Trees' Engineering Skills

Taking advantage of a unique labyrinth of Texas caves festooned with tree roots, Duke biologists have given trees the most exacting root-to-twig physical of their circulatory system yet.

The scientists' findings reveal the impressive adaptive engineering of deep-rooted trees in adjusting the size and structure of their piping, or xylem, to maximize water uptake, maintain their integrity, and avoid flow-blocking embolisms. In particular, the findings reveal how the deepest roots develop the largest conduits in order to draw deep water most efficiently.

The researchers, led by biologist Robert Jackson, a professor of environmental sciences and biology in the Nicholas School of the Environment and Earth Sciences, published their findings in the September 2004 issue of the journal New Phytologist.

In their study, the researchers took advantage of the fact that a well-mapped system of limestone caves runs beneath the Edwards Plateau in central Texas. Thrusting into those caves from above are the roots of trees, some of which reach depths of sixty feet. Jackson and his colleagues descended into the caves and sampled roots of four tree species--juniper, evergreen oak, deciduous oak, and deciduous bumelia lanuginosa, also known as chittamwood.

The researchers found that trees adjust their root anatomy very effectively to work in different environments. Shallow roots and especially stems have thicker walls and smaller interior diameters because they must support the tree more, and because they experience more drying and temperature extremes that cause cavitation, the arboreal equivalent of developing an embolism. Deeper roots, which are better protected from such demands, can have larger diameters and thinner walls to maximize the conduit size and enhance water flow.

"The data are interesting because they show that the same individual plant can adjust its anatomy and physiology to maximize water transport deep underground, where the plant is less likely to experience drought or freezing conditions, which cause cavitation," he says.