At about this time every year, many Americans bring a small evergreen tree into their home to help celebrate the holidays, perhaps as a symbol of life's refusal to surrender during the long bleak winter.
But why are evergreens in fact always green?
The answer is a matter of how evergreens and broad-leaf trees over the millennia have adopted different survival strategies, particularly in how they handle the vital question of water.
In temperate climates, most evergreens are conifers, or cone-bearing trees. And conifers -- pines, firs and spruces -- are among the largest, oldest and most tenacious of the planet's organisms. There are about 550 species of them, and many live in cold, drought-plagued and generally inhospitable habitats:
Some species in Canada and Siberia thrive where temperatures range from minus 95 degrees Fahrenheit in the winter to 85 degrees in the summer. How tough are they? The Monterey pine disdains to open its cones and spread its seeds until it's been scorched in a forest fire. Some of the bristlecone pines of Inyo National Forest near Bishop, Calif., are 4,600 years old, making them rough contemporaries of the pyramids.
There is one reason evergreens are so tough: their needles.
It's important to know that a conifer needle is just a round, stiff leaf with a very sharp point.
Then, it's important to know what a leaf does: Like all plants, trees are basically solar engines, and their leaves or needles are power cells -- designed by evolution to gather the maximum number of photons of sunlight. Trees, in turn, are designed to produce a maximum number of leaves.
An averaged-size sugar maple, for instance, will cover a footprint of perhaps 1/100th of an acre of ground. The surface area of that same tree's leaves will typically cover about 1/3 of an acre.
Leaves use sunlight to power photosynthesis. In this chemical process, chlorophyll in the leaves takes carbon dioxide from the air and combines it with water and minerals from the soil to manufacture food. And leaves have to breathe drawing in carbon dioxide and expelling oxygen as a waste product. To do this, each leaf has small pores in its epidermis, openings called stomata.
Each leaf must also limit the loss of water to the air through evaporation, or risk dehydration. So each leaf has a waxy cuticle that acts as a kind of waterproof seal.
In areas with mild climates, broad leaves perform all three functions just fine. They turn their wide, flat faces to the abundant sunlight. Their stomata sit near the leaf's surface, letting carbon dioxide flow in and oxygen drift out. The thin cuticle on the leaf regulates the amount of water lost to the air, even as the tree draws fresh water in through its roots.
When shortening periods of sunlight signal the approach of cold weather, and the threat to freeze and kill a deciduous tree's leaves, the organism makes a tactical retreat. Chlorophyl productions stops, nuturients migrate from the leaves into the branches; then the trees drops the leaves.
The tree settles down to a long winter's nap.
But in some climates, things don't work so smoothly.
Much of the western United States, for example, gets little rainfall or is subject to long droughts. In northern forests, the long winter freezes the ground water, making it impossible for trees to absorb much through the soil. Frigid winds would quickly desiccate thin, unprotected branches.
So the conifer has adapted by trading in its leaves for needles. Needles have a far smaller surface area. They generally have deep-set stomata. Their waxy covering, or cuticle, is very thick, and backed up by one or more layers of thick-walled cells -- a layer called the hypodermis. All these features make needles more watertight than leaves. They lose far less moisture through evaporation.
Having less surface area also limits the capture of sunlight. So a given needle can't produce as much food as a given broad leaf. If conifers grew only during the warm weather months, they might quickly get crowded out in some areas by deciduous trees that shoot up every summer.
But the features that protect needles against drought also protect them against cold, so they can remain active even in cold weather. That, of course, is exactly what these small solar cells do hum along in the brief, feeble light of winter.
Conifers have still other adaptations to cold: The conical shapes and stubby branches protect the trees from being knocked over or damaged in heavy snow or high wind. As a strategy for spreading seeds, cones are better adapted to cold, dry conditions than, say, fruit. (Conifers are part of a group of plants called 'gymnosperms,' which means 'naked seed.')
So that holiday totem -- festooned with tinsel and lights, hung with plastic gee-gaws -- is worth a second look.
Before it was abducted from its forest or tree farm, that gymnosperm was probably better suited to survive the long winter than you were.
Pub Date: 12/18/96