Smooch-inducing mistletoe can indeed be a menace – and not only as a Justin Bieber song. All species of mistletoe are parasitic, extracting water and carbohydrates directly from the host tries they live on. Dwarf mistletoe (Arceuthobium tsugense), common throughout coastal B.C. forests, can cause risks for other forest inhabitants too. Infected trees swell and grow abnormally, branching into broom-like balls. The swellings can cause branches to break and fall.
Dwarf Mistletoe’s (Arceuthobium tsugense) most common host is Western Hemlock, and the big, branching swellings that it creates are commonly called “witches’ broom.” Despite its (hemi)parasitic habits, Dwarf mistletoe has many redeeming qualities. It provides an important nesting habitat and food source for many birds and mammals. It also provides an excellent, ecology-filled way to teach osmosis.
Let’s tackle first things first. What does it mean to be a hemiparasite? This means that mistletoe, though mostly a parasite, can survive on its own for a small amount of time at the beginning of its life. Mistletoe species will photosynthesize early in their life cycle, before becoming fully parasitic once they infect a host tree.
The second thing to know about mistletoe is that it gives us a new way to think about parasites. We often think of parasites as bad, but mistletoe species may actually be keystone species in their ecosystems. In coastal B.C. forests, for example, the “witches’ broom” that Dwarf mistletoe forms in infected Hemlock trees provides nesting habitat for endangered birds, like spotted owls and marbled murrelets. They are also mutualists, supporting many species of insectivorous (insect-eating), nectivorour (nectar-eating), and fructivorous (fruit-eating) birds and mammals, which in turn help with reproduction between male and female plants and with dispersal of the mistletoe seeds.
Finally, we think that mistletoe can make learning (and teaching) about osmosis a little more interesting. We know that solvents (like water) move from areas of higher concentrations of solutes to areas of lower concentration, until they balance out. Dwarf mistletoe species (Arceuthobium sp.) get all of their water by tapping into the host ‘xylem’ (the arteries of the plant vascular system, which carry water). To keep a steady flow of water, Dwarf mistletoe are especially good at ‘transpiration’. Transpiration is when plants continually lose water by evaporation, so that they can continually gain water through their roots. In this case, the mistletoe ‘roots’ tap into the host plant. Mistletoe is so good at transpiration, that it can even continue to draw water from its host via osmosis when the host is drought-stressed. Pique your students’ interest in osmosis by telling them a tale of the parasitic mistletoe.
Featured image by Stan Shebs