgewater and Route 24 itself. The first three lie 'straight through' the Hockomock, while most of Route 24 follows a peninsula of naturally higher ground except where the Town River itself crosses beneath it. The first three function as low, but potent earthen dams that greatly alter the depth and movement of water in the swamp; and in doing so the habitat for trees and wetland vegetation.
Should we care? I think so. Waterflow in the Hockomock travels from west to east. All three of the first barriers constrain this water movement, forcing it to flow through small culverts, now mostly clogged with debris. Walking on these barriers, their west side is obvious the "wet side" and the opposite side is noticeably drier. Water flowing through the swamp from west to east is forced to pool up against these dams and flow parallel to it until it finds a small box culvert that is still not totally clogged with debris.
A cursory examination of the railroad grade east of Route 138 shows a high density of cedar swamp on the west side of the grade and a total absence of cedars on the east side remarkably coincident with the railroad grade itself. This railroad grade was built c. 1875-1880. The MBTA prizes this overgrown railroad grade because it wants to use it to build a high speed railroad line from Stoughton to New Bedford. But now, because of the enormous, and admitted, negative effects of raising and rebuilding the grade to accommodate 80 mph passenger rail, MBTA has proposed putting the rail line through the Hockomock on stilts, or as they say, a 6,400 foot long 'trestle.'
Well, fine. This means that whether the train ever passes through or not, the existing railbed can be dug up and removed, restoring the swamp's natural flowage pattern. I am not concerned at how this might affect existing ATC and dirt bike use, since this use is being conducted illegally anyways.
The old Maple Street/Hall Street 'grade' from Raynham to West Bridgewater is another story. Except as a conduit for illegal ATV traffic, it serves no purpose except to radically and negatively alter the drainage pattern of the Swamp. As you can tell by walking along it from the Raynham end, it forces Black Brook to flow parallel to it for more than 1,000 feet, making the west side of the berm unnaturally wet and the east side unnaturally dry. In a place like the Hockomock, where just a few inches of difference in standing water and water table changes the entire tree species assemblage, the berm has a decidedly negative effect, heaped upon the fact the grade/berm never should have been built in the first place and will never carry any traffic ever again.
It would take one summer season with a couple excavators and a few dump trucks to remove both of these post 1880 'dams' that cross the width and breadth of the Hockomock, just by digging them down to swamp level and removing and trucking out the artificial fill used to create them. The cost would be less than repaving a similarly long section of Route 24 which is done almost every year. And unlike repaving Route 24, once done it would be done in perpetuity. The natural drainage pattern of the Hockomock would be permanently restored and rescued from a time when people wanted to destroy it but only lacked the capital to do so. Then put in boardwalks so people can walk through.
Time to End the Trope of the Abused Child
Efforts to save what is left of the Hockomock date back to the 1960s. For obvious reasons, which I do not knock, the emphasis was placed on not allowing the rest of the Swamp to be filled and destroyed, as was certainly the tenor of those times. But that was nearly 50 years ago. The available land was bought and a fairly good-sized chunk of the Swamp was placed under state ownership. The path toward the destruction of those purchased lands was averted. The folks involved rightly breathed a sigh of relief. They deserve immense credit for what they did.
But this is 50 years later. During this time the uplands and swamp directly adjacent to the state-owned part of the Swamp have been chewed and ravaged by umpteen subsequent encroachments. All of which trend negatively on those values for which the Hockomock was first preserved: as a wild, clean and unspoiled place amidst a helter-skelter of urban-suburban pavement of ugliness.
I use the "Abused Child" analogy here in the sense that it is good for the police to come and stop your husband or boyfriend from beating your kid to death with a tire iron; but that interdictment alone does little to help the kid figure out how to save her sense of self and make it in the world. There is a difference between a tourniquet and triage and a stable existence. The preservation efforts on the Hockomock in the 1960s were the tourniquet and Medivac triage. But it is foolish to confuse these efforts as commensurate with a stable, normal and healthy post-traumatic existence.