Mistletoe

A single mistletoe plant is growing near the top of an ash tree behind Shenanigans and Woody’s Bikes. Mistletoe is not common in Sewanee, although it can be quite abundant on the lower slopes of the coves.

American mistletoe, Phoradendron leucarpum (literally, the "white-seeded tree thief")

Recently, biologists have realized that mistletoe is a very important part of the ecology of forests. In fact, mistletoe has such a major effect on other species that it has been called a “keystone species,” one of the major determinants of a forest community’s vitality. How so? Mistletoe steals some of its host tree’s food and combines this with the food that it makes for itself through photosynthesis. This combination of a big trust fund (the tree) and a steady job (the mistletoe’s own leaves) allows the plant to live large, offering abundant nectar in the early spring and fat, nutritious fruit later in the year.

Bees love the nectar which comes earlier in the year than the nectar of most other flower species. Birds and many climbing mammals adore the fruit. The pulp of the fruit is sticky, ensuring that it will stick to branches after it has passed through the bird. More, the seed is often so gummy that it sticks to the birds’ feathers. Birds have to grab the seed, then wipe it off on a branch — a perfect way for the mistletoe seed to get placed exactly where it needs to be. So, this small plant is used by dozens of invertebrate and vertebrate animals. One butterfly, the great purple hairstreak, has gone so far as to become wholly dependent on the plant. Its caterpillars will eat nothing else.

In other parts of the world, including the western U.S., different mistletoe species provide a similar range of services. Some even provide favored nesting areas for many birds. A review of mistletoe biology a few years ago stated that, “…the widespread perception of mistletoes as destructive weeds needs to be challenged. Many landholders, managers, and even biologists regard mistletoes as invasive pests, damaging to individual trees and detrimental to forest health. [But] …mistletoes have a substantial positive role in many forests and woodlands, and should be given appropriate recognition.” (from: Watson, D. M.. 2001. Mistletoe — a keystone resource in forests and woodlands worldwide. Annu. Rev. Ecol. Syst. 32:219–49)

Of course, mistletoe has other roles in the ecology of our world. So, I’ll sit and do my end-of-semester grading under the thief, ever hopeful that songbirds will visit.

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