Christmas: Mistletoe

For years, mistletoe (Genus Phoradendron) has suffered from a split reputation: either the decorative prelude to a Christmas kiss or the tree-killing parasite that must be mercilessly excised for the good of the forests. At least 24 different species of Mistletoe grow in the United States.

Now a recent scientific studies (Mistletoe as a keystone resource) have come up with new understanding of the evergreen plant: It is a key to keeping forest life healthy. Not only should it not be cut out of the forests it affects, but it could also be introduced in injured woodlands to restore them to health.

The mistletoe makeover stems from an experiment started in 2004 in a small woods surrounded by farmland in the upper Billabong Creek area of Australia’s New South Wales. David Watson, an ecologist at Charles Sturt University in Albury, New South Wales, reasoned that the only way to discern the role of mistletoe was to remove it from 17 woodlands and compare them with 11 woodlands where the mistletoe remained and 12 woodlands naturally devoid of the plant.

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It was a massive task to eradicate the parasitic mistletoe, involving cherry-pickers, loppers, a dozen people and two seasons of work, made all the tougher because the Australian mistletoe mimics the trees it takes root on. Moreover, while mistletoe, with its 1,400 species in five families, lives on every continent except Antarctica, it is sparse within each forest. Dr. Watson found only a few plants in every acre in the woodlands where he worked.

In all, his team members removed more than 40 tons of the plant, leaving it on the ground for livestock to consume. Then they waited for three years.

Dr. Watson, known in academic circles as “the mistletoe guy,” had long suspected that his favorite plant was a keystone species, meaning it punches above its weight, ecologically speaking, but even he was unprepared for the results. He had supposed that creatures that fed or nested on mistletoe would be affected by its removal. Instead, he found that the whole woodland community in the mistletoe-free forests declined.

Three years after the mistletoe vanished, so had more than a third of the bird species, including those that fed on insects. Bird diversity is considered an indicator of overall diversity. Where mistletoe remained, bird species increased slightly. It was a similar story for some mammals and reptiles, but, in another surprise, particularly for those that fed on insects on the forest floor.

Analysis showed that species of mistletoe play an important role in moving nutrients around the forest food web. That has to do with their status as parasites.

Nonparasitic plants suck nutrients out of their own leaves before they let them fall, sending dry containers to the ground. But because the vampiric mistletoe draws water and nutrients from the tree stem or branch it attaches to, it is more nonchalant about leaving that nutrition in falling leaves. That means the fallen leaves still contain nutrients that feed creatures on the forest floor.

Not only that, but mistletoes make and drop leaves three or four times as rapidly as the trees they live off of. As evergreens, they also do it throughout the year, even when trees are dormant. It is like a round-the-calendar mistletoe banquet.

While no similar mistletoe excision experiments have been performed in North America, where fossil pollen grains suggest the plants have lived for millions of years, scientists in the United States say they, too, have noticed mistletoe's positive effect on forest life.

For example, van-sized “witches’ brooms” formations in old-growth Douglas fir trees in the northwest United States produced by dwarf mistletoe parasites were once pruned away. Today, the brooms are protected because they are important nesting sites for the endangered northern spotted owl.

ome scientists believe it is possible that introducing mistletoe into a damaged forest could help restore it to health.

But introducing mistletoe onto trees could prove controversial. While the parasites are like Robin Hood, stealing from rich trees to feed the forest poor, they can spoil individual trees for lumber. That is especially true of the deforming dwarf mistletoe. Mistletoe is still widely known as the “thief of trees.”

Still, recent studies add a touch of science to the folkloric view of mistletoe as a tantalizer, inducing people to wait under it for a kiss at Christmas. The custom stems from the ancient Druids, who believed mistletoe could work magic because it grew high in bare oak trees in midwinter where nothing else did, seemingly out of thin air. They cut it down with golden sickles, never letting it touch the ground, and hung it in homes to foster fertility.

The Australian study suggests that the plant does seem to work ecological magic of a sort.

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