It’s an interesting question when you think about it. The same species of tree can vary its height wildly depending on the conditions of its climate. Take, for example, eucalyptus trees in Victoria, Australia, which vary in height from 4 feet all the way up to 300.
There are many variables that affect how high a tree can grow. Some of it has to do with gravity and the work that has to be done to transport water hundreds of feet in the air. The rest comes down to resource allocation; is the tree spending its energy making roots to soak up more water or growing upward to soak in more sunlight?
You need three things for tall trees—fertile soil, a lot of rain and a humid atmosphere. In moist areas, trees can devote fewer resources to growing roots. And if the sun isn’t beating down and removing the moisture from the surfaces of the leaves, plants can open the stomata that exchange gases with the atmosphere and make more energy.
This is all well and good, but recently Thomas Givnish, a professor of botany at the University of Wisconsin, wondered which factors were the most important.
“Since Galileo’s time,” Givnish says, “people have wondered what determines maximum tree height: ‘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.”
To figure it out, Givnish looked at the ratios of carbon isotopes in the wood in an extremely moist environment of Australia. His thinking was that heavier carbon isotopes should accumulate in moister areas where faster photosynthesis can balance the costs of building higher into the sky.
And that’s exactly what they found.
This indicates that both fighting gravity and how much moisture can stick to a tree’s leaves govern the ultimate height of trees. In drier conditions, gravity stops the trees from moving upward. In moist conditions, it’s all about how long the moisture sits on the leaves.
Most studies of tree height have focused on finding the tallest trees and explaining why they live where they do, Givnish says. “This study was the first to ask, ‘How does the maximum tree height vary with the environment, and why?'”
The study, “Determinants of maximum tree height in Eucalyptus species along a rainfall gradient in Victoria, Australia,” was published in the journal Ecology by Givnish and colleagues Suen Chin Wong, Hilary Stuart-Williams, Meisha Holloway-Phillips and Graham D. Farquhar.