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River Scenics - Spring 2009
RIVERSIDE PLANT COMMUNITIES ARE AMONG the richest in the Northeast. Where large rivers run swiftly through narrow valleys, a formidable number of species thrive in a variety of habitats that depend on river dynamics, especially ice-scouring and flooding. According to botanist Tom Rawinski of the U.S. Forest Service in Durham, N.H., these habitats dot the banks of the Connecticut River, which forms a border between Vermont and New Hampshire, and more than 1,000 vascular plant species may be found on its shores. “Although riverine communities are nationally rare,” says Rawinski, “they occur on many major rivers in New England and the East.” Among them: the Gauley, Potomac, Susquehanna, Delaware, Merrimack, Kennebec and St. John.
Depending on the nature of the shoreline, these plant communities may be riverside seeps, outcrops, cobble shores or grasslands. A handful of such sites in New York state are known as riverside ice meadows, federally listed by the U.S. Fish and Wildlife Service as a threatened ecosystem. They’re shaped by an ice-pack, typically 8 to 10 feet thick, that persists well beyond the first blush of spring in neighboring forests. Arguably the best example thrives on a steep, rugged Hudson River shoreline about 40 miles south of the river’s source in the mountains of the Adirondack Park. With the melting of the ice that blankets the rocky shore for as much as six months of the year, a stunning mosaic of wildflowers begins to bloom. They continue well into fall, when the fragile white stalks of nodding ladies’-tresses (Spiranthes cernua) appear. Geologists, botanists and other wildflower enthusiasts find the ice meadows endlessly fascinating. Parts of the site are also enjoyed by many – but not too many – who come to swim, fish or picnic.
Known as “South of The Glen,” or simply “The Glen,” these 165 acres were purchased by The Nature Conservancy from a power company in 1994. The site, which straddles the Hudson, became known as a botanically interesting area in the 1960s. In the 1980s, the combination of rare natural communities and plants at The Glen was brought to the attention of conservationists. Since then, more than 400 plant species have been identified there, according to Steve Young, chief botanist for the New York Natural Heritage Program (NYNHP), a partnership between The Nature Conservancy and the New York State Department of Environmental Conservation.
“We have dry and wet species, acidlovers and calciphiles all within a short walk,” says Greg Edinger, NYNHP chief ecologist. “It is one of the few places in the state where you can see all these species in one place,” adds Young.
Edinger believes that the heavy ice pack compresses and scours the plant life, stressing and effectively pruning woody species and keeping the area in full sun. He’s convinced that a particular kind of ice, called frazil ice, also contributes minerals and organic material that nourish the rich variety of flora. “The key thing about frazil ice,” he says, “is that it forms in suspension in the water in a super-cold environment and accumulates on the river bank. As it warms up, it doesn’t float away but melts in place.”
Embedded in the ice, Edinger explains, nutrients remain in place throughout the winter (as long as six months) and, with melt, are deposited on the shore. “With frazil ice, because it melts in place, it’s as if you put fertilizer on your field on top of the snow. As the snow melts it seeps gradually and steadily into the ground. If you put it right on the ground, it’s much more easily washed away by rain,” he adds. Frazil ice can be found on many rivers, but here it joins with a complex of other factors, including slope, bedrocktype rock and soil chemistry that combine to produce an unusually plant-rich site.
“There is seasonal flooding throughout the year, similar to a floodplain forest, except here it is more confined and the nutrient deposits are in a very narrow area. When the river floods, some nutrients do wash away, but persistent ice keeps them on site longer – which makes it different from other flood areas.”
Rawinski, on the other hand, sees the primary role of the ice “as a bulldozer that scours the bank, damaging weaker plants, breaking brittle stems, and keeping seeps and rock outcrops exposed.” This issue remains to be studied, but whatever the role of the ice we know it is essential to this ecosystem and that we can no longer rely on its annual appearance in its present form. In the Adirondacks, while little change has been noted in summer temperatures over the past century, winter temperatures have risen significantly, and snowfall and snow cover have decreased, according to “The Adirondack Atlas” by Jerry B. Jenkins with Andy Keal.
Although ice-out records have not been kept for the upper Hudson, early ice-out has been documented on many rivers in New England. In an analysis of more than 50 years of continuous records on 16 rural rivers in Maine, New Hampshire and Vermont, USGS hydrologists Glenn Hodgkins and Robert Dudley found that on the nine rivers with the longest records, the ice went out 11 days earlier in 2000 than in 1936. “Most of the change occurred from the 1960s to 2000,” says Hodgkins.
“We also have good evidence of rising mean water temperatures in the Hudson at Poughkeepsie [110 miles downstream],” says Dr. William H. Schlesinger, president of the Cary Institute of Ecosystem Studies in Millbrook, New York. “All this would lead me to believe that the ice will disappear earlier each spring, and that if this community depends on it, the community could be threatened.”
Visiting the Glen on a sunny summer day, it’s hard to think much about ice. “It’s a heat-sink,” says Rawinski. There is no shade, because the ice destroys or dwarfs most woody plants, and those like the dwarf sand cherry (Prunus pumila var. depressa) that cling to their bit of soil grow close to the ground, their branches spreading over the rocks rather than gaining height.
This harsh environment is home to more than a dozen seldom-seen species, including the New England violet (Viola novae-angliae), found nowhere else in the state, sticky false asphodel (Triantha glutinosa) and half a dozen sedges, such as brown bog sedge (Carex buxbaumii).
“These rare species are adapted to the extremes of this environment,” explains Rawinski. Others, like Canadian burnet (Sanguisorba canadensis), which unfurls its fringed white flag of petals in August, are very uncommon in the lower 48 states. Like the New England violet and the sticky false asphodel, the burnet is a more northerly species. “In Labrador, it’s everywhere,” says Rawinski. There are southerners, too, like the striking orange butterfly weed (Asclepias tuberosa), almost never seen this far north. “The element of time plays a role here,” Rawinski explains. “These river shores have been open areas for thousands of years. The more southerly species have had extended opportunities to colonize these sites. Or they might be relics of a time when the climate was warmer here.”
One thinks of a meadow as expansive, offering a broad perspective, and The Glen does that, yet it is also a very intimate landscape. One can’t see much detail from any distance, but to the close observer each step reveals a new world. Tiny harebell (Campanula rotundifolia), brook lobelia (Lobelia kalmii) and the delicate pink petals of smooth rose (Rosa blanda) peer out from between the cobbles near the gently lapping water.
The rocks, especially the outcrops, bear testament to the enormous energy of moving water. As if the river’s motion has literally been sculpted into them, their patterns – in shape, color and texture – seem to move and flow. Very few plant species occupy turf of substantial size; most tuck themselves between pebbles and cobbles or rise from silt and gravel a few at a time. Some, like Canadian burnet, can be seen in only two or three places. Others, typically the more common species like thin-leaved sunflower (Helianthus decapetalus), appear in small clumps throughout the landscape.
The nature of the rocks – stunning marble outcrops in some areas and pinkish boulders of granitic gneiss in others, cobble shores and gravelly bars – all contribute to the huge diversity and the juxtaposition of acid-loving plants and those found only in a higher-pH environment.
There are also minerals in the seeps – rivulets, pools and wet spots fed by groundwater. Rawinski notes three broad types of seeps that indicate soil and water chemistry: acidic, calcareous and mafic – a term used to indicate high magnesium and iron environments, similar to those found in certain fens in the Midwest.
“These are harsh microhabitats,” says Rawinski. And that’s where most of the rare species are found. “Often, the rarities are northern species that evolved in a similarly harsh environment,” he adds. “They persist here in the absence of competition from more common plants, which demand more nutrients than such areas can provide.“
Riverside habitats often have several distinct vegetation zones. A seepy area along the water may have wetland species such as sweet-gale (Myrica gale). Where the cobble shore is broad, there may be a zone of dry grassland, where little bluestem (Schizachyrium scoparium) and big bluestem (Adropogon gerardii) are prominent. Small shrubs may pop up throughout but gather farthest from the shore, where New Jersey tea (Ceanothus americanus) may grow side by side with interrupted fern (Osmunda claytoniana).
Much remains to be learned about the ecology of riverbank plant species and plant communities, not least of which is how they will respond to shorter winters. “How much less ice would it take to see an impact? We don’t know. Does it have to be 8 feet thick? Or is 5 feet enough to keep the woody species down?” asks Steve Young. “And how long does it have to last?”
Terrestrial Invasives Project Coordinator Steven Flint of the Adirondack Nature Conservancy has noticed some changes already. “What I’m seeing with this climate spike is that over the past 10 years some invasives are emerging 10 to 15 days earlier in spring and going dormant 10 to 15 days later in fall. In the past, we had harsh cold and an early hard freeze that protected us somewhat. That definitive shut-down is no longer taking place.”
None of the scientists we spoke to have noticed other changes, but amateur naturalist Evelyn Greene, a volunteer steward for The Glen, does something no one else, to our knowledge, has done: She has been observing the behavior of the ice there for two decades and trying to understand its relation to the plants. “In recent years, I’ve seen mid-winter washouts of seven miles of frazil ice buildup because of warm weather flooding,” she says.
There is constant change in every habitat. The composition of plant communities undergoes natural shifts, and severe wet or dry spells have many impacts. But it would be sad indeed to see places like the Glen transformed into, say, gentle grasslands.
Consider the dwarf sand cherry, utterly prone, hugging the rocks. “It reminds me of Japanese bonsai,” says Rawinski. “Its shape reflects the severity of its environment, how much stress it has endured. When we observe life forms in extreme environments, we wonder what adaptations allow them to defy the odds – how is this possible, that they survive?”
Perhaps such observations can help us tap our own resilience. It never hurts to be reminded that the human species has been known to endure some tough times too.