The heavy rains of late have turned some homeowners in our region into hydraulic engineers, their sump pumps working overtime to pump rainwater out of flooded basements. When saddled with such a chore, it's hard not to look up at the maple tree in the yard with a hint of envy, trees being masters of the task.
Raising water and setting it free into the air – a process known in plants as transpiration – has been the subject of research for many years, though it's hard to fully appreciate it because we can't often see it. I used to hold my students' attention by asking them to imagine that a tree was a giant fountain spraying water into the air. Exaggerated, of course, but they opened their eyes a little. Then I suggested they lean against an apple tree on a summer day and imagine they are listening to four gallons of watery sap gurgling upward every hour. Eyes opened somewhat wider.
Suppose you could cover an acre of grass with a tarp, collect all the evaporated water, and funnel it into a measuring device. Rutherford Platt, a botanist, once calculated that by mid-summer you would be collecting six and one-half tons of water each day. A stalk of corn, a giant grass, needs over 50 gallons of water in its lifespan, and an acre of corn requires more than half a million gallons during its growing season. If an apple tree – small as trees go – raises thirty or forty gallons a day with a thousand leaves, what would a healthy sugar maple be capable of with its tens or even hundreds of thousands of leaves? Now imagine what an entire forest moves.
Comparing a tree to a sump pump makes the tree seem like an inanimate plumbing device, but that's far from the case. A green plant is a living system that ceases working as soon as the tree dies; it is just as sophisticated in a hydraulic sense as our own circulatory system, although the dynamics and construction of the two are entirely different. In a sense, the plant's way is superior because it relies upon physical laws instead of a muscular pump, and a system obeying principles of physics doesn't get tired or wear out.
How does the “pump” work? If you've ever closed the end of a drinking straw with your finger and lifted a column, you know that water molecules cleave to one another; this cohesion allows water to be drawn upwards through the tree's vascular tissues (xylem).
But why does the column move upward? Because evaporation takes place, molecule by molecule, at the upper end of the column. Imagine a long metal chain hanging vertically: someone at the top removes links one by one and someone at the bottom snaps new links on. The entire chain ascends whenever there is a vacancy at the top. As the water molecules depart through stomata on the undersides of the leaves, new molecular links leave the soil, enter the roots, and join the column of water.
Sixteen million tons of water rain down upon the earth every second, and every second sixteen million tons must rise from the earth to balance the system. Over and over, climatologists and plant ecologists attempt to estimate the amount of water that ascends into the atmosphere as a result of plant activity. The figures vary, yet they all astound, and not one is minimal enough to disregard. Certainly, most water evaporates from oceans, but if the contribution of plants was much reduced, climate, weather, and the balance of atmospheric water would be drastically altered, even to the extent the whole system eventually could go out of whack, finis for life on earth.
I'm fond of trees. They are great old living things. When I lie in my hammock in summer months looking at the leaves above, I've been known to reach out and touch one of the supporting maples with a friendly pat, acknowledging our kinship and my dependency. I don't let anyone see me do it, but now I've said it.
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