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 College of Forestry professor emeritus Dave Shaw, 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."
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 learn about mistletoe hosts and gain insight into mistletoe management.
Common from Baja California to the northern Willamette Valley, western oak mistletoe is one of more than 1,400 species of mistletoe, a type of flowering plant that attaches to the branches of trees and shrubs around the globe.
Western oak mistletoe berries (on the female plants only, and toxic to humans) ripen in late fall or early winter and are eaten by western bluebirds and other birds, who disperse the seeds, most commonly on larger trees.
Mistletoe seeds are covered in a sticky substance that allows them to cling to branches. Mistletoes siphon food and water from their hosts via a bark-penetrating, root-like structure, sometimes to the detriment of the host tree.
"Western oak mistletoe is probably a benefit to wildlife in urban forests," Shaw said. "On the other hand, there is the potential for negative impacts on amenity trees, which is why it's important for urban forest managers to have assessments of mistletoe host range, both for future tree planting decisions and managing current tree populations."
Western oak mistletoe occurs on native oaks and a collection of other hosts – including acacia, alder, aspen, birch, chestnut, locust, pear, poplar, walnut and willow – so vast that no definitive host list exists.
Shaw notes that urban forests often include a variety of non-native trees, planted historically for reasons that include aesthetics, adaptability, and rapid growth of shade-producing canopy. For this study, the researchers focused on introduced tree species in Salem, Corvallis, Eugene, Medford, Central Point, Rogue River and Ashland.
The scientists conducted their surveys by driving, walking and biking city streets, parks, university campuses and arboreta, with the goal of visually examining all non-native tree crowns for the presence of mistletoe plants.
"Urban forests are unique settings that allow for a wide assortment of potential hosts to be exposed to mistletoe seed," Shaw said. "We observed western oak mistletoe in 227 non-native trees, of which 85% were pin oak or northern red oak."
Among the rest, 12% were other oak species, meaning just 3% were not some kind of oak tree – even though non-oaks were in the majority.
Western oak mistletoe infested trees of all size classes, but only six of the 227 infested trees were less than 10 centimeters in diameter and just nine were less than 10 meters tall. Only one infested tree looked to be in poor condition, 14 were in moderate condition, and 212 appeared to be in good condition.
"That suggests mistletoe, at the levels of infestation we saw, is not adversely impacting tree health," Shaw said. "Of the 42 trees with greater than 20 mistletoe plants in their crowns, none was in poor condition, one was moderate and 41 were in apparently good condition."
Mistletoe has been a symbol of health, love, vitality and fertility in multiple cultures since ancient times, and the tradition of kissing under mistletoe has its roots in 18th century England.
"The word 'parasite' can carry negative connotations, but mistletoe is a remarkable and beautiful plant with centuries of cultural importance behind it," Shaw said. "It was nice that our survey showed 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. The other collaborators were retired Forest Service researcher Don Goheen, retired Oregon Department of Forestry scientist Alan Kanaskie, and current ODF scientist Scott Altenhoff.
