Desert shrubs increased water use efficiency to survive a mega-drought
Shrubs in the desert southwest increased their water-use efficiency to some of the highest rates ever to cope with a decades-long mega-drought. That’s the conclusion of a new study by researchers at the University of Utah, who found that while shrub efficiency increases are unprecedented and heroic, they might not be enough to s adapt to the long-term drought trend in the West.
“The Southwest is in for a really dry spell,” says Steven Kannenberg, postdoctoral research associate in the University’s School of Biological Sciences, “and this is further evidence of the impact of ‘mega- drought” on the functioning of plants and the anomaly of this period. “
Water use efficiency
The key metric to know for this study is intrinsic water use efficiency, or iWUE. It is a ratio between the amount of photosynthesis in a plant and the opening of the plants stomata. Stomata are small openings in leaves that allow carbon dioxide to enter and water vapor to exit. Think of them as the nostrils of plants.
When iWUE is high, plants perform photosynthesis with minimal stomatal opening. Several factors can affect a plant’s iWUE. If carbon dioxide levels in the atmosphere increase, plants don’t need to open their stomata as much to bring in the same amount of carbon. If temperatures rise or water availability decreases, the plant will limit the opening of stomata so as not to lose too much water.
It therefore stands to reason that the current increase in carbon dioxide levels and warming and drying trends would lead to an increase in iWUE. But by how much? And how does it compare to iWUE trends of the past?
“Unfortunately, there are almost no published long-term recordings of iWUE in the Southwest,” says Kannenberg. “Furthermore, the vast majority of long-term data are from trees, so the sensitivity of iWUE in other plant types is unknown.”
Build a long-term case
Fortunately, University of Utah scientists have traveled to the Mojave Desert every year during the last two weeks of March for nearly 40 years to collect leaf samples from shrubs. At two sites in California and one in Arizona, researchers are harvesting fresh leaves from Encelia farinosa (also called brittlebush), Encelia frutescens (also called brittlebush button) and Ambrosia salsola (also called cheesebush or burrobush). The eminent professor Jim Ehleringer began the annual sampling trips both to establish long-term records of the health of shrubs in the desert – and to go somewhere warm during the winter.
The study period covered what is being called a “mega-drought” in the American Southwest – “one of the region’s most severe multi-decadal drought events in the past 1,200 years,” the authors write. researchers. Thus, the long-term study not only shows how plants react to variations in weather conditions from year to year, but also how they cope with a large-scale extreme weather event.
Researchers bring leaves back to U and analyze their stable isotopes, chemical signatures that offer clues to how plants get and use water and carbon dioxide. From these measurements, the team was able to calculate the iWUE of the shrubs each year.
They also collected samples of ponderosa pines from 16 sites in the American West, to compare the water use of shrubs with that of trees.
“We used a statistical approach to directly compare the sensitivity of shrubs and trees to variability in CO2, rainfall and aridity,” says Kannenberg.
A race to adapt
Climate data at the study sites showed no significant changes in average annual precipitation over 30 years, from 1989 to 2019. But at nine of the sites, the vapor pressure deficit, a key measure of drought air, has increased considerably. In fact, since 2010, the rate of increase in vapor pressure deficit has increased sevenfold. The southwestern desert has begun to dry out MUCH faster.
So shrubs increased their iWUE to match – also MUCH faster. In comparison with another study of plants in desert environments, iWUE in the Mojave Desert increased 6-10 times faster than in other regions. In fact, the rate of increase in iWUE in shrubs was so high that the researchers found only one other recorded case of a faster increase in iWUE, in a conifer.
The results suggest that shrubs are much more sensitive to increased aridity than trees. “Given the rapid increase in aridity in the region over the past few decades,” says Kannenberg, “this has resulted in an increase in shrub iWUE (as far as we can find) the second highest rate rapid never documented for all plant species”.
Does it work?
Kannenberg adds that the results are surprising. Scientists had noticed an overall increase in iWUE linked to the increase in atmospheric CO2 levels. They had assumed that the rate of increase of iWUE would stabilize over time.
“However, our results imply that the overall increase in aridity could compensate for this in the future,” says Kannenberg. Hence the frantic and historic increases in the iWUE of the shrub to cope with the mega-drought in the West.
Does it work?
All things being equal, yes, increased water use efficiency is good for plant health and for adapting to changing weather conditions. But forecasts suggest the southwest will likely become drier with less plant growth. Increasing iWUE may not be enough to save desert shrubs. Their fate, the researchers write, depends on whether the intensity and duration of the drought exceeds the adaptive capacity of the plants.
The study is published in Proceedings of the National Academy of Sciences and was funded by the National Science Foundation, US Department of Energy, US Department of Agriculture, and the David and Lucille Packard Foundation.