Female flower of the eastern dwarf mistletoe growing on a spruce branch.
The season's tiny tot
Our native mistletoe is a teensy shrub whose seeds are launched with a powerful punch.
As we gear up for the upcoming holiday season, some will hang mistletoe near gathering areas in hopes of getting a quick smooch. The traditional mistletoe produces large leaves and big berries, but this leafy type of "kissing mistletoe" is not native to Wisconsin. Our only native mistletoe species is not showy at all; in fact eastern dwarf mistletoe (Arceuthobium pusillum) only grows to heights of a couple of centimeters, easily making it Wisconsin's tiniest shrub.
Eastern dwarf mistletoe is a hemi-parasitic plant, meaning it derives some of its nutrients from photosynthesis, but also robs some nutrients from the host plant. In nature, its hosts are primarily black spruce trees and occasionally white spruce and tamarack. The vast majority of mistletoe populations occur in the swamps of Northern Wisconsin.
Eastern dwarf mistletoe derives its nutrition, especially amino acids and sugars, from the host. The mistletoe seeps a growth-inhibiting hormone into the host tree's branches, upsets the hormonal balance and causes proliferation of twigs in dense clumps called a witches' broom. The deformed tree, weakened by the loss of nutrients, eventually succumbs.
Although this mistletoe will in time kill a tree it infects, it should not be considered a threat to forest health. It is important to realize that eastern dwarf mistletoe is not a disease, but rather a slow-growing shrub. Diseases can easily kill countless plants in a short time and can spread over vast areas like wildfire. This parasitic shrub moves more slowly. One study indicated that a 1.5 acre patch of mistletoe took 60-70 years to form. Moreover, the death of an individual tree from dwarf mistletoe can take several decades and more widespread infestation of a forest stand may take centuries.
Instead of a bane to the forest, many ecologists consider mistletoe beneficial. Witches' brooms provide nesting sites for birds. Plus, the buds of infected spruce trees often open very early in spring when other buds are still closed. This yields sugars for ants and other insects, which in turn serve as food for birds. When infected trees eventually die and fall to their swampy floor, they open up the forest canopy to sunlight that gives other trees the opportunity to grow. The small trees interspersed with more mature growth creates a more uneven aged forest stand that enhances the health and ecology of the whole forest. Since a witches' broom can make a tree more top heavy and the roots of most swamp dwelling trees are shallow, the tree is more susceptible to toppling. Wind-felled trees in swampy areas often cause the roots to come out of the ground. When this happens all of the soil that covered the roots is eventually deposited in a very small area near the base of the fallen tree. Since swamps typically have a very thin topsoil layer, this piled soil creates ideal habitats for many of our beautiful native orchid species.
Dwarf mistletoe plants are dioecious, meaning that male and female flowers bloom on separate plants. Wind and insects carry the pollen in April and May. In the fall, mature fruits fill with so much fluid that they build up enough pressure to burst and cast seeds around 20 feet at about 50 miles per hour. Seeds that land on suitable host twigs will germinate in spring. Another fascinating adaptation of these seeds is that instead of having a hard seed coat, mistletoe seeds are coated with a sticky viscin, which glues them to host surfaces. Unlike most germinating seeds, mistletoe seeds are not sensitive to the pull of gravity or the tug of light. Their dominant growth impulse is to grow away from the light. Thus, a seed attached to the bottom of a twig will grow up into the twig, while one atop a twig grows down. In either case they grow into the twig to infect the host tree.
Although eastern dwarf mistletoe may not be the showiest or most elegant plant of the season, its diminutive size and fascinating life cycle more than make up for its unsightly form. Though few Wisconsinites will be scouring the swamps for some eastern dwarf mistletoe to hang over a doorway, we can all raise a glass of eggnog and toast this tiny, most interesting native shrub.
Matthew Wagner is an amateur botanist and writes from Summit Lake.