One of the great scientific tasks of the day is to understand how and why trees die. It may seem like a question that would have been answered many decades ago, but it was not—at least not at a detailed physiological level. Now, amid growing signs worldwide that forests are at risk as the climate changes, scientists are trying to catch up to events.
Lately, more and more evidence is pointing toward a mechanism known as hydraulic failure as the culprit in many large-scale forest die-backs. This occurs when drought reduces the flow of water into tree roots. The trees take measures to limit the loss of water through their leaves, but trees need water flowing through them as much as humans need blood. Eventually, if the drought is bad enough, the tiny tubes that carry water up the trunk of the plant can fill with air bubbles.
Detailed understanding of this mechanism may still be developing, but anybody who has forgotten to water a house plant has seen the consequences. The flow of water through the body of the plant is interrupted, and unless moisture is restored to the soil, it can droop and eventually die. Now comes a surprising new paper from an international research team presenting ominous findings about the risks to forests from global warming and its accompanying water stress.
For the study, released online on Thursday by the journal Nature, Brendan Choat of the University of Western Sydney in Australia, Steven Jansen of Ulm University in Germany, and a large group of their colleagues compiled data from 226 forest species at 81 sites worldwide. They found that around 70% of the species operate with only a narrow margin of safety when it comes to their water supply. In other words, many of the world’s important forest species are vulnerable to hydraulic failure.
In effect, the trees have adopted an aggressive evolutionary strategy, creating robust water-moving machinery that allows them to grow quickly and out-compete other trees during times of adequate rainfall, but putting them at risk of dying when water is scarce.
That means that virtually all types of forests, even in regions that seem to get plenty of rain today, are vulnerable to increased drought and increased evaporation driven by higher temperatures. If the changes in rainfall and soil moisture in coming decades turn out to be as big as many scientists fear, the Choat-Jansen paper implies that