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"The Southwest is in a really dry period," says Steven Kannenberg, postdoctoral research associate in the U's School of Biological Sciences, "and this is further evidence of how much the 'megadrought' has impacted plant functioning and how anomalous this period is."

Water Use Efficiency

The key metric to know for this study is intrinsic water-use efficiency, or iWUE. It's a ratio of the amount of photosynthesis in a plant to how open the plants' stomata are. Stomata are small openings in leaves that allow carbon dioxide to come in and water vapor to go out. Think of them as the plants' nostrils.

When iWUE is high, then plants carry out photosynthesis with minimal stomatal opening. Several factors can affect a plant's iWUE. If carbon dioxide levels in the atmosphere go up, then the plants don't need to open their stomata as much to bring in the same amount of carbon. If temperatures rise or water availability drops, the plant will restrict stomatal opening so as to not lose too much water.

It would stand to reason, then, that current rising carbon dioxide levels and warming and drying trends would lead to increases in iWUE. But by how much? And how does it compare to iWUE trends in the past?