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Maps published in new study show effects of California drought on trees

New maps published in a recent study reveal that the drought in California, which has lasted four years, has caused signifiant water losses in millions of the state’s trees. Californian trees are of significant ecological and economic importance being some of the tallest and oldest trees in the world and are frequently used as lumber, so continued drought could have negative consequences on the state. This study is the first to measure water content in the canopies of forests in California during its four-year drought.

“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 Gregory Asner, lead author of the study “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.”

The study used airborne spectroscopy, which can be used to detect certain molecules, combined with environmental and satellite data to measure the water content in the canopy. Since water is vital to plant growth and function, water content in the canopy is a good indicator of forest health.

Major changes were found in the amount of water contained in the canopy. Approximately 10.6 million hectares of forest showed losses in water content in California in the drought years, which includes almost 900 million trees.

A map showing canopy water content in California. Areas in red indicate low water content. Grey and black areas were not measured. Credit: Asner et. al. 2015
A map showing canopy water content in California. Areas in red indicate low water content. Grey and black areas were not measured. Credit: Asner et. al. 2015

Of these 888 million trees though, about 58 million have experienced water loses of greater than 30 percent, indicating they are more vulnerable and may be at risk of death.

The image to the left shows areas of high and low water content in tree canopy, while the one below shows the level of stress of forested areas.

Water loss was very dependant on several factors including the species of tree and the landscape. Trees in low land areas experienced significant decreases in canopy water content, especially in redwoods. Several species such as pines, firs and black oak showed water loss at higher elevational areas.

Studies by the US Forest Service have estimated a loss of 27 million trees during the drought years of 2012-2015. This study suggests this number will continue to climb as water loss continues in these trees.

A map of the showing the most and least stressed forested areas in California. Credit: Greg Asner
A map of the showing the most and least stressed forested areas in California. Credit: Greg Asner

“So much of the forest underwent a loss of canopy water,” says Asner. “Within that there are lots of pockets of deeply stressed landscapes — not just individual trees, but entire landscapes of trees.”

The data collected in this study can’t be used directly to determine tree mortality, but according to the researchers, tree mortality can be estimated by combining this data with field inventory data.

The increased rainfall brought on by this year’s El Nino will provide some relief to the landscape but probably won’t be enough to restore the forest.

About Harry H

Harry H
Harry is currently studying biology and chemistry in University and hopes to go to grad school for evolutionary biology. He enjoys writing about sciences and sports and is a big fan of hockey and soccer. Some of his other interests are reading and rock climbing. Contact Harry: harry.h@youthindependent.com