Nathan McDowell, of the Los Alamos Earth and Environmental Sciences Division, examines an old, large tree, which could be impacted by future droughts. Photo: LANL

[cf. Scientists say climate change could cause a ‘massive’ tree die-off in the U.S. Southwest and Drought-induced tree mortality accelerating in forests]

By Diana Madson
18 April 2016

(Yale Climate Connections) – New research predicts that nearly all coniferous forests in the American Southwest could be lost to climate change by the end of the century.

Dr. Nathan McDowell, who led the Los Alamos National Laboratory study, says the projected mass die-off of trees like junipers and piñon pines will be widespread within the next thirty-four years.

MCDOWELL: “The key take away from a study like this is that while the exact numbers are tenuous, the general trajectories they project are pretty robust. All of the different research is pointing in the same direction, which is that we’re going to lose forests around the world.”

What’s going to kill so many trees? The big culprits will be more frequent and intense droughts and heat waves.

MCDOWELL: “There’s one smoking gun: we must curb CO2 emissions dramatically and the Paris climate talks last December are a great step in that direction.”

And smart forest management is critical since drought also increases the risk of wildfire. McDowell advises prescribed burns to control grasses while keeping old trees alive, limiting the chances of a mega-blaze and reducing competition for water.

McDowell says such practices could ultimately buy more time for our southwest forests while we address the root causes of global warming.

Forest Die-Offs Predicted in U.S. Southwest

LOS ALAMOS, N.M., 19 May 2015 – Researchers at Los Alamos National Laboratory have found that drought and heat-induced tree mortality is accelerating in many forest biomes as a consequence of a warming climate in their paper “Darcy's law predicts widespread forest mortality under climate warming,” published in the journal Nature Climate Change.

“The warming climate is creating a threat to global forests unlike any in recorded history,” said Nathan McDowell, of Los Alamos’ Earth and Environmental Sciences Division. “Forests store the majority of terrestrial carbon and their loss may have significant and sustained impacts on the global carbon cycle.”

To predict characteristics of plants that will survive and die during drought in future warmer climates, scientists used Darcy’s Law, a core principle of vascular plant physiology. Darcy’s Law is an equation that describes the flow of liquid through a porous medium, which is how trees take in water.

They found that tall plants with low hydraulic conductance and high leaf area are most likely to die from future drought stress. This means that tall trees in old forests are at the greatest risk, which could negatively affect the earth’s carbon storage.

“This work is another line of evidence indicating that historic forests in general, and irreplaceable, ancient trees may be increasingly at risk from hotter droughts if global climate warms as projected,” said Craig Allen, of the United States Geological Survey and coauthor of the paper.

McDowell's and Allen’s research indicates today’s forests should be replaced by shorter and more xeric plants, which need less water, due to future droughts and associated wildfires, and pest attacks.

The application of Darcy’s Law provides a simple and robust framework for informing forest management interventions needed to promote the survival of current forests.

“Today’s forests will see continued increases in mortality rates that will result in substantial reorganization of their structure and carbon storage,” McDowell said.

Drought-induced tree mortality accelerating in forests



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