Floodplain Forests: Background Exploration Floodplain Forest: A Definition
Floodplain Forests grow alongside streams on soil created by stream
deposits (alluvial soil) and are subject to occasional flooding by the
adjacent stream. Consequently, their plants and animals experience
extreme changes in environmental conditions throughout the year. A
spring flood might cover the ground for a few days with rushing water,
several feet deep, which retreats back into the stream bed as quickly
as it went overboard. Alternatively, the floodwater might get caught in
quiet backwater areas and take longer to recede through slow
percolation through the soil. However, most of the time, the soil of
floodplain forests is well aerated and, consequently, floodplain
forests are usually not considered wetlands. The big difference between
floodplain forests and swamp forests (which, confusingly, can sometimes
also be found in floodplains) is that the soil of swamp forests are
saturated most of the time, making them true wetlands. Floodplain Forests as Rare Habitat
Three decades ago, it had been estimated that 70% of the natural
riparian plant communities in the United States had already been
destroyed 1). Our own studies in Dutchess and Columbia counties 3,4) confirmed
that floodplain forests are rare in our region, and currently cover
only 1/3 of the suitable soils along bottomlands of larger streams and
some of their (2 nd and 3 rd order) tributaries in both counties. Map
of Columbia County indicating floodplain soils (green), as well as
ancient (red) and recently reforested (yellow) floodplain forests (to enlarge image, please click on map) Map
of Dutchess County indicating floodplain soils (green), as well as
ancient (red) and recently reforested (yellow) floodplain forests (to enlarge image, please click on map)
Ancient floodplain forests, those which have likely never been
completely cleared for agriculture (although they may have seen a
variety of human activities, including selective logging and garbage
dumping), are even rarer. Less than half of the currently forested
floodplain areas in Columbia County and less than a third of those in
Dutchess County represent ancient floodplain forest.
Furthermore, most floodplain forests, and especially the remnants of
ancient floodplain forests, occur in small, isolated patches.  Patch Size of Remnant Ancient Floodplain Forests in Dutchess and Columbia Counties(to enlarge, please click on image) Floodplain Forests as Biological Hotspots Floodplain forests can be biological hotspots for species diversity 2,5).
During our two-year study of 31 floodplain forest sites in Columbia and Dutchess counties, we documented a large
diversity of plants and animals in these habitats.  Green Dragon, a relative of the more common Jack-in-the-Pulpit, is a plant unique to floodplain forests.
We recorded 442 species of vascular plants (including 47 regionally
rare or uncommon species) and 25
species of mammals (plus a land owner’s report of the rare Indiana Bat), 46 species of birds, 12
species of amphibians and reptiles (including the rare Box and Wood
Turtles), more than 20 species of butterflies
(including the rare Hackberry Emperor and American Snout, and the
uncommon Question Mark and Spicebush Swallowtail), and 45 species of dragon- and damselflies (more than
10 of these were new county records, including Brook Snaketail,
Spine-Crowned Clubtail, Arrow Clubtail, and Blue-tipped Dancer, all
species of greatest conservation need), 59
species of native bees (mostly new county records), and 85 species of ground beetles (35 of which are considered
rare or uncommon in our region).  The
Hackberry Emperor (above) and American Snout (below) are two
regionally-rare butterflies closely associated with floodplain
forests. Their caterpillars feed exclusively on the leaves of
Hackberry, which occurs mostly in floodplain forests in our region.   Spicebush
Swallowtail is one of the showier butterflies sometimes encountered in
floodplain forests. Their caterpillars feed exclusively on the leaves
of the Spicebush, a native shrub that favors rich moist soils,
including floodplain forests.  Eastern Comma caterpillars feed on nettles, a group of plants commonly encountered in floodplain forests.  Brook
Snaketail is one of several dragonfly species closely associated with
streams. Its aquatic larva develop in clear, running water.
For many of these species, floodplain forests are not the only habitat
where they occur. However, we found more than 50 plant species which,
in our experience, occur almost exclusively or mostly along streams, at
least in Dutchess and Columbia counties. More than half of the
documented dragon- and butterfly species were classified as stream or
river species, whose aquatic larvae develop in running water.
Half of the native bees recorded in the floodplains were not found in
adjacent farm fields. More than half of the ground beetle
species were classified as associated with water. For other
species (many birds and mammals, including bats), forested stream
corridors provide migration routes and resource-rich areas where they
come to feed. Floodplain Forests Provide Ecological Services In addition to being hotspots for biodiversity, floodplain forests provide a variety of important ecological services 2,5).
For example, they supply high quality organic detritus to the stream
where it creates shelter and serves as the base of the food web for
stream organisms. Forested river banks help to minimize soil erosion
and filter surface runoff before it reaches the stream, thereby
maintaining stream water quality. Floodplain forests might also play a
role in diffusing the down-stream intensity of flooding. Different Types of Floodplain Forests
The floodplain forests in our study grouped into 5 forest types
according to their dominant tree species:
‘Sugar Maple Dominated Floodplain Forests’ were ancient forests on
relatively stable terraces where they might get flooded on average less
than once a year.  A Sugar Maple - dominated Floodplain Forest along the Taghkanic Creek.
‘Elm – Sugar Maple – Bitternut Floodplain Forests’ and ‘Elm – Ash –
Black Cherry Floodplain Forests’ were located lower in the floodplain
where, on average, they may receive at least one flood each year, which
might last for several days. The latter type occurred in locations
where stream activity frequently reshaped the channel. It may represent
an earlier successional stage of the former type, which was found in
somewhat more stable locations.  An
Elm - Ash - Black Cherry Floodplain Forest along the Roeliff-Jansen
Kill. The herbaceous layer is dominated by Ostrich Fern, a plant that
almost exclusively is found in floodplain forests.(cont. bottom part of right column)
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| 1/27 column: "Forests Under the Spell of Streams"
by Claudia Knab-Vispo
The
depth of winter is a good time to go explore a floodplain forest. Put
on your snow-shoes or warm winter boots and tromp right into one of
those magical forests that lie alongside streams big enough to
occasionally go overboard and inundate their low banks and forests. If
the going is easy and not many thorn bushes bar your way, you will
likely find yourself in an “ancient” floodplain forest: a place where
trees have reigned for a long, long time and their shade prevented the
establishment of a significant shrub layer. These ancient floodplain
forests are rare in Columbia County, both because their fertile soils
were often cleared for agricultural fields, and because their vicinity
to a stream carries the risk that trees might be prematurely destroyed
by a flood.
Such a forest might be dominated by towering
Sycamore trees that create a cathedral-like feel with their straight
trunks characterized by light colored, smooth bark that peels off in
plate-sized pieces and leaves the tree bole looking distinctly blotchy.
This signature tree of certain floodplain forests is also very familiar
as one of the most beautiful ornamental trees in city parks. Looking up
into the Sycamore branches, you might still see the little pompons that
will eventually release the tiny dandelion-like seeds, which germinate
best on the sunny beaches and gravel bars of streams.
Sycamore
is often accompanied by another tree of gigantic dimensions, the
Cottonwood. Without raising your eyes to the canopy, the Cottonwood
trunks are easily distinguished from those of Sycamore by their darker,
thick and deeply furrowed bark. Come back to the same spot in early
June, and you might think it is winter again, judging from the white
fluff covering the ground all around the Cottonwood trees. Upon closer
inspection, one can see that this white fluff helps the Cottonwood
seeds ride on the wind and so escape the shade of their parents and
find a suitable spot to germinate on those same sunny, gravelly beaches
favored by the Sycamore seedlings.
We were fortunate to have
been able to spend two summers in a variety of floodplain forests in
Columbia and Dutchess County, documenting the plants and animals that
live in them and determining which species occur exclusively in this
habitat and nowhere else. During our studies, we learned that not all
floodplain forests are characterized by the same tree communities. For
example, the Sycamore- and Cottonwood-populated forest described above
tends to grow in areas where the river can develop great force and
deposit coarse materials, such as sand and pebbles during floods. By
contrast, Silver Maple trees, often with multiple trunks of enormous
girth, tend to be found around quiet backwater areas where they are
exposed to deep and prolonged flooding with slower-flowing water that
leaves behind fine-textured, silty deposits.
During a winter
tromp through a floodplain forest, you might be impressed by the piles
of debris, natural and not so natural, that have accumulated on the
upriver side of some of the tree trunks and are evidence of past
floods. Spring floods can be awesome and a great opportunity to
experience the untrammeled power of the elements. Most mind-boggling to
me is the realization that, under this calamity of gushing and swirling
waters, there is life. The tubers and bulbs of many plant species
patiently ride out the storm, ready to present their leaves and flowers
to the world as soon as the flood has subsided. For example, during May
in these forests, one can find an array of delicate native spring
flowers, such as Bloodroot, Dutchman’s Breeches, Toothworth, Trout
Lily, and Spring Beauty, as well as the robust Green Dragon, which
looks like it could be a tropical plant escaped from a flower pot. I am
particularly fond of the inconspicuous, little, green False Mermaid
Weed, which, unique among the spring-flowering plants of the
floodplains, is an annual plant that germinates each April from seeds
buried in the silty deposits. After covering the ground with its
dense, yet delicate green carpet, the False Mermaid Weed completes its
entire life cycle from seed to seed within less than three months, and
by mid-summer not a trace of it can be found… The Green Dragon and the
False Mermaid Weed are floodplain forest specialists that never grow
anywhere else and, even then, are not found in all floodplains.
But I am getting ahead of myself, dreaming about what lies asleep below
the thick cover of mid-winter snow. Right now, aside from the stolid
trees, there are other, more active signs of life. If the stream is not
frozen solid, one might be lucky enough to catch a glimpse of a
Kingfisher as it flies along the stream-highway in its characteristic
undulating line of flight. It often announces its arrival with a loud,
persistent, woodpecker-like chatter. To think that such a small bird
relies on diving into the ice-cold water to find its food in the middle
of winter!
Another lucky sight in a floodplain forest is the
occasional evidence that an animal had some plain old fun in the snow.
River Otters love to slide down steep, snowy stream banks. Their
careless tracks in the snow are often the most visible evidence that
these rare and secretive creatures inhabit a certain stretch of stream.
A much smaller and even harder to find inhabitant of the wintery
floodplain is the Water Shrew, a mouse-sized furry creature that is
active year round, hunting in and out of the water for insects and
other invertebrates.
There is definitely not enough space here
to share all the marvels that we were privileged to observe in this
special habitat of our back-yards. But if this little essay has made
you curious about floodplain forests, there are a few publicly
accessible examples of floodplain forests in the county that you can
visit any time to make your own observations: Crellin Park along Stony
Kill in Chatham, River Street Park along the Kinderhook Creek in
Valatie, and the Siegel-Kline Kill Columbia Land Conservancy Public
Conservation Area along the Kline Kill in Ghent.
If you would
like to read more about floodplain forests, see a map of ancient and
recently reforested floodplain forests in the County, learn how to
distinguish the five different types of floodplain forests found in our
area, discover which rare plants and animals live in them, explore the
subtle and not so subtle differences between ancient and recently
reforested floodplain forests, and better understand why it is
important that we make a special effort to preserve our last remaining
examples of ancient floodplain forests, you can access our floodplain
forest reports on www.hawthornevalleyfarm.org/fep/floodplains.html. The floodplain forest research was supported by the Hudson River Estuary Program and the Biodiversity Research Institute.
Additional background information on this and previous “Perspectives on Place” columns is available on our website, at /fep/columns.html. We invite your comments, questions, and observations of the landscape. Please send to: fep@hawthornevalleyfarm.org
A lone Sycamore tree in a floodplain pasture at Hawthorne Valley Farm.
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Background Exploration (cont.)
The Elm
– Sugar Maple – Bitternut forests might in turn, over time and with
increasing distance from the creek and decreasing disturbance, succeed
into a Sugar Maple – dominated forest. ‘Black Locust – Sycamore –
Cottonwood Floodplain Forests’ were recent forests in the most dynamic
locations within the floodplain, where they colonized mineral soil that
had been deposited in major events of creek bed re-working.
‘Green Ash
– Silver Maple Floodplain Forests’ largely occupied the relatively
quiet backwater parts of the floodplain and, barring major re-working
of the creek bed, seemed to be quite stable, largely self-perpetuating
communities.
A Green Ash - Silver Maple Floodplain Forest along the Roeliff-Jansen Kill
Each of these forest types had a different set of herbaceous indicator
species and somewhat different physical
characteristics.
Within each of the five
forest types, we distinguished seven micro-habitats based on their herbaceous plant indicator species, density of
herb cover, elevation within the floodplain, distance from creek, soil
texture, and canopy cover. Most of these micro-habitats occurred across
all forest types, but differed in their frequency among forest types.
Ancient Floodplain Forests are Unique
Ancient floodplain forests had a significantly higher diversity of
native herbaceous plants than recently reforested floodplains.
Comparison
of native herb diversity in ancient (black bars) and recently
reforested (grey bars) floodplain forest transects in the different
forest types. Please click image to enlarge.
They also had significantly lower densities of invasive shrubs.
Comparison
of invasive shrub density in ancient (black bars) and recently
reforested (grey bars) floodplain forest transects in different forest
types. Please click image to enlarge.
We suspect that the
lower native herb diversity in the recent floodplain forests resulted
from a combination of lack of colonization, competition from invasive
shrubs, and, possibly, the impact of deer browsing on vulnerable, newly
colonizing plant populations.
We conclude that ancient floodplain
forest remnants are ecologically unique and potentially irreplaceable.
Only 16% and 10% of the original extent of this habitat type remain in
Columbia and Dutchess County, respectively. Ancient floodplain forests
deserve high priority for conservation, especially in the few areas
where large ancient floodplain forests remain. Nobody knows where the
natural succession of recent floodplain forests will lead, but the
re-colonization of their native herb communities might be actively
promoted by removal of dense invasive shrubs and the introduction of
seeds or enrichment planting, especially if deer browsing can be
limited.
Ancient Green Ash - Silver Maple Floodplain Forest along the Roeliff-Jansen Kill
Acknowledgements
The floodplain forests study was partly supported by grants from the
Hudson River Estuary Program and the Biodiversity Research Institute.
We thank the landowners who welcomed the study of their forests, the
teams of interns and volunteers involved in this fieldwork and data
processing, our colleagues from Hudsonia Ltd for their work on the
Dutchess County floodplains, as well as all the other scientists and
institutions who helped with different aspects of this study.
References
1)Brinson, M.M., Swift, B.L., Plantico, R.C., and J.S. Barclay. 1981. Riparian ecosystems: their
ecology and status.US Fish and Wildlife Service OBS-81/17. Washington.
2)Burton, M. L. 2006. Riparian woody plant diversity, composition, and structure across an urban-rural land use gradient in the piedmont of Georgia, US. Ph.D. Thesis, Auburn University, Alabama. 152p.
3)Knab-Vispo, C. and C. Vispo. 2009.The Plant and Animal Diversity of Columbia County, NY Floodplain Forests: Composition and Patterns. Farmscape Ecology Program, Hawthorne Valley Farm, Ghent NY. 63p + appendix. LINK TO WEB-site
4)Knab-Vispo, C. and C. Vispo. 2010.Floodplain Forests of Columbia and Dutchess Counties, NY: Distribution, Biodiversity, Classification, and Conservation. Hawthorne Valley Farmscape Ecology Program in cooperation with Hudsonia Ltd., Ghent NY. 67p + appendix. LINK TO WEB-site
5)Naiman, R.J., Décamps, H., and M.E. McClain. 2005. Riparia: Ecology, Conservation, and Management of Streamside Communities. Elsevier Academic Press, Amsterdam. 430p.
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