Plant of the Week
Lodgepole Pine Dwarf Mistletoe (Arceuthobium americanum)
By Walter Fertig
Mistletoes are parasitic herbaceous plants that grow from the stems of other woody species. The mistletoe habit has evolved separately in several vascular plant families found in tropical rain forests, deserts, and conifer woods. Only two genera of mistletoes (Viscaceae family) occur in North America. Species of Phoradendron (literally “tree thief” in Greek) parasitize various oaks, junipers, cottonwoods, acacias, and mesquites. These mistletoes are considered partial parasites because some species have green stems and fleshy leaves capable of their own photosynthesis in addition to robbing nutrients and water from their hosts. The dwarf-mistletoes (genus Arceuthobium) are strictly parasitic and mostly infest conifers. Typically, each species has a specific genus or species as a host, thus there are dwarf-mistletoes that mostly attack white pines, red pines, pinyons, true firs, spruces, or Douglas-firs. Knowing the identity of the tree victim is a good way to help identify dwarf-mistletoes to species, as all are similar in appearance.
As suggested by its common name, lodgepole pine dwarf mistletoe, also called American dwarf mistletoe, (A. americanum) grows primarily on lodgepole pine (Pinus contorta), the characteristic two-needle pine species of much of the Rocky Mountains. In Canada, this mistletoe can also infect Jack pine (P. banksiana) and in the western U.S. is occasionally found on Ponderosa pine (P. ponderosa). Like most parasites, the lodgepole pine dwarf-mistletoe has a greatly simplified body plan consisting of short, bushy whorls of yellowish, leaf-less stems tipped by petal-less flowers. Specialized stems penetrate the bark of the host plant to tap into the food-transporting phloem tissue and water-conducting xylem, as well as provide a solid anchor. Dense clumps of mistletoes are sometimes called witches’-brooms, though these structures can also be formed by rust fungi and amoeba-like microbes. These structures can provide important nesting habitat for songbirds, raptors, and squirrels, as well as forage for other wildlife.
Lodgepole pine dwarf-mistletoes produce one-seeded berries that take more than a year to fully ripen. During that time, water pressure slowly builds up inside the fruit until its wall finally bursts and the seed is explosively discharged 15 to 50 feet (at a speed of 60 miles per hour!). The seeds are extremely sticky or viscous (this helps explain the name of the family, the Viscaceae) and will adhere to the stem of a neighboring branch or tree long enough for the seed to germinate and send its root-like stems into a new host.
Since ancient times, humans have been fascinated by the peculiar growth habits of mistletoes. These plants were often thought to be omens of good fortune and so were frequently hung indoors during winter to foster positive spirits. Over time, this tradition morphed into the practice of hanging mistletoe over doorjambs at Christmas time. In Norway, tradition dictates that a man must remove one berry for each stolen kiss until all the berries are gone (and smooching must cease).
Foresters feel decidedly less romantic about mistletoes. Each year an estimated 3.3 billion board feet of timber is lost to dwarf-mistletoe infections that weaken the vigor of a host tree, damage wood, and increase drought stress and susceptibility to fire and attack by pine beetles. (For perspective, 13,600 board feet of lumber goes into a typical 2,000 square foot home.) Some researchers believe that dwarf-mistletoe has actually become more abundant over the past century as a result of fire suppression and selective cutting of larger, uninfected trees.