The tallest tree in the land

Knowing how tall trees can grow in any given region can give ecologists a wealth of information, from the potential density of a forest and size of its tree canopy to the amount of carbon stored in woodlands and the overall health of an ecosystem. Now an MIT researcher, along with colleagues at the University of Maryland and the Santa Fe Institute in New Mexico, has come up with a simple model to predict the maximum tree height in different environments across the United States. The researchers’ results have been published in the journal PLoS One.

The model takes in basic meteorological data — such as average annual temperature, precipitation, humidity and solar radiation — and computes how tall a tree is likely to grow under those conditions. The concept sounds simple enough, but lead author Chris Kempes, a PhD student in MIT’s Department of Earth, Atmospheric and Planetary Sciences, says it took an understanding of plant mechanics, as well as fractal geometry, to accurately link tree height with meteorology.

“The branches of a tree really form a fractal, where if you cut off one of the limbs … and blow it up to the size of the tree, it’ll look like the whole tree,” Kempes says. “If you nail down that network structure correctly, then you can use it to predict how things change with size.”

Kempes describes a tree’s network as a vascular “highway system” of xylem and phloem that pumps water and nutrients from the soil up to the leaves, which in turn transport the sugary byproducts of photosynthesis back down to the roots. This pumping capacity, or fluid flow rate, is a bit of a balancing act: Trees of a given size have certain rates of metabolism that pump enough water and nutrients to survive without using up all of the available resources, such as light and water.