Within dark and damp forests that hold rich organic soils lives a plant, oft unnoticed, that is changing the face of forest evolution. No, we are not talking about an old or new invasive exotic plant that ravishes the natural landscape but rather a new hero amongst plants; the first known species to co-evolve with an invasive exotic plant in such a way that it flourishes in the midst of an enemy. In an age when herbicides are the only known way of battling invasive exotic plants, poisons so toxic that they threaten the very environment that the same people using them are trying to protect, this is a very hopeful and wonderful development.
Clearweed (Pilea pumila) is a common understory plant that, for some reason or another, lives without much human attention, unless of course, it invades a shaded garden. It is common, easy to identify, and has a host of cousins throughout North America, particularly in northern zones. But despite its ubiquitous nature few people can name it when asked. Perhaps the plant’s low physical profile, that is it is a ground cover that is often trod upon, is what keeps it from being noticed. Despite its ability to hide from human observation, despite the fact that it is clearly in view, it is a very easy plant to key and identify. This ground cover plant often grows in dense mats, has opposite leaves that are deeply toothed, and holds a translucent stem, hence its name clearweed. Clearweed can grow taller in lightly shaded gardens but in the dark cover of the forest it often hugs the ground. The plant bears very small, white flowers that yield a tiny green fruit with purple markings; both found at the joint between the leaf stem and main stalk. The plant turns a glorious bright yellow in the autumn, hardly a camouflage as the beauty of the plant might be considered head turning. Clearweed can be confused with a cousin, Stinging Nettle (Urtica dioica) which has similar leaves but grows much taller and has hairs along the stem that have oils that can cause a severe and painful rash on human skin.
This succulent plant is a favorite food of the white tailed deer. On many occasions I have found areas of clearweed where deer have bedded in down on the dense mats and munched away at the leaves. The caterpillars of several butterflies use leaves for nourishment, the most common of which is the Comma Butterfly. Several species of aphids also enjoy the juicy stems of clearweed and one leafhopper uses clearweed as a primary host. It is truly a valuable forage plant amongst others in our rich New England forests.
What distinguishes this common plant from all others in the forest, and perhaps from all other plants in North America, is that it is the first and only plant to have evolved to survive the onslaught of one of the most predatory invasive exotic plants. Clearweed has learned to coexist with garlic mustard (Allilaria petiolata). This noxious plant is one of the most invasive plants found in the forests of North America. Originally introduced as a garden plant in New York, and later Chicago, it takes over the understory plant community in forests, particularly those with rich soils. It is so effective that is known to completely eliminate every other understory plant species. Garlic Mustard produces a chemical known as sinigrin. When garlic mustard comes in contact with competing understory plant species it conducts chemical warfare by emitting gross amounts of this chemical into the soil. Sinigrin kills the mychorrhizal fungi in the soil which are used by most understory plants in a symbiotic relationship where the fungi’s hyphae breaks down nutrients to easily used forms while using the root structure of the plant as a host. The loss of this soil fungi starves plants dependent upon this long held coevolutionary process. It is a deadly and very effective strategy that Garlic Mustard employs without abandon whenever necessary.
This strategy of garlic mustard has proven to be nearly invincible when it comes to the survival of other forest understory plants until recently. Enter our plant hero, clearweed, which has rapidly evolved to resisting the effects of sinigrin and has learned to coexist with the invasive plant. The mechanism that has allowed it to coexist with garlic mustard is a bit unclear. The mystery is that the clearweed that has made this evolutionary adaptation does not do well when transplanted to areas that do not contain garlic mustard. It seems to only do well in areas infested with this noxious plant. Nevertheless clearweed may provide ecologists with enough clues so that we can better understand how other plants might evolve to living with this invasive exotic plant. The key to a healthy forest is diversified plant species in every stratum (plant layer) in the forest from the herbaceous layer through the shrub/sapling layer to the upper canopy where trees dominate the botanical community.
Once again mother nature provides a lesson for the human species. By paying attention to evolutionary adaptation we can understand the processes by which plants change to continue living in an environment and perhaps eventually prevent legions of well intentioned human do gooders from spreading many metric tons of toxic herbicides in an otherwise pristine landscape.
A lesson worth paying attention to, wouldn’t you say?
Originally written for the Heath Herald in September of 2014.