Researchers find why junipers are drought tolerant; relevance for the bioeconomy

The emerging bioeconomy's success is partly based on a stringent analysis of the regional availability of biomass resources that can be transformed efficiently into energy, biofuels, green chemicals and bioproducts. Large drought-stricken and semi-arid environments are taken up in this geospatial analysis, as these zones could in the future become suitable regions to grow newly developed, drought-tolerant crops. But before scientists start greening the desert, more research is needed into the mechanisms that allow some plants to thrive in water-starved environments.

Researchers from Duke University have contributed to this work by finding the hydraulic mechanism responsible for the unusual drought resistance of junipers. Junipers are coniferous plants in the genus Juniperus of the cypress family Cupressaceae. There are between 50-67 species of juniper, widely distributed throughout the northern hemisphere, from the Arctic to tropical Africa and to the mountains of Central America.

An ability to avoid the plant equivalent of vapor lock and a favorable evolutionary history may explain the unusual drought resistance of junipers, some varieties of which are now spreading rapidly in water-starved regions of the western United States, the Duke University study has found. The new report is published in the American Journal of Botany's online edition.

According to Robert Jackson, professor of global environmental change and biology at Duke's Nicholas School of the Environment and Earth Sciences junipers are the most drought-resistant group that has ever been studied.

The researchers examined 14 species from the U.S. and the Caribbean, all relatively drought-resistant. Even the trees in the mountains of Jamaica that get hundreds of inches of rain a year, belong to the drought tolerant group.

The plants have been expanding for about 100 years in some places, and drought plays a role in this process. Recent droughts have decimated pinyon pine populations in pinyon-juniper woodlands of the Southwestern U.S. but left the junipers relatively unscathed. Many juniper species - including several popularly known as cedars - are invading drier habitats and increasing in abundance where they already exist by surviving droughts that other conifers cannot.

To understand why junipers are so successful, Jackson's graduate student Cynthia Willson and Duke associate biology professor Paul Manos assessed structural and genetic features in the 14 species that can explain their special drought tolerance. They found a key structural adaptation in junipers: resistance to what scientists call "cavitation" - a tendency for bubbles to form in the water-conducting xylem tissues of plants:
energy :: sustainability :: biomass :: bioenergy :: biofuels :: drought-tolerance :: water stress :: xylem :: juniper :: semi-arid :: arid :: desert ::

Water is sucked through xylem tissues under a partial vacuum, almost like a rubber band being stretched out. The drier the conditions, the greater the tension on that 'rubber band' and the more likely that it will snap. If it snaps, air bubbles can get into the xylem.

The scientists found that xylem tissues of juniper species tend to be reinforced with extra woody material to prevent rupture. Such rupturing can introduce bubble-forming air either through seepage from adjacent cavities or by coming out of solution from the water itself.

They also determined that the more cavitation-resistant Juniper species have thicker but narrower leaves - a trait known as low specific leaf area (SLA) - and live primarily in the western United States.

According to Cynthia Willson, the study's first author, who having completed her Ph.D. at Duke is now a student at North Carolina State University's College of Veterinary Medicine, plants in drier environments typically have lower SLA. She and the team found that junipers from the driest environments were more drought resistant and also had the lowest SLA.

Their research found that the most cavitation-resistant species is the California juniper, which grows in California's Mojave Desert, while the least resistant is the eastern red cedar - the most widespread conifer in the relatively-moist eastern U.S.

While less drought-tolerant than other junipers, eastern red cedars still handle dry spells well and are in fact invading into Midwestern states including Nebraska. Juniper species growing in wet parts of the Caribbean also benefit from drought tolerance because they tend to grow in shallow, rocky soils that don't hold a lot of water.

In parts of the Southwest undergoing an extended drying period, junipers are edging out another hardy, water-thrifty conifer - the pinyon pine. These trees are both very drought-resistant, but the pinyons aren't as resistant as the junipers are.

The scientists further investigated how and where these tree types evolved their collective drought tolerance by analyzing each juniper species' DNA. That analysis found that junipers evolved into different species relatively recently, separating into eastern and western groups - technically called "clades."

The center of diversity for junipers is in arid regions of Mexico. The fact that many juniper species seem to be more drought-resistant than necessary for their current range suggests that a common ancestor of those two clades was also quite drought-resistant.

The work was funded by the National Science Foundation, Duke University and the Andrew W. Mellon Foundation.


Note, the importance of biomass resources in semi-arid and arid environments should not be underestimated. Harvesting drought-tolerant shrubs from deserts to use them as a bioenergy source for energy is already competitive in some cases.

Recently, the VTT (one of Europe's largest research organisations) released a report about the invasive Acacia species that thrive on 10 million hectares of desert land in Namibia. They found that the biomass can be harvested profitably and holds a very large potential.

If Namibia were to harvest all the shrubs, which can be done in an efficient manner, it would obtain an amount of renwable energy 400 times greater than all its current energy needs (previous post).

Picture: Juniperus californica, the most drough-tolerant of the 14 different juniper species studied.

References:
Cynthia J. Willson, Paul S. Manos and Robert B. Jackson, "Hydraulic traits are influenced by phylogenetic history in the drought-resistant, invasive genus Juniperus (Cupressaceae)", American Journal of Botany, 2008, 95:299-314.

Eurekalert: Why juniper trees can live on less water - February 27, 2008.

Biopact: Researchers: invasive bush biomass in Namibia has high energy production potential - January 14, 2008