Mistletoe: the shrub in the sky – They are more than just tree killers.
Mistletoe: the shrub in the sky – March 2014
Did you kiss under the mistletoe last Christmas? According to folklore, any girls lingering under the mistletoe bough suspended over the doorway during the Christmas festivities were allowed to be kissed. Despite the ample supply of mistletoe in our district, this quaint custom seems to have disappeared. I suppose kisses are so freely given and received nowadays, that rationing them with special permissions, and a limited open season seems strangely irrelevant.
Mistletoes can be readily seen in the landscape, and are rendered conspicuous in eucalypts by their denser yellowish foliage that contrasts with the lighter grey-green leaves of their host. Many ecologists, and other animals, will be glad that there is no annual Christmas mistletoe harvest, as these plants provide a rich wildlife habitat. Think of them as a dense shrub, suspended in the sky. They attach to the tree with a knobbly, woody haustorium, through which they extract water and nutrients from their host. While they can perform photosynthesize for themselves, they also steal some of the carbohydrates that their hosts make.
The knobbly base, dense branches and abundant leaves provide sheltered, stable nesting and resting sites. They have fruit, nectar, flowers and leaves, all which are consumed eagerly by insects, birds, possums and the like. The beautiful red and blue/black Mistletoe bird has a particular relationship with the plant. It specialises in eating the sweet pulpy fruit, but the mistletoe also gains. The sticky seeds rapidly pass though the bird’s gut and, as they come out the other end, a snappy shake of the tail sticks them on to other branches. Thus the seeds are well positioned to start new mistletoes.
Mistletoes take on even greater significance to fauna when the land is drought-stricken or heavily grazed, and the shrubs are eaten out. They are a bit like Robin Hood, stealing from the trees that are privileged enough to be protected from grazing and which can access water and nutrients from deepest in the ground, and then passing the bounty on to insects, bird and mammals.
But there is a downside, as mistletoe can overwhelm eucalypts and kill them. Red box and Blakely’s red gum seem to be especially susceptible. Mature trees with hollows are a scarce resource on the landscape, and it is quite upsetting to see mistletoes felling the last few large trees in a paddock.
We had this problem at Gang Gang. The handful of adult Red Box that had been left in our otherwise treeless areas were carrying a very heavy burden, and there were skeletons of dead trees with the knobbly evidence of too many mistletoes. If we lost the remaining trees we would lose our only chance of natural regeneration and the mistletoes would lose their future means of support.
My intrepid and impressively well-balanced husband made a rope ladder and climbed up in his Dunlop Volleys and cut off many mistletoes with a saw on a stick. A number of significant trees were saved this way and have since gone on to regenerate vigorously when the rains eventually came. At first I just used to tell him to “be careful”, but then I went on to insisting on a safety harness. If you are cutting any, it is worth knowing that the ones growing furthest out on the tips of the branches do less damage than the ones closer to the trunk. A safety tip.
When might you need to act to save a tree? How much mistletoe can accumulate before the life of a tree is threatened?
Nick Reid from the University of New England has studied mistletoes for much of his career and has shown that mistletoes can kill trees and their removal can forestall death of the host trees. If you want to keep a particular tree, start acting when more than half of the tree’s crown comprises mistletoe leaves. If 85% or more of the leaves forming the crown are mistletoe leaves, there is probably not much that can be done, the tree will die. This is obviously a misjudgement on behalf of the mistletoes, because of course they will also die.