The Farmscape Ecology Program * KYPP Nugget 15: Feb.18th, 2011

KYPP Nugget: Floodplain Forests

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 destroyed1). Our own studies in Dutchess and Columbia counties3,4) 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 (2nd and 3rd 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 diversity2,5). 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 services2,5). 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)

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 our web site. The floodplain forest research was supported by the Hudson River Estuary Program and the Biodiversity Research Institute.

A lone Sycamore tree in a floodplain pasture at Hawthorne Valley Farm.

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

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.

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.
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.
5)Naiman, R.J., Décamps, H., and M.E. McClain. 2005. Riparia: Ecology, Conservation, and Management of Streamside Communities. Elsevier Academic Press, Amsterdam. 430p.