Record-setting heat and dryness plagued many of the lower 48 states in 2012. While 2013 hasn’t been as extreme, half of the country continues to suffer from dry to exceptional drought, according to the U.S. drought monitor. It’s a trend that’s likely to continue as the global climate changes, and it could have big impacts on trees across the West.
Near his office in Lincoln, Nebraska, Mark Harrell examines a brown-looking Eastern White Pine. He’s the Forest Health Program Leader for the Nebraska Forest Service. He points to dried resin on the bark. “That’s kind of a typical symptom of fungus disease affecting the branches,” he says. “And those are very common on trees stressed from drought.”
Harrell says that because of disease—likely brought on by drought—the tree may not survive the summer. And it isn’t the only one. Foresters across the state say they’re losing trees not only to drought-induced disease and dehydration but also to heat that exacerbates the situation and increases the severity of wildfires. Drier, warmer weather has also given insects like bark beetles more time to populate and spread into climate-stressed trees where they destroy nutrient-bearing tissues.
A Fatal Thirst
Trees nourish themselves by pulling water up from the ground to the leaves. There it reacts with sunlight and carbon dioxide, converting that energy into food in the form of carbohydrates. But drought hampers that critical process, according to U.S. Geological Survey research ecologist Craig Allen.
“When there starts to be limited water available in the soil, it gets more difficult. It takes more tension to pull water out of the soil and get it into the tree and get it all the way up to where the leaves are in the canopy,” he says.
In fact, that tension can get so strong that the tissues in the tree—think of them as little straws—get air bubbles, or embolisms, which prevent water from moving up the trunk. Trees have many ways of dealing with periods of drought and other stress, which is one reason they can live so long, says Allen.
“By avoiding dehydrating to death, they can end up starving, essentially,” says Allen.
Allen, who is based near Santa Fe, New Mexico, says the Southwest has been in a prolonged drought since about 2000. And warmer temperatures in the last 20 years have exacerbated the situation.
“The world is getting warmer, has been getting warmer in recent decades. And it appears there’s a signal that this is amplifying the drought stress on trees in many parts of the world. We’re starting to see more tree mortality in these areas,” says Allen.
Higher temperatures—partly the result of more carbon dioxide in the air—means the atmosphere needs more water. Think of the warmer atmosphere as a sponge, increasing evaporation and pulling more water from leaves and the soil.
According to Allen, adding hotter temperatures to ongoing drought is often a fatal combination for trees. Many species around the world already toe the line of high temperatures they can withstand to maximize their ability to survive. And in many places, trees have already used up deeper reserves of subsoil moisture stored from previous wet periods.
Mark Svoboda, a climatologist with the National Drought Mitigation Center, says those places now need serious moisture.
“When you get into the second or third year of drought, the cumulative impacts begin to ramp up, like we’re seeing particularly in Oklahoma, Texas, Colorado, Kansas. New Mexico has been very, very hard hit.
You need well above sustained rains, not just a good month or a good week,” says Svoboda.
More Intense Wildfires
Drier winters have also led to longer, more intense wildfires sweeping through western forests.
“Cumulatively, we’re seeing this much longer fire season in just a few decades, and the severity of fire seems to be related to
temperature as well, so we’re seeing more high-severity fire,” says Allen.
These intensely hot fires can sometimes kill huge swaths of mature trees, rather than the spotty burn patterns of more normal wildfires.
As a result, Allen and other forest researchers now worry about losing that native seed stock, and the trees’ ability to repopulate the landscape by distributing their seeds as they have after previous die-offs. The sudden absence of these trees would likely impact the ecosystem, including plant and animal biodiversity, as shrubs and grasses take over.
Along with efforts like thinning forests to reduce competition for water, researchers are searching for new species that can withstand the changing conditions.
The Search for Tougher Trees
Justin Evertson, Green Infrastructure Coordinator for the Nebraska Forest Service, says they think they need to look south for trees that can withstand warmer temperatures and drier conditions, though he admits it’s hard to know just how things will change in the future.
Evertson works with Bob Henrickson at the Nebraska Statewide Arboretum in Lincoln. Along with nearly 100 affiliate sites around the state, Henrickson routinely gathers seeds from a wide range of trees—particularly native varieties—and cultivates and distributes them to determine what trees do best in the region’s variable and changing climate. With more drought and heat projected for the future, moving away from cooler-weather species might be an option for maintaining a healthy tree population in the state.
At the arboretum’s test garden, Evertson walks over to a dwarf chinkapin oak grown from acorns collected nearly a decade ago. He says this oak occurs on tough, gravelly soils.
“It’s a tree that has proven itself to be tough and adaptable across the region,” he says.
Evertson says orders for chinkapin oak and other hardy trees routinely come in from
numerous nearby states. The Nebraska Forest Service and Kansas Forest Service are starting a cooperative effort to collect and evaluate other tough trees that can handle changing climate conditions. Bob Henrickson says that’s exactly what he envisions for the arboretum’s future.
“A lot of native trees and shrubs in Nebraska are native to other states but meet their natural range limit in our state,” he says. “So we feel we’re set up with some of the best seed sources in the country to distribute these plants throughout (the region) that can take these tough conditions.”
Distributing seeds to other states might be necessary if we lose a lot of the current forests in the Southwest as a result of the combined threat of drought, heat, and fire.
After reviewing thousands of years of historic tree-ring data in the Southwest, Allen and other researchers found there have been two other periods of extended, “mega-drought” that caused trees to die out on a large scale and likely forced native people to leave the region.
“The current drought we’re seeing is approaching the severity of those two most severe mega-droughts in the last 1,000 years, and it’s because it’s warmer,” says Allen.
Mark Svoboda agrees but says it’s still too early to know if we’re at the beginning of another mega-drought.
“There’s just no way to tell until you look back on it. But it’s certainly been shown that it’s very, very possible,” he says.
Ultimately, Allen says we should expect change. “We’re in a period where the new normal is just a transition period -- we can’t see to what yet. But it looks like, for this century at least, there will be this climate changing and so the systems are changing as they must.”
That could mean cold-weather trees slowly repopulating in higher elevations, while lower areas shift to more scrub- and grass-based ecosystems. Only time—and the climate—will tell.