To deal with a decades-long megadrought, shrubs in the desert Southwest have boosted their water consumption efficiency at some of the highest rates ever documented. According to a new study by University of Utah researchers, the shrubs’ efficiency increases are unparalleled and heroic, but they may not be enough to adapt to the West’s long-term drying trend.
“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 intrinsic water-use efficiency, or iWUE, is the most important parameter to understand for this study. It’s a proportion of how much photosynthesis a plant does to how open its stomata are. Stomata are tiny holes in leaves that allow carbon dioxide and water vapor to pass through. Consider them the plant’s nostrils.
Plants carry out photosynthesis with the minimum stomatal opening when iWUE is high. The iWUE of a plant can be influenced by a number of things. If the amount of carbon dioxide in the atmosphere rises, plants don’t need to open their stomata as much to take in the same amount of carbon.
If the temperature rises or the amount of water available decreases, the plant will reduce the stomatal opening to avoid losing too much water. It stands to reason that rising carbon dioxide levels, as well as warming and drying trends, would result in an increase in iWUE. But how much is it? And how does it relate to previous iWUE trends?
The Southwest is in a really dry period, and this is further evidence of how much the ‘megadrought’ has impacted plant functioning and how anomalous this period is.
Steven Kannenberg
“Unfortunately, there are almost no published long-term records of iWUE in the Southwest,” Kannenberg says. “Additionally, the vast majority of long-term data are on trees, so the sensitivity of iWUE in other plant types is unknown.”
Building a long-term record
Fortunately, University of Utah scientists have been collecting shrub leaf samples in the Mojave Desert for nearly 40 years during the last two weeks of March. Researchers collect fresh leaves of Encelia farinosa (also known as brittlebush), Encelia frutescens (also known as button brittlebush), and Ambrosia salsola at two locations in California and one in Arizona (also called cheesebush or burrobush).
The annual sampling expeditions were started by renowned professor Jim Ehleringer in order to keep long-term records of how desert shrubs were faring as well as to get away from the cold during the winter.
The study period encompassed a “megadrought” in the American Southwest, which the researchers describe as “one of the most devastating multi-decadal drought occurrences in the region over the last 1200 years.” As a result, the long-term study not only demonstrates how plants respond to changes in climate from year to year but also how they cope with a large-scale extreme weather event.
The researchers return to the University of Utah to examine the leaves’ stable isotopes, chemical traces that reveal how plants receive and utilize water and carbon dioxide. The team was able to compute the bushes’ annual iWUE using these metrics.
They also took samples from ponderosa pines at 16 locations across the American West to compare how the shrubs and trees use water.
“We used a statistical approach to directly compare how sensitive the shrubs and the trees were to variability in CO2, precipitation, and aridity,” Kannenberg says.
A race to adapt
Over a 30-year period, from 1989 to 2019, climate data at the research sites revealed no significant changes in mean annual precipitation. However, vapor pressure deficit, a fundamental indicator of how dry the air is, increased significantly at nine of the sites.
In fact, the pace of vapor pressure deficit rise has increased seven-fold since 2010. The desert Southwest began to dry out much more quickly.
As a result, the bushes increased their iWUE to match a LOT faster. iWUE in the Mojave Arid increased 6-10 times quicker than in other locations, according to another research of plants in desert conditions. In fact, the shrubs’ rate of iWUE rise was so quick that the researchers only found one other example of a faster iWUE increase, in a conifer.
The results suggest that shrubs are much more sensitive to increasing aridity than trees are. “Given the rapid uptick in aridity across the region in recent decades,” Kannenberg says, “this has resulted in shrub iWUE increasing at (as far we can find) the second-fastest rate ever documented in any plant species.”
Is it working?
Kannenberg goes on to say that the results are unexpected. Scientists have discovered a global increase in iWUE, which they linked to rising CO2 levels in the atmosphere. They had predicted that the rate of iWUE growth would slow down with time.
“However, our results imply that global increases in aridity may offset that in the future,” Kannenberg says. Hence the shrub’s frantic and historic increases in iWUE to cope with the West’s megadrought.
Is it working?
Yes, higher water use efficiency is beneficial to plant health and adaptation to changing climate conditions when all other factors are equal. However, estimates indicate that the Southwest will likely become drier as plant growth declines.
It’s possible that increasing iWUE won’t be enough to save the desert bushes. The researchers argue that whether the drought’s intensity and duration exceed the plants’ ability to adapt will determine their fate.
The study was financed by the National Science Foundation, the US Department of Energy, the US Department of Agriculture, and the David and Lucille Packard Foundation, and was published in Proceedings of the National Academy of Sciences.
