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Trees that can see

by Timber Press on October 9, 2017

in Natural History

Trees don’t have two eyes like we do, yet they can see. They know how much light is hitting their leaves, and they know the quality of that light, too.

They know if it’s summer or winter by the length of the day, and they know if it’s noon or afternoon by the wavelength of the light. (The noon light is stronger and more direct, while the afternoon light is softened by the additional distance of atmosphere it must travel through when the sun is at a lower angle.) It’s as though the whole plant is covered with millions of tiny eyes—with each photosynthetic cell in each leaf acting as a miniature light receptor. Both animals and plants sense light when specific molecules are triggered by specific light wavelengths. Humans have some of the same receptor molecules that plants have. Red wavelengths? It must be morning or afternoon. Blue wavelengths? It must be midday. Plants respond to these wavelengths and to day lengths, too, by producing various hormones. Days are getting shorter? It’s fall, time to drop the leaves. Midday in summer? Ripen the fruit.

Plants have evolved this incredible sensitivity to light because light is everything to them. Without it they die. Light is a key ingredient in every morsel of their food (sugar made from sunlight, water, and carbon dioxide). We are used to the idea that plants need sunlight, but when we think about that concept more deeply we find that it has many implications. For instance, if the leaves in the top of a tree’s canopy intercepted all the light, no branches or leaves would be needed lower down and our trees would look like green umbrellas. Instead trees have evolved an architecture that allows them to share the light throughout.

And not only does the light sharing go on along a branch, but the branches themselves are arranged so they won’t completely block the light for the branches below. Along a trunk the branches alternate or grow opposite each other, or perhaps even spiral around, but they’re never stacked one immediately above the other like a ladder. Eventually, as a tree grows taller—indeed, as the entire forest gets taller—very little light reaches the leaves on the bottom branches. The tree then gets only a small energetic return from the resources it put into making those leaves. Finally it makes no sense to even have a branch there at all, so the tree stops sending resources to the branch. This process may happen very slowly, but finally the branch dies as a result of the low light, and perhaps in the next windstorm that branch will fall. Many decades of this lead to a tall tree with most branches on the top and very few lower down. We look at trees like these and know they are “forest-grown” trees. They have literally been shaped by light, or the lack of it.

If a tree grows in an environment with abundant light and no shading from other trees, it may be able to keep its branches all the way to the ground. Think of a grand live oak. This phenomenon of trees losing lower branches, or not, depending on light levels, gives us clues to the past. When we see a big tree in the middle of a forest that has large lower branches—living or dead—we can surmise that that tree once grew in the open. Most likely the tree stood in a pasture and shaded field animals, and perhaps field workers, from the harsh sun. In time, for various reasons, the pasture was left ungrazed and unmown and the gentle creep of succession resulted in a forest springing up where none had recently been. After a few decades the grand old pasture tree became a part of the forest, but its lower branches, deprived of light, began their slow decline.

Tree people call these wolf trees. Perhaps this term came from the idea that wolves gobble up all the food around them, and foresters saw these trees as gobbling up lots of space that could have been the domain of the tall, thin trees that their mills were built for. In the past it was thought that these trees, like the animals they were named after, should be eliminated. But the wolf trees were oen unmanageable, and so they were left. Many of these ancient ones have been witness to Native American encampments, to the now-extinct passenger pigeon, and to the birth of the United States. Today some of these trees are our largest, oldest, and most beloved. Birds and other animals are found in much greater abundance in these wolf trees than in the ones ready for factory production lines.

Unless you understand the primal urge of trees to grow toward the light, you might be tempted to look at a forest canopy and think that two nearby trees are purposely sharing the space. But when you see as a tree does, you realize that one is avoiding the other tree’s branches because the branch is dark, not light. More buds develop on the sunny side of the plant, so that’s the direction in which it will grow.

Click image for a look inside this book.

Robert Llewellyn’s photographs have been featured in major art exhibits, and more than thirty books currently in print. His book, Empires of the Forest: Jamestown and the Beginning of America, won five national awards in nonfiction and photography, and Washington: The Capital was an official diplomatic gift of the White House and State Department.



Joan Maloof is a scientist, writer, and the founder and director of the Old-Growth Forest network, a nonprofit organization creating a network of forests across the U.S. that will remain forever unlogged and open to the public. She studied plant science at the University of Delaware, environmental science at the University of Maryland Eastern Shore, and ecology at the University of Maryland College Park.



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