July 18, 2011

New Model Predicts Tree Height Across The US

MIT researchers have created a new model that predicts maximum tree height across the U.S.

According to the researchers, knowing how tall trees can grow in a region gives ecologists knowledge about 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.

The new model takes in basic meteorological data like average annual temperature, precipitation, humidity and solar radiation.  The team said the model computes how tall a tree is likely to grow under those conditions.

"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," lead author Chris Kempes, a PhD student in MIT's Department of Earth, Atmospheric and Planetary Sciences, said in a statement. "If you nail down that network structure correctly, then you can use it to predict how things change with size."

Kempes said a tree's network is a vascular "highway system" of xylem and phloem that pumps water and nutrients from the soil up to the leaves.

The team drew up an "idealized tree" to represent an average model of all tree species in the U.S. and developed equations to represent the relationships between fluid flow rate, tree size, and meteorological factors like average temperatures, rainfall, humidity and sunlight.

The group predicted the height of the tallest trees in the U.S. by using local meteorological data. 

The predictions from the model matched up with the Forest Service data, except for two geographic regions:  the arid Southwest and parts of New England.

The group used the same model to predict what would happen to tree height in the event of global temperature changes, and found that with an increase of 35 degrees Fahrenheit, the average height of the tallest trees would shrink by 11 percent. 

James Brown, Distinguished Professor of Biology at the University of New Mexico, says the model's real strength is in its equations.

"One of the goals that a lot of ecologists have in addressing questions of global change is to work out the relationship between the structure and function of plant communities and the climate," Brown, who was not involved in the study, said in a statement. "This model lays the foundation for that."

The researchers plan to use the model to "see the forest from the tree," by using maximum-tree-height data to extrapolate the heights of other, smaller trees in the forest.

"If you take a really small juniper tree that lives in the desert and you put it in the Northwest, it'll grow to perhaps four times as tall," Kempes said in a statement.

"But it won't grow to be the height of a redwood. And this is what all of ecology is interested in: How much of your existence is determined by the environment versus your genetics? Now we can concretely say this is the environmental side of things, and now we want to go after the species."

The researchers published their study in the journal PLoS One.


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