Can the Southwest’s forests survive global warming?

New study projects widespread forest mortality by 2100

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Vast swaths of spruce trees have died in southern Colorado during the past few years. Photo courtesy U.S. Forest Service.
Forest ecosystems around the world are under the gun from climate change, development, insect invasions and conversion to agriculture. This stand of lodgepoles in Colorado was clear cut after pine beetles killed most of the trees.
Forest ecosystems around the world are under the gun from climate change, development, insect invasions and conversion to agriculture. This stand of lodgepoles in Colorado was clear cut after pine beetles killed most of the trees. @bberwyn photo.

Staff Report

Unless deep greenhouse gas cuts happen soon, last month’s historic climate agreement may be too little, too late for some forests in the American Southwest, where scientists are projecting a widespread die-off of needleleaf evergreens — including pine, spruce, piñon and juniper trees — by 2100.

After combining data from field observations with climate model projections, the research team concluded that  72 percent of the region’s needleleaf evergreen forests will die by 2050, with nearly 100 percent mortality by 2100.

“No matter how we investigated the problem, we got the same result. This consensus gives us confidence in this projection of forest mortality,” said Sara Rauscher, assistant professor of geography in University of Delaware’s College of Earth, Ocean, and Environment.

It’s pretty clear that the die-off is already under way. Even drought-resistant species like piñon pines were hit hard by a drought in the early 2000s. A massive bark beetle infestation wiped out millions of acres of lodgepole pines in the southern Rockies, and spruce beetles have taken a big toll on once-lush spruce forests in southern Colorado.

These events, coupled with similar reports from around the world indicating increasing tree death, prompted the current group of scientists to look at whether this was related to climate change.

Loss of broad-scale forest cover over the Southwest could contribute additional carbon to the atmosphere, creating additional warming. This is because trees and understory vegetation, such as shrubs and bushes, sequester carbon from the atmosphere. Less vegetation means less carbon capture, which can create a negative feedback loop that can accelerate climate change, Rauscher said.

“The rise in juniper mortality likelihood has alarming implications for conifers in general because juniper historically experienced far less mortality than other conifers in droughts,” the researchers wrote in the paper.

The study included results from field experiments in a piñon-juniper woodland in New Mexico. Over the five-year study, they restricted nearly 50 percent of the rainfall from three one-mile square plots to mimic drought conditions. During the study period, 80 percent of the mature piñon pines in the three plots died, with other trees experiencing associated drought stress effects.

According to Rauscher, the study may be one of the first examples where observational data of tree mortality is combined with results from not just one but many models of differing complexities. The simulations considered the worst “business-as-usual” scenario for greenhouse gas emissions.

“This region of the U.S. has beautiful, old forests with historic trees like Ponderosa pine that you don’t find in many other places. A treeless Southwest would be a major change not only to the landscape, but to the overall ecosystem,” Rauscher said. “There is always hope that if we reduce carbon emissions, if we continue to address climate change, then perhaps these dire projections won’t come to pass.”

The models didn’t take into account was whether the vegetation will adapt to warmer conditions or reduced precipitation levels to survive. Another possibility is the existence of refugia — areas of relatively unaltered climate — that could help some species survive until future climate changes allowed them to grow again.

Increases in the frequency and severity of wildfires, acceleration of insect populations, and failure of seedlings to take root, could also affect the projections, the researchers said.

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4 thoughts on “Can the Southwest’s forests survive global warming?

  1. Bob, can you please give me a reference for this report. What is UD? U of Denver or something else. I am appointed to a planning commission for the Wasatch and take exception to the reforestation plans, and really need to have a proper reference for this material, which is observed truth anyhow….

    Thanks, I keep on enjoying your very helpful work. Linda

    Linda Johnson, 1356 E 4500 S, SLC 84117; home 801-277-4499; mobile 801-870-5006

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