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 notat 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 worlds 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 the result could be massive die-backs, shifts in the composition of forests, and a transition from forest to grassland in many regions.
That may sound alarmist, but a developing body of evidence suggests that it is already starting to happen. Last year, for example, I wrote about the large forest die-backs that are being seen in the American West and the Pacific Northwest because of mountain pine beetles, an insect pest that is moving farther north because of global warming.
We are also seeing huge impacts on forests from water stress in the Mediterranean, the Amazon and many other regions.
William RL Anderegg, a Stanford University researcher who was uninvolved in the new paper but is doing related work, told me he saw the new research as a major step forward in gaining a more complete global understanding of the risks to forests from climate change.
The new paper tells us that many, many tree species live close to the dry edge of what they can tolerate, even if they live in a very wet area, Anderegg said in an e-mail. It makes evolutionary sense, he added, because no matter your environment as a tree, you would want to maximise your growth in order to compete with other trees, while still narrowly avoiding death from water stress. The practical and critical outcome of this is that trees and forests, globally, appear to all be relatively vulnerable to drought-induced mortality.
Climate change puts at risk not only the rich diversity of life in the worlds forests, but also the ability of those forests to suck carbon dioxide out of the air, as they do today in immense volume, helping to limit global warming. In other words, if forests start dying from global warming, that means the warming will get worse, presumably killing more forestsa dangerous feedback loop.
The consequences of longer droughts and higher temperatures are potentially dramatic, Dr Choat, Dr Jansen and their colleagues wrote in the new paper. For example, rapid forest collapse as a result of drought could convert the worlds tropical forests from a net carbon sink into a large carbon source during this century.
The big question now is how much ability the worlds trees have to adapt. In theory, one might imagine that young trees growing under drought stress would shift their architecture in ways that would limit their risk. But whether they really have the genetic capacity to do this, or to do it quickly enough to keep up with the rapid climatic shifts projected for coming decades, is an open issue.
A distinct possibility, the scientists wrote, is that the rapid pace of climate change may outstrip the capacity of populations to adapt.
In a commentary accompanying the paper, Bettina MJ Engelbrecht of the University of Bayreuth in Germany, who was not involved in the research, writes that the accumulating scientific evidence sounds a warning bell that we can expect to see forest diebacks become more widespread, more frequent and more severeand that no forests are immune.