A parasitic flowering plant with leathery leaves and watery berry-like fruit, American Christmas mistletoe often grows on oaks in southern states. (Photo: Copyright Jeff McMillian for USDA-NRCS)
A parasitic flowering plant with leathery leaves and watery berry-like fruit, American Christmas mistletoe often grows on oaks in southern states. (Photo: Copyright Jeff McMillian for USDA-NRCS)
Mistletoe is mostly known as the plant hung over doorways as an invitation to kiss whoever walks beneath it — a custom perhaps best avoided this year in government buildings and at office Christmas parties.

The truth is that the role of mistletoe has changed considerably over the past 2,000 years, and it was never so innocent as the Norman Rockwell version portrayed in mid-20th-century America.

For one thing, mistletoe is poisonous. It’s rarely lethal, but it’s poisonous enough to curdle the holiday cheer if the kids eat it.

Mistletoe is also a parasite. The plant grows on tree limbs and sinks its roots deep into the living wood of the tree and sucks out the nutrients. That’s how mistletoe remains leafy and green at Christmastime when the trees have lost their leaves.

In Europe, the story of mistletoe goes back to the tree-worshipping Druids of seventh-century Germany.

The Druids climbed bare oak trees in midwinter to harvest mistletoe with sacred golden knives and hung sprigs of the leafy greenery with its white berry-like fruit around the house for the good magic it brought — they believed it could only be magic that allowed the mistletoe to remain green in winter when all else had lost flowers, fruit and leaves.

St. Boniface did away with those mysteries when he came over from England as a Christian missionary set on eradicating pagan worship. He cut down a sacrad oak to convince the Druids they were just telling stories, then convinced them the fir tree that sprang up next to the fallen oak represented the Christ child.

Mistletoe got a lot of attention from northern Europe to the Mediterranean. The Greeks saw it as a symbol of fertility and used it in marriage ceremonies, and the Scandinavians hung mistletoe to create a sort of portable DMZ where warring peoples or spouses could come together and reconcile their differences.

It wasn’t until the eighth century that the Plant of Peace became a Kissing Ball to hang in the entry hall where it was difficult to avoid the uncle lurking near the umbrella stand.

Modern mistletoe stories lack the same level of romantic mystery. For one thing, harvesters in the American South don’t use golden knives to collect the American Christmas mistletoe (Phoradendron spp).

In Virginia, they blast it out of the treetops with a shotgun.

No one is shooting Christmas mistletoe out of oak trees in Maine, however.

Christmas mistletoe grows in a geographic range from New York south and west to New Mexico. It does not grow this far north, according to Barry Logan, a Bowdoin College forest ecologist who has been studying Maine’s eastern dwarf mistletoe (Arceuthobium pussillum) for 20 years.

Eastern dwarf mistletoe is a sort of Grinch version of its leafy cousin. It has no obvious leaves and appears to have a preference for white spruce — a tree that most will want to avoid using as a Christmas tree since its other name, cat spruce, comes from its odor.

Dwarf mistletoe does, however, have its own unfolding story on the nearby islands of Penobscot Bay.

Logan and his student, Hanna Baldecchi, are looking into the role the tree parasite plays in how island forests work, focusing their studies on Islesboro with the help of the Islesboro Island Trust.

Logan knows that white spruce (Picea glauca) are mostly defenseless against the dwarf mistletoe, which digs its roots into a white spruce branch and literally sucks the life out of the tree.

“The infection upsets the hormone metabolism of the tree and the mistletoe parasite benefits,” said Logan.

Logan discovered the mistletoe sends a dose of growth hormone into the branch it has infected. In response, the white spruce sends most of its energy resources over to the branch with the mistletoe, thus starving the rest of the tree. One of the hallmarks of a white spruce infected by dwarf mistletoe is a brushy-looking witch’s broom that forms on the infected branch.

It may be tempting to malign the dwarf mistletoe because of its impact on individual trees; however, the plant is a native species, not an invasive outsider that has migrated in-state to upset things.

“Mistletoe provides forage for birds and the plant has an ecological role in creating greater diversity in the forest,” said Logan.

Logan has focused his attention on the difference between how white and red spruce trees appear to respond to mistletoe.

Red spruce (Picea rubens) — which is one of Maine’s most valuable trees for making lumber and musical instruments — also gets infected by the dwarf mistletoe and forms witch’s brooms in response. Unlike white spruce, red spruce appears to fight back.

 Logan hypothesized that red spruce trees shut down the mistletoe-infected branch, essentially turning off the nutrient supply lines from the rest of the tree. Red spruce appears to kill off the branch in order to save the tree.

It’s Baldecchi’s mission to collect the data to show if Logan’s scientific guess is accurate. The results of the Islesboro study will be out next spring, said Logan.

Logan has also researched dwarf mistletoe on Monhegan, which has a forest dominated by white spruce.

Monhegan islanders may think natural forces created the large, old white spruce forests with their criss-crossed windthrown trees and forest shadows. They didn’t. Monhegan probably had a mixed forest of spruce, oaks, maples and other trees before settlers cut them down for farming and sheep.

When islanders fled for New England factories and Midwestern farms in the 19th century, the island forests did not grow back the same way. The sheep and clearcuts had created an unnatural advantage for white spruce on many islands from Penobscot Bay south.

On Monhegan, white spruce crowded out the other trees and grew up in one big generation that has now grown old pretty much at the same time in a Baby Boom kind of forest.

The larger numbers of white spruce trees give the dwarf mistletoe a big advantage. Almost every tree became a potential host for the sticky seeds that burst out of the plump berry-like fruits of the mistletoe.

Weakened and with mistletoe clinging to their supply lines, white spruce also become more vulnerable to the force of wind.

The white spruce forests with their mossy floors and fairy houses on Monhegan may be nearing the end of their chapter in a long story of a changing forest. The dwarf mistletoe’s role in hastening the end may well be nature’s means of recreating a more balanced forest that is more resilient to the changes yet to come.