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The River is a Restless Spirit - Life in the floodplain forest
by Gayle Goddard-Taylor
from Mass Audubon's Sanctuary magazine, March 2006 edition
(see www.massaudubon.org/support/support_benefits.php for membership info)
Added to website April 3, 2006
The River is a Restless Spirit
Life in the floodplain forest
by Gayle Goddard-Taylor
northeastern corner of Belmont in the floodplain of the Little River sits a
pocket-sized forest made up almost entirely of silver maples. A few of its
trees are approaching 150 years of age, relics of a bygone era when the land
upon which they stood was cropland or pasture that was regularly enriched with
the spring floods. Freed of competition from other trees and bathed in
sunlight, these trees grew fat in bole and branch and earned their keep by
providing shade for cows and sheep.
owned Belmont Uplands—some fifteen acres bounded by Route 2 to the north,
Little Pond to the west, Little River to the south, and commercial buildings to
the east—is no longer pastureland. The venerable old pasture maples are now
surrounded and overtopped by their much younger progeny. Some have lost massive
limbs or, in the case of the Mother Tree, have endured worse.
“Half of it fell
last spring,” says Ellen Mass, president of the Friends of Alewife Reservation
(FAR), inspecting the deeply lobed matriarch. It has been cleaved in two to
expose a hollowing core, half lying on its side, but both halves still
impressive in their girth. Last year, seven people could encircle the trunk
with arms outstretched and just touch hands.
Before it split,
the 119-foot-tall Mother Tree was considered one of the three largest silver
maples in the state. But what is perhaps more significant is that it is a
silver maple in a monoculture of its species that is the only forest of this
kind in the heart of metropolitan Boston. Friends of Alewife Reservation and
Belmont officials have been leading an effort to convince the state Department
of Conservation and Recreation (DCR) to ensure the land remains forested.
That this northern
floodplain forest is unique in an urban setting is only one of the reasons FAR
wants to keep the Belmont Uplands from disappearing beneath a proposed
condominium complex. Another is flooding, always a concern in the Alewife
watershed of Belmont, Arlington, and Cambridge, which has become more severe in
recent years, due in part to the loss of naturally pervious surface to pavement
and drainage construction. As small as it is, the Belmont Uplands still provides
valuable floodwater storage, according to preservation advocates.
woods and marshes there serve as an important stopover for migrating birds, as
well as habitat for a wildlife population that ranges from muskrats and minks
to foxes and coyotes. Also, the site’s merit as an urban outdoor classroom has
prompted area educators to bring students there for hands-on learning.
forest—a woodland that is regularly inundated with water that moves into the
lowlands bordering a river—is a disappearing ecological treasure. While
flooding usually occurs in spring when snows melt and rainfall is more
abundant, it can also happen at other times of year with major rainfall events,
as was evident here in New England last fall. Floodwaters carry sediments from
higher up along the river’s course; and, as the force of the flow lessens, the
nutrient-laden sediments are deposited in order of weight, with the heaviest
rocks falling first and the fine silts last. This cyclical flooding regime continually
refertilizes the floodplain and nourishes the well-adapted trees and understory.
“It’s a rough
environment,” says Pat Swain, an ecologist with the Division of Fisheries and
Wildlife Natural Heritage and Endangered Species Program. “The trees that live
there have to be adapted to being flooded for months at a time and having
sediment around their bases.”
tend to be those that mature and set seed quickly. Silver and red maple,
willows, river birch, box elder, sycamore, and cottonwood are some of the more
common trees in this environment. “The floodplain forest tends to be less
diverse than its associated uplands,” says Swain, “because there are simply
fewer species that can survive here.”
a variety of floodplain forest types have been identified, but they can, for
the most part, be lumped into three categories. Major-river floodplain forests
exist along such watercourses as the Connecticut, Deerfield, and Housatonic
rivers in the western part of the state while small-river forests can be found
along the Ipswich, Concord, and Three-Mile rivers to the east. Transitional
floodplain forest occurs adjacent to smaller tributaries to the Connecticut and
along portions of the Housatonic.
While silver maple
trees dominate all three types of forest, the severity of flooding, soil
texture, and soil drainage dictate what other types of vegetation grow in each.
Cottonwoods and American elms are present along major rivers while, in
intermediate-river and small-river forests, cottonwoods are absent, replaced by
green ash and elm.
Flooding along the
major rivers is usually severe and produces permeable sandy soils through which
drainage is rapid. The powerful annual flooding regime means that shrubs find
little to welcome them. The habitat is suitable, however, for a herbaceous
layer consisting of wood nettle and ferns.
corridors make for rich birding opportunities; warblers, thrushes and other
songbirds feed in the insect-rich habitat and nest in riverside trees. The shady
pools along the bank attract wood ducks and hooded mergansers while larger
trees also provide perches for fish-hunting raptors.
Shrub layers are
still lacking, for the most part, in intermediate-river forests, but the soils
are somewhat different. Sandy or silty loam is more the norm, making drainage
less rapid, and the number of herbaceous species is greater. In the poorly
drained, small-river forest, some shrubs can get a foothold, most notably silky
dogwood and buttonbush. The herbaceous layer here is far more diverse and can
include water hemlock, water parsnip, and swamp candles.
At one time, the
forests that stretched along the rivers in southern New England were a major
ecosystem. With farming and subsequent development, floodplain forests have been
reduced to isolated patches and narrow strips. But, while the owners of homes
built upon floodplains may believe that dams and channels have tamed the
rivers, that notion could not be more untrue. The prospect of damage from
annual flooding may have been reduced, but the potential for the kind of havoc
wreaked by a large-scale flood still remains. In fact, the loss of floodplain
forest increases the prospect of damage because it decreases the land’s
water-storage and sediment-filtering capacity.
house on a floodplain is like putting your house on a railroad track,” says
John Kricher, biology professor at Wheaton College and author of A Field
Guide to Eastern Forests. “A floodplain forest should not conjure visions
of terra firma,” he says, noting that such woodlands are neither aquatic nor
terrestrial. “The important thing to remember is that a floodplain forest is
part of the river. It exists because of flooding from the river.”
A river is
restless. Over time, its waters move its course by grinding away the soil on
the windward side and depositing it on the other. Sometimes these changes
happen quickly, undermining embankments and toppling trees. Where those trees
finally come to rest, soil becomes trapped and builds up, assisting in changing
where the river runs. Landscape features, such as oxbows, form as the river
slowly carves a serpentine path and then, in times of raging floods, shortcuts
across land to leave a meander scar.
tell you that over time a river will occupy every part of the floodplain,” says
Pat Swain. “A river is constantly cutting and leaving meander scars and
detritus that invades the adjacent forest will often snap the trunks of young
trees, which, like the Hydra’s head, regenerate by sending up multiple trunks.
Where the floodwaters are at their fastest, the river’s banks are
scoured—stripped of vegetation, soil, even rocks and trees. And when the water
spreads out over the floodplain, it brings not only minerals but also organic
matter—from fallen leaves to fish—whose decomposition also boosts the soil’s
A forest that has
been scoured of vegetation in severe flooding regenerates through a variety of
adaptations. The floodwater actually aids in dispersing seeds, which in some
cases produce plants that send down deep taproots. Certain trees such as
cottonwoods and birches have short life cycles that allow them to grow to
maturity and disperse seeds quickly. Others waste no time sending down deep
taproots, and many species seed bank generations to come, waiting until
conditions are right, and then promptly take hold.
area is also vulnerable to invasion by exotic species such as purple
loosestrife, Japanese knotweed, and Japanese bamboo, all of which take
advantage of disturbed areas. “Along the Housatonic and Connecticut rivers,”
says Swain, “Japanese bamboo has gone from hardly there at all to being a major
problem. Once it gets in, the natives have a hard time germinating.”
Along the upper
reaches of the Connecticut, New England’s largest river, what remains of a
grand forest tapestry that once stretched unbroken along the river’s floodplain
is slowly being rewoven. The Department of Conservation and Recreation has been
acquiring key forested parcels as opportunities arise.
“There’s not a lot
of floodplain forest left,” says Terry Blunt, director of the DCR’s Connecticut
Valley Action Program. “Much of the agricultural land extends to within 150
feet of the river.”
But in the North
Hadley and Northampton areas, the effort has created three large forested areas
totaling well over 200 acres. One of these tracts, the 112-acre Floodplain
Forest Reserve in Hadley, has earned the designation of Representative Natural
Area, a distinction that implements cutting and management restrictions and
allows natural processes to take their course.
The bulk of the
reserve, a puzzle whose 12 to 14 separate pieces were reassembled over a
ten-year-period, is located where the Fort River joins the Connecticut. This
forest community—large cottonwoods reaching 30 inches in diameter, silver and
red maples, willows, and a sprinkling of sycamore and ash—will now be used for
research and limited recreation.
“It’s a prime
example of what the floodplain forest would have looked like before people
began encroaching,” says Blunt.
“Apart from preserving
ecological habitat, a wildlife corridor, and a greenway,” says Blunt, “the DCR
has some very practical reasons for restoring these communities.” The broad
areas of the floodplain forest are natural water-storage areas. During
significant floods—and such events can occur any time of the year—water
overspreads the flat wetlands and loses velocity. The trees act as filter
strips, holding back sediments and reducing the extent of downstream damage.
“There are still
privately held parcels that could be acquired,” Blunt says. “The biggest threat
isn’t housing because most communities have at least some kind of prohibitions
for building in a floodplain,” says Blunt. “The big issue is recreational
sales. Everyone wants their own riverfront campsite.”
For the tiny
silver maple forest known as the Belmont Uplands, the only campsites are those
occasionally used by the homeless. Housing, however, is a looming
threat. In February, a proposal for 300 apartment units on four and a half of
the parcel’s fifteen acres received approval for financing from the
Massachusetts Development Finance Authority.
Opponents want to
see the forest annexed to the Alewife Reservation. One scientific estimate
indicates that a development of this size would swell stormwater runoff by 2.26
million gallons a year. Additionally, they also cite a recent preliminary study
placing the hundred-year-flood elevation 28 inches higher than previously
established by the Federal Emergency Management Agency. This would put the
lowest floors of the buildings below state building-code requirements.
advocates have cited a variety of funding sources that could be tapped to
acquire the land and contend that leaving the naturally occurring floodplain
forest processes in place better exemplifies smart growth.
“Putting housing here
would be totally un-smart growth,” says FAR’s Ellen Mass. “Basically, the
people living here would be surrounded by a mosquito marsh.”
Gayle Goddard-Taylor is a field
editor for Sanctuary magazine.