SCIENCE:

Experiments in tree mortality suggest grasslands may replace Southwestern forests

In the steel and Plexiglas chambers that make up Nathan McDowell's research station near the Bandelier National Monument in New Mexico, the future of regional forests is playing out in miniature. McDowell is baking trees.

Inside the chambers, temperatures climb 9 degrees Fahrenheit above the desert's already blistering averages. Meanwhile, precipitation, never abundant in this corner of the country, is cut by half.

This is the kind of climate that the United Nations' Intergovernmental Panel on Climate Change (IPCC) projects for the Southwest within the next half-century, as global warming drives heat and aridity in an already hot, dry region.

And in these chambers, as in that predicted future, the region's trees are dying. By studying the plants' mortality in this simulated environment, McDowell hopes to better understand how the future climate may affect forests, possibly even altering the landscape.

"We need to understand the mechanistic side if we're going to model the effects of climate on a large scale," he said. "We need to understand why and where trees die. When we can do that accurately, we'll have a shot at knowing the broader effects."

Though scientists have long expected climate change to elevate levels of drought and heat stress for much of the world's forests, precise modeling of those changes has been limited, in part because the mechanisms behind tree mortality are not fully understood.

Although the factors that lead to a tree's eventual demise -- primarily water loss, carbon starvation and the invasion of biological agents -- are well-known, the ways these stresses can interact to overwhelm a tree's defenses are not, McDowell said.

Seeing tolerance, resistance and death

The Bandelier Monument site is McDowell's second research project to orchestrate tree death and the first to include temperature control as a facet of the experiment. Inside the chambers, juniper, pinyon pine and other plant species are exposed to elevated levels of heat and water stress, while researchers monitor their declining health.

Juniper and pinyon provide optimal research specimens because they're abundant, they're small enough to be contained within built structures and, most importantly, they exemplify two behaviors that characterize plants' response to drought: tolerance and resistance.

In extreme heat events, drought-resistant plants like pinyon -- along with most other coniferous species -- go into a kind of hibernation, closing the tiny, porelike holes through which respiration occurs. Because the stomata are sealed, less water escapes the plant, lessening its chances of hydraulic failure.

The problem with this strategy is that along with respiration, the pinyons also have to put photosynthesis on hold. Photosynthesis requires carbon, which plants acquire primarily through respiration. That carbon is used to create carbohydrates, the energy packets that power all life.

If plants leave their stomata closed too long -- in the case of a severe, extended drought or heat wave -- they eventually run out of carbohydrates and starve to death.

Drought-tolerant plants like juniper may close their stomata partway but continue respiration -- in effect, trading some of their water reserves for the ability to continue producing food.

This, too, is a gamble. If severe conditions continue for three years, the hydrological deficits within the juniper can lead to air bubbles forming within the tree, interrupting its transfer of nutrients and killing it.

Can even the fittest survive a 'superdrought'?

Juniper tends to do better during droughts than pinyon pine, McDowell said. A severe dry spell a decade ago resulted in the loss of many acres of pinyon in New Mexico, and today much of that area is now covered by juniper savannas.

However, in the face of the kind of sustained "superdrought" some climatologists predict will occur toward the end of this century, both species would be heavily affected, he said.

"If we had a significant, prolonged drought in the future, we might actually lose both of them," he said. "We could see a lot of the areas that are forests today become grasslands."

Although both carbon starvation and hydraulic failure will kill a tree eventually, it is often some other agent, such as a parasite, that delivers the killing blow, he said. Biotic agents like bark beetles, and the blue-stain fungus the beetle introduces to the tree, act like a third wave of assault, attacking weakened trees and overwhelming their defenses.

Healthy trees can often fight off beetle attacks, drowning invaders in secretions of sap. But some evidence suggests that carbon and water are both necessary ingredients in the production of sap, indicating that the depletion of either can leave a tree less able to defend itself.

"These mechanisms all interact," McDowell said. "Whether the tree succumbs to carbon starvation, water loss or biota, it's just a matter of different mechanisms in different places."

In many parts of the West, a spate of uncommonly dry years has already led to the worst bark beetle outbreak in historical record.

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