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Trees and Mistletoe

Germinating mistletoe seeds on a tree.
Tomasz Klejdysz/Getty Images/iStockphoto
Germinating mistletoe seeds on a tree.

Given the biological purpose of mistletoe, Nalini Nadkarni finds it pretty strange that this parasite is also a symbol of love.

The Celtic Druids observed that mistletoe could flourish in sacred oak trees during the harsh winters of the British Isles. It became their symbol of vitality and fertility. The custom of kissing under mistletoe arose in England in the mid-1800s, and today, we chime in on popular songs about mistletoe like the tune, “I Saw Mommy Kissing Santa Claus.”

Knowing the biology of mistletoe reveals some contradictions about it as a symbol for love. After all, mistletoes are parasites. They infect trees to get resources from their hosts by penetrating the bark with a root-like tissue called a haustorium, which connects the mistletoe to the tree’s transport systems, diverting water, nutrients and sugars to itself.

Mistletoe can reduce the growth of some economically important trees, like Lodgepole pine and Douglas fir. But from an evolutionary standpoint, all parasites — from tapeworms to malarial vectors — are moderate in their takings, since diverting too much from the host would mean literally eating themselves out of house and home.

Nearly 250 species of birds disperse mistletoe. The seeds are surrounded by a sticky goo called viscin, which cements the seeds where they fall. Some birds wipe their bills onto branches; other birds excrete seeds in their droppings, manifesting the Old English origin of the word mistletoe: “mistel” meaning dung and “tan” meaning twig. Hence, dung-on-a-stick.

Many other animals — including porcupines and lizards — eat mistletoe berries and leaves as a critical part of their diet. In one experiment in Australia, mistletoe was removed entirely from study sites. Those areas ended up with 25% fewer bird species than neighboring sites where mistletoe was left intact.

So, as with many interactions with trees, the meaning of mistletoe is wonderfully complex. Mistletoes can reduce growth of their hosts, but they also provide a bounty of resources for the organisms surrounding them.

That’s something to reflect on when you’re invited beneath that dung-on-a-stick for a holiday kiss.

Dr. Nalini Nadkarni is an emeritus professor of both The Evergreen State College and the University of Utah, one of the world’s leading ecologists and a popular science communicator. Dr. Nadkarni’s research and public engagement work is supported by the National Geographic Society and the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation. @nalininadkarni
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