Old Trees Too Large for Their Own Good

In our Autumn 2004 Discoveries, we reported on research suggesting that friction and gravity constrain evapotranspiration in trees and thus limit tree height – to 426.51 feet in the redwoods studied, though the principle is thought to operate in every species of tree. Tree height and growth rates are known to slow as trees age, and though the reason has not been clear to scientists, the struggle for water to reach the tops of ever-taller trees is one possible explanation.

New evidence from Maurizio Mencuccini and his team at the University of Edinburgh bolsters the theory that tree size, and not necessarily age, is the most important factor in determining a tree’s vitality. The main reason, according to the researchers, is, again, the increased stress associated with supplying all parts of the tree with nutrients and water as it grows in height and width.

While the theory of senescense – the loss of vitality suffered by aging organisms at the cellular level, including the breakdown of DNA – is thought to apply to most plants and animals, increasing evidence that it may not apply to trees led Mencuccini and his team to undertake this particular study. They studied four tree species of diverse types, representing different evolutionary histories, water-transport mechanisms, leaf types, and management histories. They then measured the growth and leaf efficiency of different-aged and -sized members of each representative specimen growing naturally in the field, while also measuring the same characteristics in uniform-sized, grafted or rooted propagules that were taken from the “parent” study trees. In essence, they grew old trees on new roots to compare with the large, genetically identical older trees growing naturally.

What Mencuccini’s group found was that in the field trees, growth and leaf efficiency (photosynthesis) declined as trees grew and aged, but the same factors did not decline in the propagated trees, suggesting that tree size, not cellular senescence, causes reduced vigor in trees.

There are several caveats to their conclusion, however. For instance, in large, old trees, “death trigger” signals may be present that didn’t get transmitted to the propagules. The propagules may thus have been tricked into thinking they were young again. However, other studies have shown that shoots from a young tree grafted onto the crowns of old, large Japanese cedars showed the same loss of vigor as the parent shoots, further bolstering the theory that, when it comes to trees, size matters most.

 
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