Just What Makes A Tree Tall?

What makes a tree tall? Or rather, what stops a tree from growing? These are questions that have kept researcher scratching their heads for centuries. Now a team of scientists claim they have it pretty much figured out.

Thomas Givnish, an expert of botany at the University of Wisconsin, recently traveled with his colleagues to Victoria state - a region in southeastern Australia - to make discoveries that would resolve the tree height debate.

"Since Galileo's time, people have wondered what determines maximum tree height," he said in a statement. "'Where are the tallest trees, and why are they so tall?' Our study talks about the kind of constraints that could limit maximum tree height, and how those constraints and maximum height vary with climate."

While at Victoria state, Givnish paid special attention to Eucalyptus trees, which exhibit almost the entire range of possible tree heights between all their different species - one tree in particular. Eucalyptus regnans, called mountain ash in Australia - different than the US mountain ash - is the tallest flowering tree in the world. The tallest of the E. regnans is 33 feet tall - only 50 feet shorter than the tallest redwood in the world.

According to a study authored by Givnish and recently published in the journal Ecology, rainfall, as suspected, plays an important role in tree height. Without enough water to fill a plant up, it cannot properly support its own weight or grow higher.

However, Givnish adds that evaporation in an area can exacerbate a tree's water demand, not simply by evaporating existing water supplies but drawing moisture out of leaves and flowers, making a tree more thirsty. This alone could suggest that we might start seeing shorter trees in some parts and taller trees in others as climate change continues to alter precipitation and drought patterns.

Root demand, the study says, is another little-considered factor where a plant must put more energy towards growing longer roots to reach important nutrients.

"In moist areas, trees can allocate less to building roots," Givnish explained. "Other things being equal, having lower overhead should allow them to achieve greater height."