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Stiffness, cells, etc

Posted: Tue Dec 24, 2019 10:31 am
by Lucas
https://northernwoodlands.org/discoveries/tall-trees

For many years, scientists have wondered what factors influence how tall a tree can grow. Much of the research has focused on how high water can be transported internally from roots to leaves, but a tree’s strength and stiffness are also critical constraints. More than two centuries ago, Swiss scientist Leonhard Euler demonstrated that tall columns must be stiffer – but not necessarily stronger – than short columns of equal diameter and density if they are to avoid bending. Many tree biologists have concluded that tree species have a fixed stiffness-to-density ratio, so tall trees adjust for increased height by adding mass to their trunks.

Richard Jagels, Professor Emeritus of Forest Resources at the University of Maine, wasn’t convinced that was true.

He investigated whether tree species that grow tall have a greater relative stiffness than do shorter growing species. For more than a decade, he collected data on the mechanical properties of trees from various regions of the world using standardized strength tests. What he found was that for all of the regions he examined, taller tree species were stiffer, on average, than shorter species of the same density. He found no relationship between strength or density and the height of a tree.

Stiffness, he said, refers to “a tree’s ability to not bend, whereas the strength phenomenon is about how much energy can be absorbed before breaking. [Increasing] stiffness isn’t the only thing a tree does to get tall – some trees become tapered or develop buttressed bases, but these are more energy consuming strategies and detract from energy that could be used to develop more leaves or roots.”

Jagels found that tall trees, especially conifers, have evolved to be stiffer through a genetically controlled anatomical mechanism. In young trees, cellulose microfibrils in wood cells are oriented at an angle from the vertical axis. As the trees age, they produce longer and wider wood cells with cellulose microfibrils oriented closer to vertical. This increases tensile strength in the outer layers of wood and enhances stiffness. The tallest trees produce the longest and stiffest cells while also increasing water-transport potential.

“This finding has important ramifications in the biology and management of forests,” Jagels said, “but it also has implications for materials science. We create a lot of composite structures that are essentially adaptations of tree composites. Finding out there is a way you can adjust this fibril angle as a way of improving stiffness may have ramifications for composite structures.”

The research touches on other issues, as well. In the Northeast, white pines are among the tallest trees – and therefore the stiffest – but due to management strategies and the historic harvesting of the tallest specimens, Jagels believes they are no longer reaching their maximum height.

“If you only harvest the biggest trees, you may be harvesting a genetic component out of the forest,” he said. “Tall trees also do best if growing in even-aged stands, but we don’t like to manage even-aged stands. To have a really tall forest, you need to have all the neighbor trees be really tall to provide for cooperative wind protection.”