Now that the leaves are down and the fall colour spectacle is behind us, we have to content ourselves with a quiet time woodland walk. Leaves rustling underfoot remind us of nutrient recycling, as the soggy leaves decompose and release essential minerals to be absorbed yet again by other growing plants. Ah, the cycles of Nature keep rolling on, all is well. But actually, it's not.
Unfortunately, I bring you bad news about our forests. As if beech bark disease, Dutch elm disease, butternut canker, emerald ash borer, tar-spot fungus and such invasive plants as garlic mustard, dog-strangling vine and buckthorn weren't enough, now forest research has pin-pointed yet another threat to the continuation of a healthy hardwoods... earthworms!
Now, if you are like me, you probably thought that earthworms are a benefit to the soil, providing aeration to the soil and an early breakdown of dead leaf matter. But we've been fooled by worms for the past few hundred years, as they are actually invaders that are slowly yet very steadily altering the natural ecosystem of a hardwood forest, and not for the better.
While North America does have native earthworms, none survived the last glacial period that enveloped the Great Lakes region. When the planet warmed up and the ice melted away about 12,000 years ago, the succession of lichens to mosses to grasses to wildflowers to shrubs to trees went along without a hitch. Lots of bacteria and fungus were at work breaking apart dead plant material, and soon millipedes, sow bugs and springtails moved in to assist with this process, thus releasing nutrients for new growing plants.
Magnificent hardwood forests eventually came to rule the landscape around the lower Great Lakes… all done without a single earthworm in sight.
"Then came the white man", to quote from that popular song by The Stampeders, and with them also came a whole boatload of species new to North America, including earthworms from Europe. To give earthworms some credit, they do indeed provide a benefit to gardens and some agricultural crop fields by burrowing through the topsoil, thus allowing air and moisture to enter. But they were not needed, nor welcome, in the hardwood forests.
So, what's the problem? A normal and healthy hardwood forest floor has layers of old leaves upon it. Top layer is the freshly fallen leaves of this autumn. Below that are the soggy brown leaves of the autumn before, which is home to salamanders and sow bugs. Below that is a layer of fragmented leaves and bits of soil, which is home to springtails, fungi and bacteria, the layer where wildflowers and ferns set their roots. Below that is the mineral rich soil.
A healthy forest floor has a spongy, leafy depth to it, as the old leaves decompose at a slightly slower rate than the new ones that are layered on top. But when earthworms move in, that layering (called duff) gets eaten up very quickly, leaving patches of bare earth showing between the maple trees.
These wriggly residents alter both the physical and chemical makeup of the soils, changing pH, nutrient and water cycles, and disrupt the symbiotic relationships between soil fungus and roots.
That bare earth cannot hold much moisture, and erosion of both nutrients and soil starts to happen. Without a soggy layer of old leaves in which to put down roots, wildflower and fern density diminishes. This is a sight that has recently become common in the woodlands of our region.
The massive Boreal Forest of both Ontario and Canada is recognized as a huge carbon sink. What this means is that the carbon sequestered by the trees (in their needles, bark and wood) is bound to the organics, and is held, for a time, from being released into the atmosphere. As our industrial factories pump out copious amounts of carbon dioxide, it is the tree’s intake that regulates the amount of left-over free-floating carbon.
Now that earthworms have reached the Boreal Forest, they are eating and digesting the old layers of needles (thus releasing the stored carbon) faster than the new tree growth can absorb carbon. Can you say climate change factor?
Earthworms are spread by people, both deliberately and unknowingly. Sometimes this happens as unused fishing bait is dumped on the ground, sometimes as garden soil is moved off property and sometimes as worm-powered compost is taken from the house and spread outdoors. Consider that Canada ships about 110-million-dollars worth of fish-catching earthworms annually to the United States and you can see how these critters have spread throughout the area. Most worm invasion areas centre around fishing resorts, lake shorelines and stream banks.
Vermicomposting, the method of using alien 'red wriggler' earthworms in a container in the house to break down kitchen waste, has also led to foreign worms being transplanted to new areas.
Good news/bad news about earthworm invasions: they spread out at a rate of half a mile per 100 years, but once settled in they can populate an area with many hundreds of worms per square yard (or square metre, if you prefer). Eradicating worms from an area is impossible, it just can't be accomplished. So, what can be done?
The best idea is to protect the forests that do not yet have earthworms present: do whatever education and action is necessary to keep earthworms from being brought into an area. A couple of examples are: don't dump unused fishing bait on the ground; put the leftover worms in the garbage (in a sealed plastic bag); and don't spread compost that contains earthworms, or was produced by vermicomposting (using 'domestic' worms) to new gardens.
For more information on this latest scourge, check out the website www.greatlakeswormwach.org which is collecting data from citizens who are reporting earthworm sightings. Also find a copy of the booklet, "Earthworms of the Great Lakes" by Cindy Hale (Kollath and Stensaas Publishing) which has great illustrations and information about 16 earthworms that have invaded our area.
Next time you are out and about and kicking up some fallen leaves, have a look at the forest duff to ensure a healthy mat of slowly decaying leaves is present. If it's not there, well, now you have something else to worry about. Sorry about that.
Dave’s Notebook: With no caterpillars eating the leaves this year, I'm caught off guard by the volume of fallen leaves in the yard. Keeps me busy!
© 2022 David J. Hawke