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ECOVIEWS: Questions about mistletoe are asked every year

December 23, 2018

December questions about ecology often involve plants and animals associated with Christmas. Following is a version of a question people have asked over the years about mistletoe.

Q. What is mistletoe? Is it a flowering plant or some kind of parasitic fungus? Are those little white berries poisonous?

A. Characterizing mistletoe is difficult because more than 200 species have been described worldwide, and numerous lifeforms and associations with other plants exist. Several kinds of mistletoe live throughout the southern United States, from the Atlantic to the Pacific and on every continent except Antarctica. All are flowering plants, but the common Christmas mistletoe people are familiar with is arguably America’s most popular parasitic plant, or as botanists call them, hemiparasites. The common mistletoe has rootlike anchors that grip tree limbs. It derives water and nutrients from the host tree but has its own green leaves for energy production through photosynthesis.

Mistletoe thrives only on living trees. The seeds are most likely to flourish if a bird deposits them on the same species of tree on which the parent plant lived, as some tree species have traits making them more suitable for mistletoe. I seldom see it growing on magnolia or pine trees in the southeastern United States. Perhaps the yearlong shading of these evergreen trees would diminish mistletoe’s exposure to sunlight. Or maybe magnolias and pine trees have internal properties making them less desirable hosts. Eastern pine trees may be resistant to mistletoe, but as Michael Crichton wrote and Jeff Goldblum said in “Jurassic Park,” “Life will find a way.” Sure enough, a different mistletoe found in the western states has no green leaves and gets all of its resources from conifers, including pine trees, killing them in the process.

Although a parasitic or hemiparasitic lifestyle is unusual among flowering plants, many aspects of mistletoe ecology are well understood. Competition to obtain water, minerals and even space itself, is highly intense among most plants. Mistletoe does not encounter such problems. Tree limbs are a ready source of water and minerals for this atypical little plant. If an oak tree has no mistletoe, the reason is probably that no bird has dropped a seed there not that there was competition from other mistletoe plants.

In the South, tiny yellowish flowers bloom on the totally green mistletoe from fall to winter. The familiar white berries (called drupes by botanists, as are plums and cherries) begin to form soon after pollination – little packets of glue surrounding tiny indigestible seeds. A mistletoe plant can be either male or female. Like a holly tree, only the female plant has berries. Mistletoe berries may be toxic to some wildlife. Many birds, however, are immune, a trait essential to the welfare of the mistletoe plant. The dispersal and propagation of mistletoe is largely dependent on birds that eat the berries but do not digest the seeds. During spring migration, a flock of cedar waxwings can ensure that newly developing mistletoe plants are many miles away from where the seeds were consumed.

Based on numerous medical accounts of people, usually children, having eaten berries or leaves, humans are not immune to mistletoe toxins. Most cases resulted in nausea and other digestive ill effects, unpleasant but not lethal. Whether anyone has ever died from eating mistletoe is uncertain. I can find no published scientific tests comparing responses of humans who made a meal of mistletoe berries with others who did not. I would be suspect of anyone who wanted to participate in such an experiment. As with unidentified mushrooms, there might be pleasure to be gained from eating them, but it’s not worth the risk.

A potentially perilous climb is one hazard of getting a sprig of mistletoe to put over your doorway for the age-old holiday tradition, which leads to another ecological question. Does mistletoe grow high in trees and seldom on lower limbs because it is palatable to deer and would soon be eaten?

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