Home & Garden Columns

Trees Manage Water to the Benefit of the Atmosphere By RON SULLIVAN Special to the Planet

Tuesday January 24, 2006

It’s only on my worst, most misanthropic days that I suspect most of my fellow humans of not noticing that the trees around us are more than overdecorated hatracks. We all know they’re alive, right? And we all know, or ought to, that they’re in motion, growing, moving (though at a pace we’re ill-equipped to see), performing, building the massive edifices of themselves with light and carbon and air.  

They move themselves mostly by building, a few cells at a time, reaching by expanding, and bending by swelling or compressing themselves on one side or another, toward light above ground and water below. In the process of making their food and substance they emit oxygen and water into the atmosphere. Turns out they’re juggling that water in a more complex and sophisticated manner than anyone had suspected, according to recent finds by various scientists.  

The heights to which they can pump water, passing it and the nutrient minerals dissolved in it along their xylem tissues, vary among species—and it’s evidently the limiting factor for many trees’ ultimate heights. In the top leaves of big old redwoods, for example, photosynthesis is much less efficient than it was when the tree was younger and smaller.  

There appears to be a simple mechanical reason for this: Water is forced along those long columns largely by the force of evaporation through the stomata, the little pores on each leaf. At the top of a big tree, water is scarcer because it’s harder to draw up, so the stomata are closed more often to limit evaporation. But leaves also take in carbon dioxide through those stomata, and can’t do so when they’re in effect holding their breath. Metabolically, the tree ends up running as fast as it can just to stay in place.  

Conifers have larger pores in the connections within their water-conducting systems, which allow them to move more water through with less internal resistance. This would be one reason those redwoods and big pines and spruces can tower over other trees in the forest.  

Plumbing isn’t the whole story; there are internal cell-growth effects of hydrostatic pressure differences, and some tropical trees that also grow more slowly when they get big, but USDA ecologist Michael Ryan and his team studying a set of eucalyptus seedlings in Hawaii for seven years think that water’s too plentiful to be the limiter. They’re still looking for that. 

Meanwhile, a set of UC Berkeley researchers has figured out that deep-rooted trees engage in water-banking: they store water in different levels of the soil for their own use. By doing this, they’re able to keep their rate of evapotranspiration—the release of water into the air around them, humidifying and cooling it—and of photosynthesis 40 percent higher in dry months than they would otherwise be, and keep their place and the planet cooler than they’d be without them. 

Todd Dawson, Jung-Eun Lee, and Inez Fung of UC Berkeley, and their colleagues including Rafael Oliveira of the Laboratório de Ecologia Isotópica at the University of Sao Paulo, Brazil, studied Amazonian trees in particular but said their model showed effects in temperate places too.  

The tap roots that some species have, which can extend more than their height underground, have more to do than anchor the tree. Fibrous feeding roots reach a broad, shallower area around the tree; tap roots, using chemical potential gradients, redistribute water downwards in rainy seasons and upwards in dry seasons to keep nurturing life processes. These processes are how trees sequester carbon from the atmosphere—another control on planetary climate extremes. 

“These trees are using their root system to redistribute water into different soil compartments,” Dawson said. “This allows the trees and the forest to sustain water use throughout the dry season. … There’s this skin on the Earth—plants—that has an effect on a global scale, pulling carbon dioxide out of the atmosphere and letting water go, in a dynamic way that has climatic implications.” 

We know that trees modify our immediate surroundings in the city—you can feel that directly, standing in a tree’s shade on a hot day, being cooled by its transpiration. Apparently they’re actively doing that favor for the whole world, too.