Recent Droughts Could Become the Norm

Evidence uncovered by a University of Tennessee, Knoxville, geography professor suggests recent droughts could be the new normal, which is particularly bad for U.S. forests.

Professor Henri Grissino-Mayer looks at the rings of trees in order to tell how droughts effected the growth of those trees he’s studied. Tree rings act as time capsules for analyzing climate conditions because they grow more slowly in periods of drought and the size of rings they produce vary accordingly. Widely spaced rings indicate wetter seasons and narrow rings indicate drier seasons. Grissini-Mayer has recently discovered that the current drought in the southwest region of the United States is one of the worst in the last 600 years.

Grissino-Mayer collaborated with a team of scientists led by Park Williams of Los Alamos National Laboratory and others from the University of Arizona and Columbia University to evaluate how drought affects productivity and survival in conifer trees in the Southwestern U.S. According to their study, the team discovered that the nation has suffered several “mega-droughts” in the southwest during the last 1,000 years.

The researchers created a tree-ring-based index that catalogs the drought stress on forests which resolves the contributions of vapor-pressure deficit and precipitation. This information has been linked to disturbances that cause changes in forests, such as bark-beetle outbreaks, mortality and wildfires and compared these data with their model projections.

Increasing temperatures impact the water balance because they exponentially influence how much water evaporates into the atmosphere. More water in the air means less water in the ground. Trees need that water to survive, especially in water-limited areas like the American Southwest. Grissino-Mayer suggests forest management practices will need to adjust to the changes, noting the increased danger for wildfires even in East Tennessee's Great Smoky Mountains.

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