High and Dry - physiological reasons why trees stop growing - Brief Article

If a tree grows too tall, it may end up with broken water pipes.

Trees are wooden giants, towering over us puny animals. For a tree, height is often the secret to success, allowing it to get out of the shade of neighboring trees and soak up sunlight. Yet after a few decades, most trees stop ascending. Why, ecologists and botanists would like to know, are trees as tall as they are and no taller?

The question of tree size isn't purely academic. As we pump billions of tons of carbon dioxide into the atmosphere, Earth's climate and organisms will respond in ways we're unable to predict with certainty. One of the unknowns is how the extra [CO.sub.2] (a crucial ingredient in photosynthesis) will affect plant growth. Will it stimulate growth and thus cause plants to draw in even more of the gas--perhaps enough to help reduce global warming? A crucial part of the equation may be just how tall trees can grow.

During photosynthesis, a tree uses the energy of the sunlight hitting its leaves to combine water, carbon dioxide, and assorted minerals in the production of carbohydrates. Water is also needed to transport nutrients, control temperature, and maintain healthy tissues. This precious liquid has to be absorbed through the tree's roots and then carried up the trunk and branches through a system of narrow tubes known as the xylem.

Each leaf is covered with stomata, tiny pores through which [CO.sub.2] enters and water evaporates. Molecules of liquid water tend to stick together, and as water evaporates from a leaf, the remaining water "pulls" on the water below it, creating an upward movement that extends all the way down through the tree to the soil. As water moves into the leaf and out into the air, tension develops in the xylem, pulling in more water from the roots.

Although a single stoma can create only a minuscule tug on the water inside a tree, all of the stomata on all of a tree's leaves can create a huge force capable of hauling about a hundred gallons of water a day up a large tree. And the most elegant feature of this design is that the tree doesn't have to put any effort into its hydraulic delivery system; evaporation (prompted by solar energy) does the work instead.

But this tremendous feat of natural engineering is not risk free. Drier air sucks the water out of a tree with greater force. As evaporation pulls a column of water upward, the liquid's molecular cohesion puts up some resistance, causing the water to stretch like a rubber band. If the force pulling up the water column is considerable, the column may snap like a rubber band as well. The result is a gap in the column, taking the form of a bubble.

Although botanists know little about this kind of bubble (it's hard to study what goes on inside a tree trunk), they're pretty sure it's a problem for a tree. Unless the break is repaired, the tree will no longer be able to draw water up from the roots. Scientists are investigating this process but don't yet understand it fully.

Having studied the sucking action of trees, biologists Barbara Bond, of Oregon State University, and Michael Ryan, with the U.S. Forest Service, believe (as do many other scientists) that prevention is part of the tree's solution. Trees, they suggest, have developed adaptations that keep water columns from snapping. When the tension that develops as water escapes stomata exceeds a certain level, some of the leaves' pores will simply close up, decreasing the pull of evaporation.

The risk of rupture is greater for big trees than for small ones, according to Bond and Ryan, because the columns in the big trees are longer and thus offer greater "hydraulic resistance." (In physics, the resistance of an object increases as its length increases.) This resistance, added to the extra gravitational force acting on the water contained in tall trees, means that these taller trees require more pulling power to draw up water.

Bond and Ryan have found circumstantial evidence of this risk by watching how stomata on both short and tall trees behave. In the morning, as the air is warmed by the rising sun, its relative humidity drops, increasing the force exerted by evaporation on the leaves of trees. Eventually, trees of all sizes will close their stomata, but tall trees close theirs earlier in the day than shorter ones.

Although tall trees may gain some protection by shutting down their pores, they do have to pay a price. Closed stomata cannot draw in air. Without [CO.sub.2], photosynthesis comes to a halt, and without photosynthesis, the tree stops growing. Exactly where this trade-off occurs in the growth of a tree depends on its physiology and its environment. Bond and Ryan think it's no coincidence, for example, that the world's tallest living trees, the redwoods, are bathed in fog coming off the Pacific Ocean. In the moist air, they propose, these trees don't lose water as quickly, so they can keep their pores open longer and perform more photosynthesis. But even for fog-shrouded redwoods, there is a limit. Sooner or later, every tree has to give up its quest for height in order not to die of thirst.