Continued Droughts Spell Bad News For Trees And Agriculture
October 16, 2012

Continued Droughts Spell Bad News For Trees And Agriculture

April Flowers for - Your Universe Online

A research team, led by a University of Tennessee, Knoxville, geography professor has evidence suggesting that recent droughts could be the new normal. This is not good news for our nation's forests and agricultural lands.

The droughts of recent years have been record breaking, which anyone looking at their yard or garden could tell you. As a result, our nation has seen a record-breaking wildfire season, with over 8 million acres burned as of September. Professor Henri Grissin-Mayer has been studying tree rings over the past 1000 years. Grissin-Mayer says that the rings show this drought is one of the worst the American Southwest has seen in 600 years. And he predicts that the worst is still to come.

The research team, which included scientists from Los Alamos National Laboratory (LANL), the U.S. Geological Survey (USGS), University of Arizona and Columbia University, studied tree ring data to evaluate how drought affects survival and productivity in conifer trees of the American Southwest. The results of their study are published in this month's Nature Climate Change.

Tree rings are reliable markers for analyzing climate conditions. They grow more slowly in times of drought and the size of the rings vary accordingly. Wet seasons are indicated by widely spaced rings, while narrow rings indicate dry seasons.

"Using a comprehensive tree-ring data set from A.D. 1000 to 2007, we found that the U.S. has suffered several 'mega-droughts' in the last 1,000 years in the Southwest," said Grissino-Mayer. "But the most recent drought that began in the late 1990s lasted through the following decade and could become one of the worst, if not the worst, in history."

The research team compiled a tree-ring-based index to catalog the drought stress on forests. The index resolves the contributions of precipitation and vapor-pressure deficit, which is the difference between the moisture in the air and how much the air can hold. Then the team linked this information to disturbances that can cause changes in forests. Such disturbances are bark-beetle outbreaks, mortality and wildfires. They compared these data with their model projections.

"Looking forward to 2050, our climate-forest stress model suggests we will see worse drought and increased tree mortality than we've seen in the past 1,000 years," Grissino-Mayer said. "This drought will be exacerbated by increasing temperatures globally, foreshadowing major changes in the structure and species composition of forests worldwide."

The water balance is effected by increasing temperatures because they exponentially influence how much water evaporates into the atmosphere. More water in the atmosphere means less water in the ground, and trees need that water to survive. This is especially true in the American Southwest, where water supplies are limited.

"We have nothing comparable in the past to today's environment and certainly tomorrow's environment," Grissino-Mayer said. "With increasing drought stress, our forests of tomorrow will hardly resemble our forests of yesterday."

Professor Grissino-Mayer suggests that forest management practices need to be adjusted to accommodate the changes. He notes the increased dangers for wildfires even in East Tennessee's Great Smokey Mountains.