CORVALLIS, Ore. – If mistletoe’s status as a nutrient-stealing freeloader has been cooling your holiday ardor, new research led by an Oregon State University scientist may help relight the fire.
A survey of urban forests in seven western Oregon cities found no observable connection between mistletoe infestation and negative health outcomes for the trees it was parasitizing.
So worry not: Your yuletide kissing tradition probably does not involve a tree killer. And as you’re setting concern aside, you might want to head outside.
“This is the best time of year to look for mistletoe because there are no leaves on the trees,” said Dave Shaw, a professor emeritus in the OSU College of Forestry and an OSU Extension Service forest health specialist. “Also, chances are it will be found in an oak tree — most other trees don’t get infested. So if you are looking for a kiss, keep an eye out for oaks.”
Studying mistletoe in urban forests
Shaw and collaborators at OSU, the U.S. Forest Service and the Oregon Department of Forestry examined the occurrence of western oak mistletoe in city forests to better understand host range and inform mistletoe management.
Common from Baja California to the northern Willamette Valley, western oak mistletoe is one of more than 1,400 species of mistletoe — flowering plants that attach to the branches of trees and shrubs worldwide.
Western oak mistletoe berries — found only on female plants and toxic to humans — ripen in late fall or early winter. Birds such as western bluebirds eat the berries and disperse the seeds, most often on larger trees.
Mistletoe seeds are coated in a sticky substance that allows them to adhere to branches. The plants draw water and nutrients from their hosts using a bark-penetrating, root-like structure that can sometimes stress the host tree.
“Western oak mistletoe is probably a benefit to wildlife in urban forests,” Shaw said. “At the same time, there is potential for impacts on amenity trees, which is why urban forest managers need good information about host range and tree health.”
What the survey found
Western oak mistletoe occurs primarily on native oaks but also has been documented on many other hosts, including acacia, alder, aspen, birch, chestnut, locust, pear, poplar, walnut and willow.
Urban forests often include many non-native tree species planted for aesthetics, adaptability or fast canopy growth. For the study, researchers focused on introduced tree species in Salem, Corvallis, Eugene, Medford, Central Point, Rogue River and Ashland.
Researchers surveyed city streets, parks, university campuses and arboreta by car, bicycle and on foot, visually inspecting non-native tree crowns for mistletoe.
They documented mistletoe on 227 non-native trees. Of those, 85% were pin oak or northern red oak, 12% were other oak species, and just 3% were non-oak trees — despite non-oaks being more common overall.
Only six infested trees were less than 10 centimeters in diameter, and just nine were under 10 meters tall. Only one tree appeared to be in poor condition; 14 were in moderate condition, and 212 appeared to be in good condition.
“That suggests mistletoe, at the levels we observed, is not adversely impacting tree health,” Shaw said. “Even among trees with heavy infestations, the vast majority were in good condition.”
A plant with a long cultural history
Mistletoe has symbolized health, fertility and love across cultures for centuries, with the tradition of kissing under mistletoe dating to 18th-century England.
“The word ‘parasite’ can carry negative connotations,” Shaw said, “but mistletoe is a remarkable plant with ecological and cultural importance. It was encouraging to see that it wasn’t causing appreciable harm to its hosts.”
OSU professor emeritus Max Bennett also participated in the study, which was published in Northwest Science. Other collaborators included retired Forest Service researcher Don Goheen, retired Oregon Department of Forestry scientist Alan Kanaskie, and current ODF scientist Scott Altenhoff.
