By Katherine McFerrin
Mistletoe is the symbol for romance during the winter holidays. The classic mistletoe, imagined with its snowy white berries and thin green leaves, is not native to the United States. Rather, it’s the European mistletoe, Viscum album, which is found across Europe and parts of southern and western Asia. Similar to European mistletoe but with slightly smaller berries and shorter and rounder leaves is the American mistletoe, Phoradendron leucarpum. These two species are the most commonly harvested mistletoe for the winter holidays. However, beyond the love and longing associated with mistletoe and its festive appearance, mistletoe’s most alluring quality is its biology; mistletoe is parasitic.
“Mistletoe” encompasses all the parasitic plants within the order Santalales which are about 1300 species worldwide. To be more specific, these plants are hemiparasitic which means they are only partially parasitic as opposed to holoparasitic plants that are completely parasitic. Since mistletoe has chlorophyll which is required for photosynthesis, it is able to get some of its energy for the sun. Thus, mistletoe is not completely reliant on another host for its energy. The energy mistletoe gets from the sun isn’t enough though, so it must get the rest of its energy from its host.
Mistletoe’s hosts are trees that the seeds happen to fall on after dispersal. Mistletoe does have specific host trees that they favor, but the species it favors is determined by the species of mistletoe. For example, European mistletoe is often found on large, deciduous trees such as oak. Dwarf mistletoe, a variety found in the western United States, grows on pines, firs and hemlock trees. Once established on the host tree’s branches, the mistletoe seeds begin to germinate meaning the seeds start to sprout. They develop haustoria which are root-like structures that perforate the tree’s bark into the xylem, the part of the tree that transports water and raw nutrients from the roots to the leaves. The mistletoe siphons off some of the tree’s water and nutrients for its own use and energy production. This process does hurt the host tree by taking away some of its essential resources and reducing the ability for the branch to grow. If there is a significant amount of mistletoe covering a tree’s branches, the tree can die.
The mistletoe, however, doesn’t just bring death. It brings life to the ecosystem. Squirrels and many birds such as Flycatchers, Bluebirds, Robins, Grouse, Mourning Doves, Evening Grosbeaks and Pigeons eat the mistletoe berries. With a conveniently close food supply, these creatures make their home nestled within the bushy growth of mistletoe between the tree branches. Raptors such as Cooper’s Hawks and Spotted Owls sometimes nest within dead host trees. Chipmunks, deer and elk also eat the leaves and berries. Even a species of moth, Celypha woodiana, known as the “marble mistletoe moth” lays its eggs on European mistletoe. In the United States, the Great Purple Hairstreak butterfly feeds on American mistletoe and lays its eggs on the plant which is the main food source for the newly hatched caterpillars.
Within the plant world, mistletoe isn’t just an enemy. Juniper often grows near mistletoe so that the birds attracted to the mistletoe berries might also eat the juniper berries. These birds and animals that eat the mistletoe, in turn, help propagate the cycle of mistletoe growth and habitat formation by unknowingly dispersing seeds that are stuck on their fur or by eating the berries and defecating the seeds onto new trees. Unlike most mistletoe that relies on birds and animals to spread its seeds, the previously mentioned dwarf mistletoe disperses seeds on its own by exploding. As its berries grow, they fill with water until the increasing pressure causes the berries to burst. This sends the seeds flying through the air at speeds up to 60 mph to land on trees 15-40 feet away.
Because so many creatures and plants depend on the mistletoe, mistletoe is a keystone species meaning that it plays an important role in its ecosystem. Thinking back to the winter holidays, just as the tradition of hanging mistletoe brings people together, in nature too, mistletoe brings together many creatures and plants.
Katherine McFerrin is a junior at Carleton College in Northfield, Minnesota, where she enjoys cross country skiing and running across snow covered trails. She studies biology with an interest in molecular biology and zoonotic diseases. She is currently socially distancing with her family in Morgantown, WV, where she is on winter break.