California drought has damaged millions of trees

Mighty redwoods.
Redwood trees in California. @bberwyn photo.

Large swaths of forest now seen as more vulnerable to future droughts

Staff Report

California’s extended drought may take a long-term toll on the state’s forests, scientists reported last month after studying severe water loss from tree canopies since 2011.

The study, published in the Proceedings of the National Academy of Sciences, says that up to 58 million large trees in California showed signs of being drought stressed, with persistently low rainfall, high temperatures and outbreaks of the destructive bark beetle combining to increase forest mortality risk.Rather than just looking at dead trees, researchers used laser-guided imaging  tools mounted on the Carnegie Airborne Observatory to measure the full impact of the drought on California’s forests. They combined the CAO data with more-traditional satellite data going back to 2011.

The data shows a progressive loss of water in California’s forest canopies during the four-year span. Mapping changes in canopy water content tells scientists when trees are under drought stress and greatly aids in predicting which trees are at greatest death and fire risk.

“California relies on its forests for water provisioning and carbon storage, as well as timber products, tourism, and recreation, so they are tremendously important ecologically, economically, and culturally,” said the Carnegie Institution’s Greg Asner. “The drought put the forests in tremendous peril, a situation that may cause long-term changes in ecosystems that could impact animal habitats and biodiversity.”

Detailed mapping showed that  about 41,000 square miles ( of forest containing up to 888 million large trees experienced measurable losses of canopy water between 2011 and 2015. Of this group, up to 58 million large trees reached water loss thresholds that the scientists deemed extremely threatening to long-term forest health.

Given the severity of the situation, even with increased precipitation due to El Nino, if drought conditions reoccur in the near future, the team predicts that there would be substantial changes to already significantly weakened forest structures and systems.

“Our high-resolution mapping approach identifies vulnerable trees and changing landscapes,” Asner said. “Continued airborne and satellite monitoring will enable actions on the ground to mitigate a cascade of negative impacts from forest losses due to drought, as well as aid in monitoring forest recovery if and when the drought subsides.”

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