Drought Can Kill Trees Even After Rainfall Resumes

Warming trend likely to kill millions of trees, force wholesale ecosystem changes across Rim Country


Grows of aspen like these off Forest Road 300 have been dying off across the Southwest as a result of a combination of drought and changes in fire patterns.

Grows of aspen like these off Forest Road 300 have been dying off across the Southwest as a result of a combination of drought and changes in fire patterns. Photo by Pete Aleshire. |

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The era of deeper, longer droughts predicted for the Southwest will spur widespread, landscape altering tree deaths by damaging the ability of the plants to pump water up out of the ground, according to the results of several, overlapping studies.

Rim Country remains stuck in the midst of one of the most severe, long-term droughts in centuries. Last year, Payson had 13 inches of rainfall, just over half of normal.

In fact, the current drought ranks as the worst in 600 years, according to a study of tree-ring data going back 1,000 years and published in the scientific journal Nature Climate Change.

The Southwest has suffered several such “mega-droughts” in the past 1,000 years, concluded the researchers from the University of Arizona, the U.S. Geological Survey and Columbia University.

However, climate projections based on both natural cycles and the buildup of carbon dioxide and other heat-trapping pollutants in the atmosphere suggest that droughts as bad or worse as the one that has maintained a grip on the region for more than a decade will become increasingly commonplace in the next century.

The researchers used the year-to-year variation in growth recorded in tree ring patterns over the whole region to calculate the “vapor pressure deficit” — which is the difference between the moisture in the air and how much water the air could hold if saturated. An increase in temperature has an immediate impact on this water balance, since it increases both evaporation and how much water the air can hold – leaving less to soak into the ground.

The researchers then linked that measurement to the year-by-year record of tree deaths from things like bark beetle outbreaks, wildfires and other causes.

As a result, they predicted that the projected rise in temperature will cause widespread tree deaths, which will likely have a huge impact on the composition of forests throughout the region. Already, Payson perches on the ecological border between the lower-elevation pinyon and juniper forests and the higher-elevation ponderosa pine forests.

A second study answered key questions about why trees die after struggling through a drought.

Scientists have long wondered why trees often die from the lingering effects of drought, even when normal rainfall resumes. Some argued the trees can’t recover from a lack of nourishment sucked up through their roots during the drought. Others speculated that the dry period damaged their ability to take in water through their roots.

A study published recently in the journal Nature put the blame squarely on damage to the ability of the trees to take in moisture through the roots and move it up to the thirsty leaves and growing core.

The international team of 24 plant scientists examined detailed studies of 226 forest species at 81 sites around the world and discovered that most trees in the forest have only a narrow margin when it comes to water supply.

The research showed that drought kills trees by effectively collapsing the straw-like suction that draws water out of the soil, up the living layer called the xylem below the bark and up and out into the leaves. The moisture difference between the wet roots and the sun-dried leaves drives the process through osmosis. However, if the roots dry out for a prolonged period, it causes “hydraulic failure.” The tree’s roots effectively suck air instead of water out of the dry soil. These air bubbles then cause the tubes that normally carry the water to collapse – like a straw that collapses if you suck too hard.

Even a minor increase in the frequency and duration of drought will cause these air bubbles to accumulate in the xylem, which means the tree can’t recover even when the rains resume. Even if the process merely reduces the ability of the tree to take in water from the roots it can lead to tree death through things like bark beetle infestations and disease.

The researchers concluded that as a result of this process, projected increases in the duration and severity of drought could cause widespread tree deaths in forests worldwide.

The final set of studies found a similar explanation for the alarming death of white-barked aspen across the west. Many researchers had previously blamed much of the decline on the sharp reduction of the frequent, low-intensity ground fires largely eliminated by a century of grazing and fire suppression throughout the west.

Researchers have noted an alarming decrease in the number of aspen in recent decades, which they’ve dubbed Sudden Aspen Decline (SAD).

The research team from Stanford and Carnegie Institute studied the impact of drought on aspen in an experimental plot.

They found that a decrease in water causes a permanent decrease in the ability of the trees to take up water once it again becomes available. They discovered that even a single summer of drought can kill off roots and decrease the tree’s ability to take up water from the soil by 70 percent.

The researchers concluded that widespread drought and the long-term effect on the water cycle of the trees probably accounts for most of the widespread death of aspen.

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