What limits the height of trees? Is it the fraction of their photosynthetic energy they devote to productive new leaves? Or is it their ability to hoist water hundreds of feet into the air, supplying the green, solar-powered sugar factories in those leaves? Both factors, resource allocation and hydraulic limitation, might play a role.
Scientific debate has arisen as to which factor (or what combination) actually sets maximum tree height, and how their relative importance varies in different parts of the world. In research to be published in the journal Ecology, and currently posted online as a preprint, Thomas Givnish, a professor of botany at the University of Wisconsin-Madison, attempts to resolve this debate by studying how tree height, resource allocation and physiology vary with climate in Victoria state, located in southeastern Australia. There, Eucalyptus species exhibit almost the entire global range in height among flowering trees, from 4 feet to more than 300 feet.
"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." One of the species under study, Eucalyptus regnans, called mountain ash in Australia, but distinct from the smaller and unrelated mountain ash found in the U.S, is the tallest flowering tree in the world. In Tasmania, an especially rainy part of southern Australia, the tallest living E. regnans is 330 feet tall.
(The tallest tree in the world is a coastal redwood in northern California that soars 380 feet above the ground.) Southern Victoria, Tasmania and northern California all share high rainfall, high humidity and low evaporation rates, underlining the importance of moisture supply to ultra-tall trees. But the new study by Givnish, Graham Farquhar of the Australian National University and others shows that rainfall alone cannot explain maximum tree height.