PLANET WATCH--Scientists in the US have identified the factors that make a tree more likely to perish in a drought, after conducting an exhaustive examination of 33 separate scientific studies of tree mortality involving 475 species and 760,000 individual trees. (Photo above: Around 12 million trees have perished in California in the last year. Credit: NoIdentity via Flickr)
The answer they come up with is that the deciding factor is how efficiently trees draw water from the ground to their leaf tips.
This is not a surprising conclusion, but scientists don’t trust the obvious: they like to check these things.
And William Anderegg, assistant professor of biology at the University of Utah, and colleagues report in the Proceedings of the National Academy of Sciences on a list of 10 tree traits that could play a role in survival or death by drought. These include simple differences such as deciduous or evergreen, rooting depth, wood density, leaf characteristics.
Adapt and survive
Such research matters. In 2002 in the southwestern US, 225 million trees died where they stood because of drought. Texas alone lost 300 million trees in 2011. In California in the last year, 12 million trees have perished.
With losses on this scale, and more drought and heat extremes in store as climates begin to change because fossil fuel combustion worldwide has increased the levels of atmospheric greenhouse gases, foresters and conservationists need to know which species are most likely to adapt and survive, and what these species have that others do not.
In fact, deciding factors centre on the ability of a tree to draw water through the piping in its tissues. The forest giants may have to pump 200 litres of water every hour at a speed of 50 metres an hour to the topmost leaves, at a pressure of 30 atmospheres.
And the process is at risk of interruption during drought by air bubbles. To put it heartlessly, trees, like humans, can perish from embolism.
“It’s a little bit akin to a tree heart attack,” Dr Anderegg says. “You can actually hear this on a hot summer day if you stick a microphone up a tree. You can hear little pings and pops as these pipes get filled with air.”
Those species already adapted to dry climates seem to be less at risk, while those that normally flourish in wetlands are more vulnerable to drought. So far, so obvious. But not all forest physiology is so obvious.
Late last year, Dr Anderegg and his fellow researchers established that it was the increasing heat of the tropic night that was most likely to change tropical forests into carbon sources, rather than carbon sinks. What mattered was not global warming of itself, but how the warming was distributed through the forest’s diurnal cycle.
And since the world’s forests fulfil a vital role as carbon sinks—sequestering 2.4 billion tons of the greenhouse gas carbon dioxide every year, which is at least a quarter of all the carbon dioxide emissions from factory chimneys, motor exhausts and other human economic activity—what happens to forests as the world warms is vital for humankind as well.
But global warming is also increasing the risk of forest loss by drought and wildfires.
“These widespread tree die-offs are a really early and visible sign of climate change already affecting our landscapes,” Dr Anderegg says.
(Tim Radford, a founding editor of Climate News Network) … where this column first appeared … worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.)