Air Bubbles Threatening Forests

Trees need water to flow through them as much as humans need blood, and a new study published online last week in the journal Nature presents bleak findings on the risks to forests from record-breaking spells of drought. For the study, an international team of 24 plant scientists, led by Brendan Choat, Ph.D., of the University of Western Sydney in Australia and Steven Jansen, Ph.D., of Ulm University in Germany, compiled data from 226 forest species at 81 sites worldwide. They found that around 70% of the forest species operate with only a narrow margin of safety when it comes to their water supply, and “therefore potentially face long-term reductions in productivity and survival if temperature and aridity increase as predicted for many regions across the globe.” Based on this data, nearly all types of forests, regardless of their current rainfall environment, will be equally vulnerable in the event of a drought.

“It would only take a small shift in terms of the moisture environment, the temperature…to push these plants across the threshold,” Choat said. “These findings provide insight into why drought-induced forest decline is occurring not only in arid regions, but also in wet forests not normally considered at drought risk.”

What actually kills a drought-stressed tree is a condition known as “hydraulic failure.” Trees suck water up from the ground all the way to their leaves, through a bundle of channels in a part of the trunk called the xylem. In a commentary of the Nature study, Bettina Engelbrecht of the University of Bayreuth, in Germany, described this process as “much like sucking water through a straw.” But when a drought dries out the soil, a tree has to suck harder through this “straw” to find water. And that can prove to be harmful, as sucking harder increases the risk of drawing air bubbles into the tree’s plumbing.

Choat explains: “As drought stress increases, you have more and more gas accumulating in the plumbing system, until they can’t get any water up into the leaves. This is really bad news for the plant because this is like having an embolism in a human blood vessel.”

Like a human embolism, the formation of gas bubbles within a tree blocks the flow of the necessary fluid and nutrients needed in order to survive. Should drought and hydraulic failure persist, the tree will suffer from thirst and starvation and eventually die. Engelbrecht reiterates that “even a minor increase in drought intensity will induce levels of xylem embolism that will impair growth and lead to tree death.”

Scientists are now keeping an eye on whether the world’s trees will somehow find a way to rapidly adapt to rising strikes of drought. One hopeful theory suggests young trees growing under drought stress could shift their framework in ways that would limit their risk, but whether they have the genetic capacity to do this, or to do it quickly enough to keep up with the high-speed climate shifts projected for the coming decades, remains to be seen.

“Basically, this tells us that we should keep climate change in check as much as possible,” Engelbrecht added. “The ramifications of this scenario are diverse and, in many respects, dire: forest mortality will be accompanied by changes in species composition, changes in ecosystem function and losses of services and biodiversity.”