New model traces missing gas in Earth's atmosphere to tree root systems

By Sarah Davidson

The root systems of trees are known to be major storage banks for carbon dioxide (CO₂), a major greenhouse gas implicated in global warming. Figuring out exactly how much of the carbon is held by these roots has been complicated by the difficulty of predicting the mass of the underground root systems.

But now Cornell professor of plant biology Karl Niklas and a colleague have proposed a mathematical model that is able to predict very accurately size-dependent relationships for small- and intermediate-size plants, from the very smallest herbaceous plants to the world's tallest trees. In doing so, the model can determine the mass of root systems.

"There's a lot of missing carbon in the Earth's ecosystem, and ecologists need to know how much of it is underground and stored in root systems," Niklas explained. "With our new model, we can provide reasonable predictions about underground plant biomass in the form of root systems. These predictions can then be introduced into global climate models to provide what I believe are more reasonable estimates of a number of climatological features."

CO₂ is an atmospheric gas that, due to the combustion of fossil fuels, has been rising steadily over the past century and a half, becoming an environmentally harmful greenhouse gas, along with tropospheric ozone, nitrous oxide and methane. Scientists have attempted to account for the rise in global CO₂ by calculating the global balance between photosynthesis -- which removes CO₂ from the atmosphere and stores it as fixed carbon -- and processes that return CO₂ to the atmosphere -- such as combustion and respiration. But such global accounting has revealed an apparent imbalance, or missing sink, for carbon that researchers are attempting to identify.

Niklas and physicist Hans-Christof Spatz of the University of Freiburg, Germany, are authors of an article on scaling relationships of plants in a recent issue of Proceedings of the National Academy of Sciences.

"Another likely application of this work," Niklas said, "is for foresters who are interested in when to harvest trees. It is very easy to measure the diameter of the base of a tree.

"Our equations allow them to predict the mass of the stems. Very simple linear dimensions allow you to make a reasonable estimate of when to harvest a tree."

The new mathematical model proposed by Niklas and Spatz describes a complex system of scaling relationships showing that trees' height and mass are governed by growth and hydraulic constraints that influence the quantities of roots, stems and leaves. "Growth and hydraulic -- not mechanical -- restraints govern scaling of tree height and mass," Niklas said. "Imagine a flagpole that continues to grow in height but not in diameter. Eventually the flagpole will reach its critical buckling height -- the point at which it begins to deflect from the vertical under its own weight."

He noted that hydraulic constraints influence how the cross-sectional areas of roots and stems are interrelated and how stems' cross-sectional areas scale with respect to the quantity of leaves that must be supplied with water on a daily basis. He explained these relationships in terms of basic laws of physics: "Plants take up water through roots, transport it through their stems and lose it through their leaves. All of these organs have to be scaled in terms of their size so that the total amount of water is conserved per unit time."

Reported and written by Sarah Davidson, a Cornell News Service science writer intern.