Global Worming

The Unexpected Threat to Northern Forests

Well friends, here we go again. There's a long list of threats to our forests that have come about as a result of our human activities. Now there's another serious invasive pest, one that we have welcomed, and thought was completely beneficial; one that has insidious behavior, yet seems so benign and desirable to have around. It's advancing into our northeastern forests, altering their composition, even destroying them. And there's nothing we can do to stop it. What hideous critter is this?


Scientists call it "Lumbricus terrestris". Most of us know it better as ........... the nightcrawler ! Actually, it's not just the nightcrawler that's a problem, but a number of earthworm species, none of which belong here.

The Forest Floor

 

Old growth forest floor

Ten thousand and more years ago, huge glaciers covered the northern portion of our continent, as far south as New Jersey in the east, west through the Great Lakes region to the west coast. No native earthworm species, if they were here prior to the glaciers, survived under the ice sheets, which were thousands of feet thick. In the post-glacial period, our forests grew back, following the melting glaciers northward. But earthworms only advance a couple dozen feet or so a year, so they didn't move northward as fast as the glaciers retreated; thus, our northern forests have existed for millennia without them.

Old growth hardwood forest-- no worm damage

Each autumn, as leaves fell, they accumulated on the forest floor; they were slowly decomposed by fungi and other organisms. But the breakdown was slow, and the organic duff layer was thick on the forest floor; in fact, that's what the duff layer is known as: "the forest floor".

Good duff layer, with green growth

Understory plants, including wildflowers, shrubs, and tree seedlings, take root in that layer, which lies above the mineral soil. They depend upon that carpet of duff for nutrients, and as a rooting medium. In a good site, without worms, that layer can be 4 or 5 inches deep. It is an insulating blanket, helping to protect from freezing temperatures in winter, and hot, drying conditions in summer. It's the zone where most rooting takes place, because the duff is well aerated and spongy (you can feel that as you walk on it), holds moisture, and contains the most organic nutrients. Roots that penetrate below the floor layer into the soil act as anchoring roots, but most nutrient uptake is done in the compost-like duff.

In an undisturbed forest, that floor layer becomes a giant web of mycorrhizae, which are root cells modified by the presence of certain fungi. Fungi can't photosynthesize like green plants do, so they can't manufacture their own food. Instead, they either obtain it by decomposing organic matter (in the case of one class of fungi), or through a complex partnership with plant roots (mycorrhizal fungi). In the latter case, the fungi help to make nutrients and water available to the plants, which in turn surrender food to the fungus. It's a symbiotic relationship, each partner benefiting from the other. (As an interesting side note, it's been reported that the largest organism on Earth is a fungus in Oregon that is thousands of years old, and occupies nearly 2400 acres!).

The largest percentage of plant and tree species (80% or more) have mycorrhizal relationships, which are highly important to plants, particularly in poor soils. Mycorrhizae excel at gathering phosphorous from the soil, and trees in nutrient-poor soil do far better when they can form mycorrhizae. That's how a proper forest should be working.

Today's Forest Floor Problem

Where the non-native earthworms have invaded the hardwood forests, they rapidly eat newly fallen leaves and twigs, consuming the duff layer, and mixing it into the lower soil layers via their movements into the ground. The duff layer is not replenished as quickly as it is eaten, and disappears. Earthworm activity dramatically decreases the mycorrhizal relationships, and thereby harms the plants that rely on them and the duff layer. The makeup of the soil layers is changed, to the detriment of the forest community that has occupied the land for millennia. Worms break up the organic layer to the point where important elements such as nitrogen, phosphorous, and carbon are leached deeper into the soil, where young plant roots simply can't reach them. Herbaceous plants can virtually disappear, leaving the ground barren and brown. Compare this photo of stark, worm-damaged woods to those of the undamaged old growth forests above:

Worm damaged forest

Pretty unattractive, isn't it?  Notice how there's no duff under the thin layer of leaves (not to mention the obvious lack of green plant life on the ground). The photo was taken in early September, 2014, at the height of the growing season, in a suburban forest.

There are some plant species that don't participate in mycorrhizal relationships, and may survive the onslaught. So you may see a diminished complex of plants, with a sparse variety of species, with things still looking green. For example, Pennsylvania sedge may take over the area. You may find Jack-in-the-pulpit, or False Solomon's Seal.

Some of the trees that appear to be particularly sensitive to the disappearance of the forest floor layer are Sugar Maple, Red Maple, Red Oak, Basswood,

Oak leaves are last to go

Serviceberry (Shadbush), and Hophornbeam. Their seedlings don't do well without the duff's benefits. Ash seedlings, however, can do well (but they're under severe attack now by the Emerald Ash Borer). Nightcrawlers prefer to eat the most nutritious leaves, so the fallen basswood and maple leaves tend to go first; when they're gone, oak leaves will be eaten. The population of worms will be greater in the forests with the greatest quantities of the leaves they prefer, so basswood/maple forests tend to be affected more than oak/beech woods.


The forest is a complex ecosystem, so animal species are also affected. With the presence of earthworms and resultant loss of the floor layer, tiny invertebrates and microscopic organisms decline, which can affect the entire food web, from amphibians and reptiles to birds, small mammals, and large mammals. Salamanders, for instance, go into decline because their young can't find enough of the duff layer-resident food they depend on. And with the drying of the uppermost ground layer, their habitat is degraded.

Where do these earthworms come from?

 

In the northern reaches of the U.S., where glaciers occurred, there are no native earthworms. It's believed that the non-native ones that are found here were first brought in ships from Europe. Soil was used as ballast in the holds of those ships, and it no doubt contained earthworms. Potted plants were also brought here over time. So that's what started it all. Now, we also have some from Asia. The movement of the invasion front radiates outward from wherever the worms are introduced; it happens much more quickly as a result of human activities than from the natural advancement of the worms.

Now that the worms have become established, we spread them by various means. We move topsoil from one location to another; fishermen may discard unused bait worms; gardeners encourage them in compost piles; well-intended community composting sites provide soil containing worms to anyone who wants it; landscaping practices move plants with soil and worms long distances. But eggs can also be transported in soil caught in the tread of tires.

See earthworm damage for yourself

 

Line of demarcation (damaged to the right)

You can see the loss of the forest floor layer for yourself, where these non-native earthworms have become established in the north. On their own, they advance slowly (thank goodness), and so there is often a fairly stark line of demarcation between the area where the worms are and where their presence ends. Look in hardwood forests (coniferous forests are less impacted) where you suspect these worms might be. During the growing season, there should normally be an obvious herbaceous plant community on the forest floor in most hardwood forests. That will probably be the case in more remote forests, where there has been little or no human development.

No duff layer under newest leaves

But in many areas with more recent human activity, you may find areas where there is little green growth on the forest floor. If so, you'll see last autumn's readily recognizable fallen leaves on the ground; brush the leaves away with your hands. If earthworms are established there, the leaf layer will be very thin,
maybe an inch or less. Immediately below them you'll likely find soil (as opposed to a thick layer of brown, compost-like material composed of less recognizable, partially decomposed leaves and twigs). If you have a compost pile, you may be accustomed to seeing lots of worm droppings that look like
roundish, crumbly bits of dirt... that's what you'll also find under the thin leaf layer in the woods. You may also see the telltale entrance/exit holes the crawlers make in the soil. If there happens to be a low, wet area adjacent to the barren area (perhaps the margins of a stream or swampy spot), you might see green growth there, because it's too wet for the worms.

Earthworm at entrance/exit hole

Now look around for the meandering boundary line where there is more herbaceous, green growth. It may be yards or more away. That's where the worms haven't gone yet.

On a recent spring forest walk along a stream with Bob Leverett (the old-growth forest expert) and Tim Whitfeld (the new Herbarium manager at Brown University), I got my first lesson in this invasion. There was an obvious lack of green plants on the lower hillsides there, only brown leaves on brown mineral soil (see the "Worm Damaged Forest" photo

Some greenery along stream; very little on higher ground

above). Tim brushed the leaves away in several spots, revealing the complete lack of any duff; he pointed out the copious earthworm "castings" (excrement) under the leaves. There was a zone of green plants in the lowest, wettest areas along the stream, but just yards away, where the land began to rise and was drier, the greenery largely stopped. Until that day, I wouldn't have even questioned the brown landscape under some forest canopies; I always assumed it was just too dry an area, or "that's just how it is here". Now I find it quite noticeable.

Earthworm, surrounded by castings

If you're in a hardwood forest in the northern states, and you're enjoying the showy wildflowers, then you're likely not in earthworm country. Those familiar flowers will mostly be gone once the worms find them. Wild Oats, Starflower, Wild Sarsaparilla, Solomon’s Seal, Twisted Stalk, and others, will fall prey. And the trees will find regeneration difficult as well.

The Mustard Test 

 

 If there's an area you suspect may be negatively affected by non-native worms, there are a couple tests you can perform to verify their presence. Brush away any leaf litter there may be, to expose the soil. If you find the worms' entrance/exit holes, and/or their castings, you know they're present. But you can also apply another, more sinister method: buy some dry yellow mustard powder, and mix it with water (about 1/3 cup powder to 1 gallon water), then pour some of the mix on one square foot of soil. If worms are there, the mustard will irritate their skin and they'll come out of the ground in a hurry. You may have to repeat the application if none come up after a minute or two. Bob Leverett and I tried this, and amazingly, several worms squirmed out of the ground almost as soon as the mustard solution hit it. It was actually quite fascinating to see.

Once you've determined worms are present, you can monitor the area to assess their spread. Unfortunately though, no one has a "cure" for the problem. At this point, the best we can do is to avoid, as best as possible, practices that spread the worms faster than their natural pace; that's easier preached than accomplished. But at least be aware that moving worm-containing soil from one place to another, or carelessly dumping out unused bait worms, will not be a good thing for our forests. Those who manage our forests will be acting responsibly if they take steps to avoid transporting worms into those forests on/in vehicles, or otherwise.

Keep it pristine