December 31, 2012 | David F. Coppedge

An Unexpected Forest Helper: Mistletoe

Long thought a tree-killing bane, parasitic mistletoe appears to do much more good than harm to a forest ecology.

David Watson, a researcher at Charles Sturt University in Albury, New South Wales, has been studying mistletoe for years, according to Stephanie Pain at New Scientist, who wrote “Marvellous mistletoe: Giving forests the kiss of life.” Here’s what Watson has concluded from his research:

“Mistletoes are the key to a rich and healthy forest. They are the engine that drives diversity from the forest floor to the canopy.

Mistletoe gets its water from the host tree, but is able to manufacture its own carbohydrates through photosynthesis. For this reason, mistletoe is called “hemiparasitic.”  Watson found that mistletoe rarely kills a tree.  More interesting, though, are the benefits mistletoe brings to the forest:

Always assured of water, evergreen mistletoes are a reliable year-round source of food for many forest dwellers, providing nutrient-packed leaves, sugary nectar and juicy, fat-rich berries. The dense clumps offer hideaways and cool places to roost, while the tangle of sturdy stems offers secret nooks for small birds to nest in and a solid platform for larger ones to build on.

Watson proved these benefits by removing all the mistletoe from a patch of eucalyptus forest in Australia.  The reduction in diversity was dramatic, not only among birds and tree-climbing animals, but even in the forest floor.  That’s because the fleshy leaves provide a nutritious food source when they drop to the ground.  “Those leaves are packed with goodies,” Watson remarked (though many are poisonous to humans).  Soils without the mistletoe are impoverished in good bacteria, insects and invertebrates.  Trees without it reduce good spots for nesting and hiding.

Mistletoe is not a single species but a category: “What links these plants is not shared ancestry, for mistletoes have evolved at least five times in different groups of plants, but their lifestyle.”  There are some 1300 species.  They live on every continent except Antarctica.  Some ecologists consider mistletoe a “keystone species, an organism that has a disproportionately pervasive influence over its community” (Wikipedia).

Stephanie Pain listed animals that use mistletoe, including porcupines and squirrels that hibernate in the clumps.  Birds of all sizes, including hummingbirds and hawks benefit from the nutritious leaves and berries.  Even gorillas and rhinos browse on mistletoe.

The seeds of some species are spread by birds (see Mistletoes in Australia).  They can survive the bird’s digestive tract, and then stick to twigs with an adhesive coating around the seed.  Other species, particularly in the genus Arceuthobium, can launch their seeds like rockets from branch to branch.  The fruit pods build up hydrostatic pressure as they mature until the pods rupture, sending the sticky seeds on a ballistic path at 55 mph, landing up to 50 feet away (source: waynesword.palomar.edu; see also American Journal of Botany).

If an athlete get’s athlete’s foot, what does an astronaut get?*  We hope you enjoyed this ecology story for year end 2012, as a reminder that things in nature are not always what they seem, and more wonderful the closer one looks.  Most people only think of mistletoe at the holidays as an excuse for stealing a kiss.  As this entry shows, there’s much more to the story.  Thank you for reading Creation-Evolution Headlines this year.  Resolve to tell your friends!

*Missile toe, if you never heard that joke in elementary school.

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