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Posted December, 1997

Dwarf Mistletoe May Be Kiss of Death for Island Spruce Forests

This mistletoe is playing Scrooge to Maine's coastal spruce forests, and its spread from Casco Bay to Penobscot Bay is already sparking a debate about tree cutting and preservation of postcard island views. Heavily attended information sessions were held last summer by the Maine Coast Heritage Trust, The Island Institute, and Maine Forest Service in Stonington, Vinal Haven and on Louds Island (Bristol) to discuss mistletoe, spruce beetle, and coastal spruce management.

The problem is a parasitic plant known as dwarf mistletoe. It's a cousin of the leafy mistletoe we hang in our homes. According to William Livingston, a tree pathologist in the University of Maine Dept. of Forest Ecosystem Science, the slow-growing plant saps the strength of infected trees and in severe cases, kills the tree.

The most recognizeable symptom, he says, is known as witches' broom, a dense collection of twigs bunched along a larger branch. Infected trees may look sickly with fewer needles, multiple witches' brooms and dead upper-story branches.

“It's like a race between the tortoise and the hare. If a tree grows fast enough, it can shade out infected branches in the lower part of the tree and survive. However, if the mistletoe gets into the top branches and keeps up with the tree, the tree is doomed,” he says.

DISEASE CONTRIBUTES TO FIRE DANGER

Livingston studied dwarf mistletoe during his graduate student work at the University of Minnesota and, in 1991, published a survey of the plant's extent in eastern Maine in the Northern Journal of Applied Forestry. Dwarf mistletoe occurs in forests across the northern United States, but nowhere is it more severe than in stands on Maine's islands. It's not a significant threat to inland forests.

On islands, the problem is reaching an acute stage as many of those trees approach the end of their lifespan, says Livingston. Already weakened by drought or poor soils, the trees are likely to die from mistletoe infections in increasing numbers. The disease multiplies fire danger by leaving swaths of dead, dry timber in its wake.

Dwarf mistletoe has a clever survival strategy of its own. Like a cancer tumor, it slowly starves the host. A germinating seed sinks its roots deep into the life-giving tissue of a tree's branches. As the parasite withdraws nutrients, the tree signals its healthy parts to send more nutrients to the benefit of the mistletoe. In its third or fourth year, the invader grows a tiny reddish-brown shoot no longer than a spruce needle. It flowers, and in the fall, it spews forth a sticky seed which lands on another branch and starts the cycle over again.

LANDOWNER ADVICE

“Mistletoe can thrive only on living trees. If landowners want to manage their forests for scenery and reduce the fire danger, they need to remove all infected trees. Since the early life stage is barely visible, it would also be wise to remove all trees within sixty feet of a large severly infected tree. That's the distance a mistletoe plant can shoot a seed,” says Livingston. “Essentially, we're talking about a small patch clearcut,” he adds.

“Another untested option could be to maximize height growth of spruce trees by thinning-out dense stands in which dwarf mistletoe brooms are restricted to the lower branches. If a spruce can maximize its height growth, the tree could outgrow the parasite. Research is needed to test if this option is viable.”

However, commercial harvesting today is often limited by the expense of getting island logs to mainland mills. Only one barge service is now available for the entire Maine coast. Landowners considering large or small scale harvesting to remove damaged wood or to improve the overall vigor of their stands should seek the advise of a licensed professional forester.

Island forests were heavily cut during the two centuring following European colonization. In the past century, the forest has returned to pastures and other areas, and those trees are now mature. “Not doing anything about pests in these forests is an option, but if you do that, be prepared to watch trees die from mistletoe or other diseases. Trees are like people. They don't live forever.”

“Long term solutions to maintaining the health of these mature forests will require long term commitments by the forest owners, foresters, loggers, pest specialists, and researchers,” suggests Livingston. “Managing the problem is possible, but it won't be easy.”

Additional information is availble from the Maine Forest Service which has has published a general alert about the declining health of island spruce forests and pest alerts about mistletoe and spruce beetle. In addition Livingston maintains photographs and information about dwarf mistletoe on the Internet at http://www.ume.maine.edu/~nfa/fes/int256/home/tpd_home.htm.

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