Shrubs in the Treetops: a Portrait of Mistletoe

By Jenn Roe, Natural History Docent at-large

Summer is gone and our world shifts gears into fall, and then winter mode. The blooms of the earlier seasons give way to fruits of their vegetative labor: seed beds, berries, nuts, and other forms of embryonic plant progeny, all hold promise of next year’s floral offspring plus an extra yield: cuisine for native wildlife in the fall and winter. Observant hikers can spy autumnal foodstuff while wandering the mountain’s grocery aisles aka trails. Coffeeberry, blackberry, toyon berry and wild grapes, madrone and manzanita fruits, bright red rosehips are just some of the menu items on offer. And oh so many acorns, the tremendous autumn and winter food source for many oak woodland residents. But acorns aren’t the only seasonal edibles found in oak branches. Look up in the tree’s crown and you might see an evergreen shrub that grows in a jumbled ball of fuzzy stems and leaves, a native plant with a rich natural and cultural history and yet routinely misunderstood: oak mistletoe.

Mistletoe conjures up the holidays in many human cultures, but for wildlife it represents an altogether different reality. Food, shelter, a place to hunt, nutrients for the soil and plant-life beneath it, a plant host for hungry caterpillars. These are all ways that oak mistletoe interacts with life around it. To know it is to love it – or at least like it more. What better reason to delve into the nature – and culture – of mistletoe?

Historically, mistletoe is a plant-celeb, well represented in international folklore and myth. Early on, oak-worshipping Druids adopted mistletoe as a sacred symbol of life and fertility evidenced by evergreen leaves and bountiful berries in the dead of frozen winter. Kissing under mistletoe by then considered an aphrodisiac began during Kronia, the Greek harvest festival later Roman Saturnalia. It was incorporated in fertility rights used in marriage ceremonies and on farms. Ancient Romans and Scandinavians thought it a plant of peace under which warring enemies and quarreling couples could call a truce. In medieval Europe, homes decked with mistletoe were protected from fire, trolls, and witches. Victorian England reworked its legend to today’s Christmas-time tradition whereby a decorated sprig overhead is permission for stolen kisses.

By comparison, oak mistletoe’s factual role in nature lacks the thrill of the supernatural, but it’s no less magical to wildlife that shares its existence.

All 13,000-plus species of mistletoe are a type of parasite, plants that feed off another without reciprocation. Parasite is a loaded term with villainous connotations amplified in this case by oak mistletoe’s scientific name, Phoradendrom villosum. Phora comes from Greek, meaning thief, and dendron, meaning tree. Villosum means hairy in reference to its leaves and stems. Thus, oak mistletoe is labeled a hairy tree thief. Not the best brand for a good first impression.

In truth, it’s a hemiparasite and therefore only half (hemi) parasitic. It takes water and minerals from its host, but not carbohydrates. Mistletoe’s tiny root-like structures are called haustoria from the Latin word haustor, meaning things that draw in. They infiltrate the host like so many teensy-tiny straws, sipping juices from the tree’s xylem, tissues that transport fluids up from roots to branches and leaves. But like other green plants, oak mistletoe makes its own energy by photosynthesizing its own vital sugars rather than stealing them from its host. According to a recent study by Walter D. Koenig, whose many credentials include Research Zoologist Emeritus at UC Berkeley, on the effects of oak mistletoe on California oaks, “the negative consequences of Phoradendron presence on their hosts are negligible … and outweighed by the important ecosystem services mistletoe provides.”

Mistletoe’s ecosystem services make it a microhabitat in its own right. For example, the abundance and diversity of birds is directly tied to the presence of mistletoe. Its elevated bundle of fuzzy stems and leaves is a well-disguised nesting site to several bird species. House wrens, mourning doves, pygmy nuthatches, hummingbirds and others, including raptors like Cooper’s hawks and some owls, all find it a fine place to nest. Its plump berries, toxic to humans, are yummy edibles especially prized by birds (more on birds later).

A flowering plant, mistletoe is a nectar and pollen resource for creatures that seek the same. Lizards and birds hunt insects in its branches. The leaves are high-protein forage for deer and other browsers, and fallen leaves infuse the soil with nitrogen, phosphorus and other nutrients that ripple through the soil food web. Mistletoe is the only host plant for the beautiful Great Purple Hairstreak butterfly. Whew - that’s a lot of ecosystem services!

So how does a shrub land itself in the tree-tops and why do some trees have some or many mistletoe clumps while others one or none? Short answer: birds.

Mistletoe’s partnership with birds was an evolutionary leap that allowed it to rise above its root-sucking parasitic ancestor, Sandalwood FYI, to today’s lofty lifestyle. Mistletoe depends heavily on birds to get around, relying on bird poop-dispersal to deliver seeds to high branches that offer it better opportunity to thrive than the shaded woodland floor. A tree-crown residence provides improved access to sunlight for photosynthesizing and more distance from the reach of earthbound leaf-nibbling animals.

Named for this bird-plant relationship, mistletoe comes from Old English “mistiltan” derived from two even older Anglo-Saxon words; mistel, meaning dung, and tan, meaning twig. Modern translation: poop-on-a-stick. Apparently, the tree-worshiping druids noticed mistletoe grew on dung-splattered tree parts.

Mistletoe berries are filled with viscous pulp that sticks like glue. In turn, sticky seeds tack onto bird beaks and feet and are wiped off (planted) onto twigs and branches. After digestion, seeds come through intact which allows birds to deposit them on places they like to roost best. So why do some oaks have a lot of mistletoe and others not? It’s a sign of a bird-favored tree, or not.

Let’s not overlook oak mistletoe’s inconspicuous, to humans, petal-less flowers that appear July through September.

Each flower is either male or female and grows on separate shrubs; hence, each plant is either male or female. Fortunately, the modest blooms don’t go unnoticed by insect pollinators. Their visitation ensures there are berries in winter when other food is scarce, good timing for hungry animals, like squirrels and deer, but especially birds. Western bluebirds, mourning doves, grosbeaks, American robins and cedar waxwings are among the many feathered fans that literally flock to mistletoe in winter to dine on its fruit.

Western bluebirds so prize mistletoe berries that an extended family will take possession of a high yielding plant defending it against all other comers. Bluebird owners of a fruitful clump stay mostly together living off their berry harvest through winter when insect fare is meagre. Female offspring usually disperse at maturity, but sons stay behind as helpers so long as food is available, and thus enjoy a bluebird form of intergenerational wealth. But when berries disappear, so do the sons. Sound familiar?

Often misunderstood and no longer revered by people, mistletoe retains its deep value in nature. Next time you walk under one, perhaps you’ll find more to appreciate than stolen kisses.