Tuesday, December 07, 2010

Status and Future of Atlantic White Cedar in Hockomock Swamp, southeastern Massachusetts.

"Of these swamps, the most notable is the Great Cedar-Swamp so prized for its timber in the early days of our history. These swampy lands have very little value now; but they contain abundant promise of making the best farming portions of the section. They only need thorough draining in order to utilize their deep, rich, vegetable deposits, and turn them into fertile fields. The day is coming when this will be done." -- W. Chaffin, History of Easton, 1886.

By Chaffin's description of the Hockomock Swamp in 1886, he established that the Hockomock Swamp was originally an Atlantic white cedar swamp and by the time of his generation, the Hockomock had been almost completely logged of its cedar and 'had little value now.'

In this recent overview, I presented aerial photographs from 2001-2008 showing the contemporary extent of Atlantic white cedar stands in the Hockomock Swamp.

The scientific literature on Atlantic white cedar swamps suggests a combination of several forces -- clear cutting of cedar, changes in drainage patterns due to road and rail bed construction, and an asymmetrical dominance relation with swamp red maple -- have conspired to create permanent changes in the balance of cedar and red maple in the Hockomock to the great detriment of cedar.

Based upon these factors, it is unclear if the extant coverage of white cedar in the Hockomock can or will increase in the future on its own; if the remnant cedar stands in the swamp today are in decline and will continue to decline; and if active remedial efforts are necessary to increase the size of the stands or, perhaps are necessary to prevent further loss over time.

Laderman (1989) offers some sobering thoughts with relevance to the Hockomock:

"Hardwood and shrub leaf litter inhibit cedar germination to less than one percent."

"The floor of a wetland previously supporting Atlantic white cedar is the most favorable substrate."

"Cedar swamps have generally higher water levels than nearby red maple swamps and are flooded for longer periods."

"It would be expected that definitive guidelines for management of a tree that has been harvested since the first Europeans settled on the continent would have been developed long ago, yet this is not so. As with many other plentiful resources in the early days of development, the supply of cedar seemed endless. When all the cedar that was easy to remove was gone, the operators moved on. If less desirable cedars remained, they were commonly taken for fence posts, shingles or even firewood. Fast growing hardwoods often replaced cedar, and the nature of the forest changed."

If Laderman's conclusions are true, this offers little hope for natural regeneration and expansion of Atlantic white cedar in vast portions of the Hockomock where it is now completely absent; and little hope for the expansion or even continued maintenance of small, isolated stands.

Atlantic white cedar are now completely absent from the Hockomock in a zone extending east from the old railroad bed, to Route 138 and to the abandoned Maple Street/Hall Street road grade from Raynham to West Bridgewater, except for one small, isolated stand just west of Maple Street. This area comprises a large portion of the Hockomock.

This aerial shows a complete absence of cedar east of the railroad grade (yellow line) to Route 138 (red line) and beyond. The dark green areas are cedar. The question is: are these green zones of cedar shrinking or growing? If they are shrinking, then we are watching the demise of the last large remnant of the Great Hockomock Cedar Swamp.

If we assume this entire area was heavily logged for cedar in the 18th and 19th centuries (which general information suggests was the case), the aerials suggest that the area west of the railroad grade successfully regenerated in cedar while the swamp east of the railroad grade did not. What is striking is the high density and large expanses of cedar all the way up to the railroad bed and its complete replacement by swamp red maple on the other side of the rail bed. Having walked through this area extensively and field checked the aerials, I can confirm that cedars are completely absent east of the rail bed to Route 138 and the swamp in this area is virtually pure red maple.

The pattern exhibited at the 1880 railroad grade suggests it dammed the swamp enough to 'dry out' the swamp east of the grade, thus making conditions more suitable for colonization by red maple which now totally dominate the Swamp to Route 138 and almost to Route 24. This fits a pattern seen across the swamp, wherein extant cedar stands are invariably found 'above' (ie. upstream) of these roadbed dams (where the water is impounded) and are absent directly below them (where the roadbed has made the swamp slightly drier). This presence/absence pattern is seen at the north/south crossings of the Old Colony railroad bed, at the abandoned roadbed at Maple Street and at Route 24.

It is difficult to look at the aerial image above and not see that the white cedar swamp originally extended well to the east of the railroad grade and was somehow, in the past, truncated by it. But if our model assumption is true that all of this area was logged for cedar in the 19th century, something other than logging alone caused cedar to fail to regenerate east of the railroad grade. The most likely answer is a change in seasonal water table due to the grade itself that favored red maple on the 'dry' side. If we assume the area on both sides of the railroad grade was logged equally, we have to assume conditions were sufficient for cedars to regenerate from seedlings and small trees on the left side of the grade but conditions had changed enough on the right side of the grade to allow red maple to get the upper hand. And according to Alderman, once red maple gains a strong foothold in a logged-out cedar swamp its dominance becomes permanent. This appears to be the case from the railroad grade to Route 138.

The Uniqueness of the Hockomock

As noted by Laderman, most Atlantic white cedar swamps in southeastern Mass. were formed in kettle hole depressions after the Wisconsinan glaciation, wherein large blocks of glacial ice were buried by outwash debris and created isolated, deep depressions in the landscape after deglaciation. The Hockomock is 1-2 orders of magnitude larger than these small cedar swamps and is much more hydrologically complex (it's a lake bottom, not a kettle hole). The large expanse of the Hockomock and its numerous lobes has made it much more susceptible to man-made water level changes (due to large-scale road building, primarily) than much smaller and hydrologically simple kettle-hole shaped cedar swamps. Despite its larger size, I believe this has made it much harder for the Hockomock to regenerate its cedar stands after severe or total cutting because drainage changes have tended to fragment the swamp and cause large scale water table changes that tend to favor regeneration by red maple over cedar at a magnitude not seen in smaller, much more confined cedar swamps.

The area surrounding Nunkets Pond in the Swamp supports this hypothesis since (assuming a 1800s heavy cutting) its cedar have regenerated and its overall drainage pattern has not been altered:

While I have not extensively investigated this hypothesis, there does seem to be a correlation between the size of a cedar swamp and the degree its drainage patterns have been altered and its ability to regenerate and maintain its original Atlantic white cedar stands after severe cutting. Smaller, more hydrologically simple and isolated cedar swamps tend to be better able to regenerate and not succumb to takeover by more aggressive and generalist swamp red maple.

As a simple matter of area, you could fit a lot of small cedar swamps into the Hockomock. It's just that big. But regeneration of cedar after cutting during the past 150-200 years seems to be much more depauperate in the Hockomock than in smaller cedar swamps. Moreover, wide swaths of the Swamp today have the appearance of a monoculture -- red maple -- and what is known of cedar swamp dynamics suggest a 'tipping point' where a swamp dominated by red maple tends to stay that way permanently due to the highly specific germination and growth needs of cedar.

This suggests a disturbing hypothesis; either the extant wide swaths of pure red maple in the Hockomock were always pure and always there (which means most historical accounts were completely wrong), or the Hock has been radically and permanently altered by severe cutting of its cedar and drainage changes due to road building that favor red maple over cedar.

Why Should We Care?

As much as I like tromping about in the vast, pure red maple stands of the Hockomock, I can't help feeling I'm walking (slogging) through a man-made monoculture. Aerial views of the Hock show actual tree species distribution patterns that are explained better by human intervention than natural distribution of tree species and suggest human intervention in the last 200 years as radically and negatively affected the habitat value of the Hockomock, which after all, was preserved in the 1960s solely for its value has habitat, particularly for rare and endangered species requiring large amounts of undisturbed natural habitat.

Laderman (1989) reports unique and high habitat values for large intact Atlantic white cedar swamps, all the more important in Massachusetts because most of its cedar swamps no longer exist:

"In the Northeast, a preferred winter browse for white tailed deer is white cedar foliage and twigs. Cottontail rabbit and meadow mouse and feed of cedar seedlings ....

"Cedar stands in the Great Dismal National Wildlife Refuge supported the great bird density of coniferous forests censused in the United States in 1981. These stands held nearly twice as many birds per unit area as a surrounding maple-gum forest. Parulid warblers are the most dominant birds in Great Dismal cedar stands; prairie, prothonotory, hooded and worm-eating warblers, oven birds and yellowthroats comprised about 3/4ths of the breeding birds found. Prairie and worm-eating warblers appear to be particularly dependent on the Great Dismal cedars. An "over mature" stand, one with most trees over 100 years old, was particularly well populated."

So as a general rule, the extent to which the Hockomock today has veered away from its natural condition detracts from the very values for which it has been recently preserved. What's missing is an analysis of to what extent and in what fashion the Hockomock no longer resembles itself, as defined by its character prior to being mauled by loggers in the 19th century and road builders in the 19th and 20th centuries.
Laderman, A.E. 1989. The Ecology of Atlantic White Cedar Wetlands: A Community Profile. U.S. Dept. of Interior. Biological Report 85 (7.21). Washington, D.C.


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