OTTAWA — Over the past decade, Trillium Woods, west of Ottawa, have become increasingly inappropriately named, with fewer and fewer of Ontario's official flower appearing every spring.
A similar decline can be found in other trillium forests. The cause is not pollution, or drought, or logging, but something we've been taught to see as a promoter of healthy soil — the ordinary earthworm.
"The old sugar maple forests that people think of as having wall-to-wall white trillium have less of it. It's not gone, but there's less of it," says Ottawa naturalist Dan Brunton.
Forests in southern Canada are subtly changing. Thinned out, and in places eliminated, are the smaller trees (seedlings, saplings and eventually the sub-canopy) and a diverse mix of forest grasses and grasslike plants, orchids, ferns, wild leeks, mosses and shrubs.
And the perpetrators are wriggling underground.
Worms don't belong in Canada. The last Ice Age killed them all as far south as Maryland and West Virginia, and moving at worm speed, they didn't return to Canada in the next 15,000 years as forests grew back and animals returned.
The Europeans brought the first worms, and the numbers have increased with arrivals in the root balls of plants from Europe and Asia, and imports for gardening.
Long established in cities and on farms, worms are moving into forests on mud stuck to logging trucks and all-terrain vehicles, and by fishermen dumping leftover bait.
So, what's wrong with worms? They fertilize and aerate the soil, which is a good thing, as every farmer and gardener knows. But in forests, they physically alter the soil in ways that destroy the system of nutrients and moisture.
Key to everything is an organic layer on the forest floor several centimetres thick known as leaf litter. Leaves that fall from maples, oaks and other hardwood trees can take up to three years to decompose in a natural forest without earthworms. There are new leaves on top, half-decomposed leaves beneath that, with white, stringy fungus attaching the bits together, organic soil next, and finally mineral soil (sand and clay).
Pennsylvania horticulturist Dennis Burton compares the leaf litter to the skin of the forest floor.
"It retains moisture, protects the organs (roots), breathes, prevents erosion, deters pathogens (non-native plants), and promotes seed germination," he explains. "A nutrient balance has evolved in this stable system between the vegetation above ground and the enormous biosphere in and below the leaf litter. When that system loses its leaf litter, it is like puncturing your skin. Erosion follows and nutrients bleed quickly from the soil."
The soil of a healthy hardwood forest is soft and spongy and full of decaying material that's rich in nutrients. That feeds insects, and it provides a steady drip of nutrients in rainwater to trilliums and other plants rooted just under this layer, in the so-called mineral soil.
The trouble with worms is that they eat most of the fallen leaves each year, exposing bare soil and releasing a burst of nutrients in a hurry, leaving none in reserve. They physically churn the litter into the soil, like miniature rototillers. They expose tree roots. A night crawler can drill holes two metres deep in the soil, which drains off surface water.
Besides losing some trilliums (and other wild flowers), the forest invaded by worms loses much of its understory, the thick mass of young trees and smaller plants that are low to the ground. In a healthy hardwood forest, these are dense enough to make walking difficult. In a worm forest, everything is more open. There may still be an array of mature trees, but there's less under them.
"Anywhere there's human settlement, there are earthworms," says Scott Loss, a post-doctoral research at the Smithsonian Bird Migration Center in Washington.
"You pretty much can't find a place without earthworms within 50 miles of a big city."
As well, the University of Minnesota tracked earthworm populations in national parks in that state, and found they are most common around boat launches, cabins and logging roads.
Once they arrive, they stay. There's no way to kill or to remove them.
"The most important thing is to try to identify earthworm-free areas and try to prevent introduction of them into these areas," Loss said.
But as well, there are many species of worms that have different habits. Some live near the surface, others live deep down. If an area only has one or two species, it would still benefit from keeping others away.
Loss has just finished a study of a surprising effect of earthworms: They change the bird population.
You won't see an ovenbird — a little brown songbird with a big voice — in the city. But it's common in forests in our region.
It builds nests on the forest floor, covering its eggs with a dome-shaped pile of leaf litter. A long time ago, someone thought the nest looked like a Dutch oven. The name stuck.
(Robert Frost calls it "a singer everyone has heard/ Loud, a midsummer and a mid-wood bird/ Who makes the solid tree trunks sound again." It's a gloomy little poem, but a good description.)
Loss's study shows the bird's population has dropped wherever earthworms have established themselves in forests.
"In the forested areas, it's one of the most common birds in North America," he said.
"It's one of these birds that have received a lot of research and conservation attention — these long-distance birds that fly from . . . northern parts of North America down to Central America and the Caribbean.
"Some people might consider them to be a good indicator of the state of forests in North America because they're common and occur across a lot (of areas)."
The reason for the ovenbird's decline may have several aspects.
The bird needs a fair bit of concealment from predators that would eat their eggs, such as squirrels, raccoons and other birds.
The survival rate of young ovenbirds rises where there's a thick understory and lots of leaf litter.
As well, less leaf litter appears to thin out the bug population. Ovenbirds eat insects. Loss thinks they can still survive, but they may need to stake out a larger territory to find enough food, which means a square kilometre of forest can support a smaller population than before the worms arrived.
There is a confounding factor in studying changing forests. Many have a second invader that's also destroying the understory: deer.
The white-tailed deer population has exploded across Ontario and Northern U.S. states, and forest managers say deer are chewing up many of the plants near ground level, not only young trees, but also shrubs (including blueberries) and flowers such as trilliums.
"Trilliums are like candy to white-tailed deer, regardless of how many other plants there are," Loss notes.
Loss still gets funny reactions when he tells people he's investigating earthworm damage.
"I guess a lot of people are told at an early age that earthworms are really good," he says. "It might sound a little strange that you have to watch out for earthworms but they really are capable of causing some significant changes in the forests of the Northern U.S. and much of Canada."
Read more: http://www.canada.com/health/Earthworms+ruin+nutrients+moisture+forest+floor+researchers/6280575/story.html#ixzz1ohWQajnk
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