Washington, February 28 : Duke University researchers say that the unusual drought resistance of junipers-some varieties of trees that have been spreading rapidly in water-starved regions of the western United States-can be attributed to their ability to avoid the plant equivalent of vapour lock, and their favourable evolutionary history.
"The take-home message is that junipers are the most drought-resistant group that has ever been studied. We examined 14 species from the U.S. and Caribbean, and they're all relatively drought-resistant -- even ones in the mountains of Jamaica that get hundreds of inches of rain a year," said Robert Jackson, a professor of global environmental change and biology at Duke's Nicholas School of the Environment and Earth Sciences.
"They've been expanding for about 100 years in some places, and drought plays a role in that. For example, recent droughts have decimated pinyon pine populations in pinyon-juniper woodlands of the Southwestern U.S. but left the junipers relatively unscathed," added Jackson, who is corresponding author of the new report published in the American Journal of Botany's online edition.
The study, wherein Jackson worked with postgraduate student Cynthia Willson and Duke associate biology professor Paul Manos, 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.
Jackson said that water is sucked through xylem tissues under a partial vacuum, "so it's almost like a rubber band being stretched out. The dryer 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."
During the study, the researchers observed that xylem tissues of juniper species tended to be reinforced with extra woody material to prevent rupture. According to Jackson, such rupturing could introduce bubble-forming air either through seepage from adjacent cavities or by coming out of solution from the water itself.
They also found that the more cavitation-resistant Juniper species live primarily in the western United States, and have thicker but narrower leaves, a trait known as low specific leaf area (SLA).
"Plants in drier environments typically have lower SLA. We found that junipers from the driest environments were more drought resistant and also had the lowest SLA," said 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.
The study also revealed that California juniper, which grows in California's Mojave Desert, is the most cavitation-resistant species. In contrast, the least cavitation-resistant species is the eastern red cedar, the most widespread conifer in the relatively-moist eastern US.
Jackson notes that despite being less drought-tolerant than other junipers, eastern red cedars still handle dry spells well, and are in fact invading into Midwestern states including Nebraska.
The researcher says that 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.
"They're both very drought- resistant, but the pinyons aren't as resistant as the junipers are," Jackson said.
The scientists also analysed each juniper species' DNA to determine how and where they evolved their collective drought tolerance. Their analysis showed that junipers evolved into different species relatively recently, separating into eastern and western groups that are technically called "clades".
"The centre 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," said Willson.