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Friday
Oct272017

The Forest Understory: Diverse. Dynamic. Difficult

Wild sarsaparilla is not a flashy forest dweller. It doesn’t soar overhead, or have showy blossoms. Or produce copious amounts of fruit or nuts that benefit wildlife. Or even envelop large areas of the forest floor.

In fact, it is quite a nondescript little plant, though one with a disconcerting, but superficial, resemblance toWild Sarsaparilla poison ivy. Andrew Whitman, an ecologist and the director of the Sustainable Economies Program at Manoment, Inc. formerly known as Manomet Center for Conservation Sciences, has done extensive research on understory plants in Maine. It’s one of his favorites.

“It has an interesting ecology. What you see is not what you get,” Whitman said.

For instance, that wild sarsaparilla plant growing over there isn’t just one plant, it’s one leaf, though it looks like many. For another, the wild sarsaparilla plant growing way over that way? Well, that’s probably not a different plant. It’s the same plant. If you dug up all the wild sarsaparilla plants in an area of forest you would “find that all those stems are likely connected,” Whitman said. “An individual might have five to 50 aboveground leaves sticking up, connected by rhizomes. One individual might cover 200 or 300 square yards.”

Wild sarsaparilla illustrates one interesting adaptation to life in the forest understory. It literally travels — its rhizomes do, anyway — through the woods looking for a suitable spot with enough light to bother putting up a new leaf. “They move around the forest,” said Whitman. “Slowly, but they move. The individual leaves sticking up above the ground, well, you can age their stems and they might be 45 years old. The plant itself might be 100 years old.”

The forest understory is vibrant and throbbing with life. But it’s too often overlooked. That’s kind of indicated by how we describe it: the common definition is pretty much anything beneath the canopy. That encompasses a lot of territory and a lot of different types of plants. Of course some scientists parse it much more finely, breaking it down into the herbaceous layer and the shrub layer and so on. For practical purposes, you might think of the understory as anything that grows from ankle high to perhaps as high as you can reach, or slightly higher.

This includes everything from forest wildflowers to shrubs to shade-dwelling tree species that might get 20 or 25 feet tall, but are usually much shorter. The understory also includes other tree species — seedlings and saplings — that are only hanging out there temporarily, waiting for more light so they can bolt for their true home in the canopy.

But no matter who you are, the forest understory isn’t an easy place to make a living.

“This environment is not a favorable one for plant growth. Light levels are often very low, and nutrients can also be limiting, partly because of competition with canopy trees. Species typical of such habitats can perhaps best be described as stress tolerant,” writes Joseph A. Antos, a biology professor at the University of Victoria in British Columbia in an article on understory plants for the Forests and Forest Plants section of UNESCO’s online Encyclopedia of Life Support Systems.

Whitman notes that canopy trees can gobble up 95 percent of the light.

Low light levels make seedling establishment really difficult, notes Antos in his paper on “Understory Plants in Temperate Forests.” But understory plants are also subject to getting buried by debris from the canopy, to getting stepped on or eaten by browsing herbivores like moose and deer, or to having their environment change overnight due to some disturbance in the canopy.

“Understory species have numerous ways of dealing with the adversities of this environment, such as large seeds, storage organs, a long lifespan and the ability to tolerate very low light levels,” writes Antos.

One of the more common adaptations, he notes, is the one employed by Whitman’s wild sarsaparilla: “Herbs and shrubs that spread via rhizomes are especially common in many forest understories, and can form large patches. Much of the patchy nature of forest understories relates to clonal growth, although variation in the canopy is also of fundamental importance in inducing spatial structure,” Antos writes.

TrilliumsAlso among the different survival strategies when it comes to light is one employed by many forest wildflowers in Maine, from trilliums to Canada mayflower to lady’s slipper: get growing early in the spring before the canopy trees leaf out. Which is why many species bloom from April to June.

Other understory wildflowers in Maine include common wood sorrel, goldthread, blue-bead lily, trout lily, twin leaf. In northern Maine there tends to be more variety in violets, said Whitman, or things like baneberry or bunchberry.

Then there are the ferns: rattlesnake fern, cinnamon fern, bracken fern, hay-scented fern, ostrich fern, intermediate wood fern and others. The shrubs include everything from beaked hazelnut to hobble bush, rosebay rhododendron to flowering dogwood, nannyberry to low-bush blueberry. Understory trees can include striped maple, hawthorn, and witch hazel.

The exact makeup of the understory will vary from place to place, depending on soils and moisture and recent disturbances. It’s not uncommon for a wildflower or shrub to overlap different forest types. “The Canada mayflower can be found in a variety of forest types in Maine,” Whitman said. “Compare that with trillium, which will be concentrated on burned over areas or places with sandy soils.”

All in all, the forest understory in Maine is quite a diverse place. In fact, it boasts many more species ofRattlesnake Fern vascular and woody plants than the canopy does tree species. The northeast, however, doesn’t tend to have the most biodiverse understory in North America.. The low to mid-latitudes, from, say the mid-Atlantic states through the Midwest, where soils tend to be richer and the growing season kinder, tend to have more diversity, Whitman said. The cove forests of the Great Smoky Mountains, with their rich, deep and non-glaciated soils and copious rainfall, have a lush understory.

All in all, understory plants do a lot to help make the forest a thriving place. “It adds a lot to species richness, in terms of biodiversity, including species that are rare in Maine,” said Whitman.

It also provides food for mammals and birds, plays an important role in nutrient cycling, and provides cover and nesting structures. Early-season plants help capture nutrients that would otherwise be lost in spring runoff. Understory plants help moderate temperature extremes in the lower levels of the forest, and help conserve moisture.

“It’s a very dynamic place,” said Whitman. In a slow-motion sort of way. “Many wildflowers slowly ramble across the forest floor and occasionally get stuck where light is too low and die in place; woody plants stay in place, expanding or shrinking or even dying as light levels fluctuate; and trees surge upward starved to get light and make it into the canopy, some never making it, seen by us as the frozen dead, saplings, or small trees, kindling for a fire.”

Whitman spent years studying the impacts of logging on understory plant communities. His research never conclusively demonstrated that timber harvesting caused certain herbaceous plants to go extinct, though his teams did find that “logging does sort of simplify the forest understory communities. The generalists do better with logging, the rare species not so well.”

Perhaps of more concern is deer, particularly in southern Maine with its high deer population. “Deer can have a profound impacts in eliminating understory species, including forest herbs, but also seedlings and saplings . . .It’s a biodiversity threat.”

Japanese BarberryAnother threat is invasive understory plants like Japanese barberry or Japanese stiltgrass, which can crowd out native understory species and literally take over.

Longer-term, Whitman said climate change might pose a risk to some species or groups of species, particularly in the form of longer and more frequent droughts. Back-to-back droughts “may have a more profound effect,” he said.

 

Joe Rankin writes on forestry, nature and sustainability for magazines and websites.