High-level danger for forest giants

The tallest trees play a vital role in combating global warming by taking carbon from the atmosphere, but they are at greatest risk from ever-increasing droughts.

When the rains fail, it doesn’t pay to be the tallest tree in the forest. Scientists have confirmed once again that when it comes to staying alive, water is more important than any other nourishment.

The finding is that, as drought strikes ever more frequently in the temperate and tropical forests, the tallest trees will be the most at risk. And since the oldest and tallest forest giants have unexpectedly turned out to be also the greatest consumers of carbon from the atmosphere, there will also be implications for climate change.

Tomorrow’s forests, repeatedly hit by extremes of heat and drought, will be stocked with smaller trees that store less carbon, which will mean more carbon dioxide in the atmosphere, and thence more warming. This is an instance of what engineers call positive feedback.

Sugar levels

Lucy Rowland, a researcher in the School of Geosciences at the University of Edinburgh, UK, and colleagues report in Nature journal that what actually caused the death of trees during a drought had not been well understood.

Researchers carried out field studies over a period of 13 years to see what drought did to the Amazon forests. They monitored growth, sugar levels and water transport from the roots through the woody trunks to the canopy.

Food supply was not the problem: the levels of sugars stored in trees during a drought for more than a decade were similar to those in trees that experienced normal water supplies during the same period.

Drought-affected specimens grew at a normal rate right up to the lingering moment of death. They had enough sugar, so it cannot have been starvation that saw them off.

Trees are nature’s most accomplished hydraulic engineers, hauling water through dense tissues to great heights. A diver would have problems breathing at pressures of three atmospheres, but trees have to pump water at 30 atmospheres.

Topmost leaves

And every hour, on a hot day, a full-grown hardwood may have to draw 200 litres of water at the rate of 50 metres an hour to its topmost leaves.

Ultimately, although the tallest trees get the most sunlight, this hydraulic engineering challenge puts a limit on any tree’s race to the top. And in a drought, the tree’s problems multiply.

According to the researchers, air bubbles get into the sap and interrupt the columns of water transported. The taller the tree, the more vulnerable it is, the scientists found.

“The mortality signal was dominated by the death of large trees, which were at a much greater risk of hydraulic deterioration than smaller trees,” the researchers say.

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