I have a terrible tree pollen allergy so I was amused to see that Olivia Judson picked the tree for her Life-form of the month for April.

Trees figure in our mythologies and metaphors — the tree of life, the tree of knowledge — and we often imagine them to harbor spirits and sprites. They also figure in a big way in our reality: forests (still) cover about 30 percent of the planet’s land, and may make up as much as 80 percent of Earth’s biomass. That is, if you were to put all the organisms on the planet on a giant set of scales, trees would account for 80 percent of the total.

Better yet, trees harbor plenty of non-imaginary beings. Birds like starlings or blue tits nest in tree holes; others, like magpies and crows, build their nests high in the branches. Chimpanzees sleep in trees. A number of fungi — truffles, anyone? — associate with tree roots. Insects like wasps make houses (galls) in the leaves. And so on.

Until last November, we had a huge Norway maple tree which covered the entire front of the house and reached to the 4th floor.  Over a hundred years old, it as beginning to learn away from the house and over the street.  It was also rotting from an old wound.  It had to come down.  I was wondering if taking it down would lessen my allergies, but so far it doesn’t seem to have made a difference.

Olivia Judson again

Yet although trees are familiar to all of us, many aspects of their biology remain enigmatic: because they grow slowly and live for so long, they’ve been hard for us to study in the laboratory. Which is why they are my nomination for Life-form of the Month: April.

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What, then, is a tree? Precise definitions vary, but most of them mention the words “tall” and “woody,” and add that a tree has a single self-supporting stem (i.e., a trunk) that branches well above the ground.

The first trees appeared more than 375 million years ago, in several different plant lineages, in a burst of evolution that some authors have termed “the scramble for the sky.” If you’d been walking through the Earth’s early forests, you might have seen club mosses that were 40 meters (131 feet) tall, as well as giant horsetails. Both types of tree are now extinct. But what’s interesting about them is that they made wood differently from, say, pine trees. Pine trees grow outwards, forming a solid woody cylinder. In contrast, the trunks of tree-horsetails were hollow tubes, like bamboo. Tree-club mosses produced trunks with a hard outer casing, and a softer interior. Meanwhile, tree-ferns evolved a fourth type of woody structure: they grow several stems that are bound together by other tissues.

However you define a tree, they are beautiful, provide shelter, and are a renewable resource if harvested carefully.  I find looking at them restful – even if they make me miserable for a couple of months a year.