Why the Juniper Thrives

By J. Peder Zane, The News & Observer, Raleigh, N.C.

Jun. 7–Cynthia Willson could only laugh as the tropical rains poured down. After all, the Duke University graduate student had traveled to Jamaica to study drought-resistant trees.

Unbowed, she and her husband, Darcy, trudged across mud-slopped roads to the heart of the island’s Blue Mountains. Proving chivalry is not dead, he shimmied up a juniper tree with a machete in his mouth to cut down a half dozen branches while she collected material from the tree’s roots.

A few days later, Willson was putting those samples through some high-tech paces in her Durham lab. By spinning them in centrifuges and sticking them with probes, she said, she and fellow researchers hoped to find out why junipers are able to thrive when the rains don’t come.

Local gardeners and farmers who have had to cope with drought and water restrictions can appreciate the importance of the research Willson conducted with Duke biology professors Robert B. Jackson and Paul S. Manos. If predictions about global warming come true, the need to understand how trees and plants respond to hotter, drier conditions will become increasingly important.

"We know that summertime droughts will become more common, putting greater stress on all types of vegetation," Jackson explained. "As conditions change, species that have thrived in certain regions will be threatened while drought-resistant species, like the junipers we studied, may become more plentiful."

Although their research focused on 14 species of junipers — including two red cedar varieties common in the Southeast — the questions Jackson and Willson raised involved the most fundamental aspects of botany. It led them to explore how air pressure acts like a giant vacuum cleaner in plants, the risky behavior that all plants engage in whenever they absorb life-giving sunlight, and how Aesop’s fable "The Tortoise and the Hare" helps explain drought resistance.

In his book-lined — yet plantless! — office at Duke, Jackson said that the study of drought begins with rain. First it is absorbed by the roots, then it makes its way up to the leaves. Junipers do not have elevators or pumps, so how does the water defy gravity to make this journey?

The answer is in the air.

A matter of balance

Start with one of the basic laws of thermodynamics that says, essentially, that nature likes balance. If the ground is wet and the air is dry, nature works to equalize the moisture in each. The most common example of this is simple evaporation. In the garden or forest, the air acts like a powerful vacuum, sucking moisture up from the ground through trees and plants.

"Plants and trees use only a tiny fraction of the water they absorb for themselves," Jackson said, "maybe a few ounces of the gallon you give them. Almost all of it ends up in the air."

Plants and trees have to work hard to hang on to those few water drops by engaging in risky behavior. The leaves, which are speckled with pores, have to open their pores to absorb the carbon dioxide they need to make the sugars they feed on. When they do this, however, air swoops in, sucking out moisture from the wet interior of the leaves. Thus, the price trees pay for the food they need is the water they must have.

This exchange works well enough, unless conditions become exceptionally hot and dry. The higher the temperature, the more moisture the air can hold. That’s why we don’t speak of cold, humid days and why we see dew in the morning: As temperatures fall during the night, the air can no longer hold all its moisture, which it deposits on the ground.

Pressed to the limit

The air is indifferent to plants’ survival. During droughts it does not give them a compassionate break. Instead, like a vampire, it works even harder to suck water out of parched leaves.

One reason junipers can withstand drought is that they have relatively thick, narrow leaves. They are better able to retain water in their thick folds.

Still, they too can be pressed to the limit. When this happens, the naked eye sees the leaves start to discolor and wither. But, Jackson said, the bigger problem occurs in the body of the juniper tree.

Water is pulled up through junipers — and all other trees and plants — via short, interconnected passages called "tracheids." Air draws moisture through them in the same way a mouth pulls soda through a straw — in this case, from the root, through the stem or trunk, through the branches and then out through the leaves.

As long as there’s plenty of water, everything is hunky-dory. But when the air is dry, it increases its pressure, just as a thirsty person will draw harder on a straw. Changing metaphors, Jackson said it’s helpful to imagine the water in the tracheids as rubber bands getting stretched tighter and tighter by dry conditions. When the pressure is so great, the water can "snap," creating an air pocket that renders the tracheid useless. For the same reason, a straw won’t work when it gets an air bubble.

Trees and plants have many tracheids. But a drought can make so many of them fail that the plant can die because they are unable to have moisture brought to their food-making leaves.

Creating bad conditions

Determining the snapping point of junipers was a main purpose of the research by Jackson, Willson and Manos. In a pivotal test, they placed the samples Willson had collected in Jamaica and other locales into a centrifuge. Mimicking drought conditions, it pulled moisture from samples in the same way a washing machine’s spin cycle draws water out of clothes.

The researchers would then stick the samples with probes to determine their ability to transport water was diminished. They then repeated the test.

They found that junipers have fewer tracheids than less drought-resistant trees — but stronger ones. This makes them less likely to snap under pressure.

Like the fable of the "Tortoise and the Hare," junipers thrive during tough conditions because they grow slowly. Trees with many thin tracheids can move water to their leaves more effectively, but the qualities that help them thrive when the rains come, make them vulnerable in a drought.

Jackson said this research will not pay immediate dividends: It will not enable scientists to make fragile plants more drought-resistant. "But as our planet gets drier and hotter, an understanding of why some species thrive and others do not will become more important," Jackson said. "And it may lead to breakthroughs we can’t see right now."

peder.zane@newsobserver.com or (919) 829-4773

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