Climate models of the impacts of forests on carbon emissions have come to wildly divergent conclusions. Some viewed forests as natural carbon "sinks" that absorb and trap carbon dioxide, thus helping to reduce the amount of greenhouse gases in the atmosphere. But others see forests as a significant source of atmospheric carbon.

This huge uncertainty came from one problem: People weren't sure exactly how many trees would be around in the future. (Beyond that, people didn't even know how many trees were on Earth right now. Until recently, scientists assumed that there were about 400 billion trees on Earth, but another study presented here put the number of trees on Earth at a whopping 3.04 trillion.)

At its heart, though, the disparity stemmed from uncertainty about whether plants would respond to the coming droughts. A plant's response to drought is enormously complicated — water shortage can kill a tree in many ways, from making it more susceptible to bark beetles to producing conditions ripe for wildfires that could burn down a whole forest, Anderegg said.

Hydraulic factors

However, one factor seemed to play an outsize role in a tree's ability to adapt to parched conditions. Plants suck water up through their xylem, the vascular system that threads through a tree's roots and branches, by creating a pressure gradient. The less water there is in the soil, the harder these plants have to suck.

"At a certain point, which varies by species and tissue, we start to get air bubbles pulled into these xylem elements," Anderegg said.

When enough of these air bubbles form in the xylem, they form an embolism, similar to the kind found in humans, that blocks the flow of needed water and nutrients, leading to "hydraulic failure," Anderegg said.

"This may be the dominant process we've got to get at to predict tree mortality," he added.