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For trees, as for all living things, life is a competition. It is a competition for space, for survival, for breeding rights, for water and for food. Since trees acquire their food through photosynthesis, it is also a competition for light. Send your leaves high enough to reach the sunlight – you feed. Get shaded by some larger tree – you starve.
Photosynthesis takes place in the chloroplasts, the green parts of the tree. The chloroplasts are concentrated in the leaves. Leaves are perfect solar collectors. They are very wide and very thin. Being wide, they present a large surface to the sun, maximizing the amount of sunlight they can collect. And since sunlight can’t penetrate very deep into the leaves, leaves don’t need to be thick. Being thin, the leaves are light in weight. This is very important to a tree as it needs to send its leaves as high as possible to outcompete a neighboring tree for sunlight..
As well as sunlight, trees need water. Water, along with carbon dioxide, are the raw materials in photosynthesis that combine to make sugar – the tree’s food. Water is also the liquid that fills up the tree’s cells. And it is the lifeblood of the tree, carrying nutrients to all parts of the tree. Trees can’t do without it.
Here in New England, this competition that is the tree’s life comes to a halt in the winter. It comes to a halt for two reasons: food and water – the two most basic needs of the tree.
The rate of photosynthesis, and therefore the amount of food a tree is producing, depends in part on temperature. As the temperature falls in winter, photosynthesis slows down until it stops altogether. The leaves are no longer producing sugars. They are no longer doing their job. Now, rather than being an asset to the tree, the leaves are a liability. They are a liability because of water.
Trees take up water through their roots. At the other end of the tree, the water evaporates from specialized cells in the leaves called stomata*. As the water evaporates from the stomata, it draws water up behind it, eventually pulling more water, as well as nutrients, in through the roots. As long as there is enough water entering the roots to replace the water exiting the leaves, everything is fine. In winter, however, the water in the ground may not be able to enter the tree because it may be frozen. And yet, water can still leave the plant through the leaves. While the stomata may be closed tight, water will evaporate through the surface of the leaf. In fact, the greater the difference in the moisture content between the inside of the leaf and the surrounding air, the faster the evaporation. Considering how dry the air is in the winter, it is not surprising that the trees’ leaves are a liability in the winter. To prevent the tree from drying out, the leaves must go.
Evergreen trees – pines, hemlocks, spruce etc. – have evolved another solution to winter dehydration. Their leaves – the needles – have much less surface area and have a thick waxy coating. This minimizes the amount of water loss. Therefore they can keep their needles year round (evergreens actually do shed their needles just like deciduous trees, but not all at once. Each fall, they shed the older needles only, the ones that are a few years old. The new needles remain). But needles don’t eliminate water loss altogether. Therefore evergreens need a larger water supply during the winter than deciduous trees and may be more susceptible to winter droughts.
So it would seem that in this competition between deciduous and evergreen trees, the deciduous strategy of shedding leaves might be a better strategy for surviving the winter. And yet, when we think of northern forests, the ones that survive long winters, we think of evergreens. The evergreen’s advantage is food.
As the weather warms and photosynthesis becomes possible again, evergreens can start to feed right away. Their needles are ready to start producing the sugars necessary for growth. Meanwhile, deciduous trees must first expend time and energy making leaves before food production can start. Therefore, everything else being equal, the evergreens have an advantage for early growth. Growing faster, they win the competition for sunlight, food and survival. If only the deciduous trees could get a head start.
Aspen trees have come up with a way to get this head start. There is no rule in nature that says that photosynthesis must happen in the leaves. It can happen any place in the plant that sunlight can reach. For instance, it could happen in the bark of the tree if the plant were to produce chlorophyll in the bark. While sunlight doesn’t penetrate the bark as easily as the leaves and photosynthesis is not as efficient in the bark as the leaves, every little bit helps.
Scratch the bark of an aspen tree and you might see a layer of green just below the bark. In fact, some aspens have a greenish tinge to their bark. This green is, of course, chlorophyll. As soon as the temperature warms enough for photosynthesis, the aspen can start to produce food in its bark. Not enough to feed the tree completely, but enough to give it just the little bit of a head start it needs to compete with the evergreens.
While researching this story, I happened to notice a young pine tree standing next to an aspen tree. The bark of the pine was green! Seems like the aspen is not the only tree looking for a head start.**
*Deciduous trees have stomata on the underside of their leaves. Some stomata are big enough to see with the naked eye. Others can be easily seen with a magnifying glass.
** Scientists have identified over 50 trees that are capable of photosynthesis in their bark.