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Geo-Joint: Giant Ash

Posted on September 08 2015


There are all kinds of record-sized trees but of course the granddaddy of pure mass is the General Sherman, a giant sequoia found in Sequoia National Park. And the tallest trees are the coast redwoods of northern California. Less well known is the giant ash (or mountain ash, or giant swamp gum) of Tasmania, Australia’s island state. It’s really not an ash but a species of eucalyptus (Eucalyptus regnans) like so many of Australia’s trees and holds the record as the tallest flowering tree at 324 feet. The big sequoias, and those coast redwoods that set the tallest-tree record at 379 feet are cone-bearing trees. The giant ashes and redwoods are both enormous (a big ash can be almost 70 feet around at its base), but the ash grows much faster, achieving its height in 350 to 450 years. The coast redwood can live almost 3,000 years. It’s speculated that the ash could live 500 years. A specimen merely 3 feet shorter than the tallest living redwood was felled in the late 1800s, and although its age was not determined, it’s possible such a tree could have claimed the height record if it were still living and growing today. Loggers in the old days were fond of cutting down the biggest thing they could find, so, sadly, the best examples of mountain ash are gone.

Growing tall gives a tree the edge on capturing sunlight before some other tree shades it out. But the taller it gets, the harder it is for water to get drawn up all the way to the top. The tree modifies the size of the pores on its leaves to preserve the water it’s already got, but this also limits its intake of carbon dioxide, and therefore sugar production. When the effort to pull water up exceeds the energy created by photosynthesis, the tree calls it quits and stops trying to get any taller, though it may continue to fill out laterally. Giant ash are also constrained in their ability to set height records when their lives are shortened by fire and fungus. Less resistant to fire than redwoods, they often succumb to the heat of a forest fire before they have reached their potential height. And a lack of defensive chemicals leaves them susceptible to fungus infection, which leads to rot and death. However, no tree grows as fast as a eucalyptus, so with the right conditions over a few hundred years, a hearty giant ash could one day claim title of “Tallest Tree in the World.”

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