California Forests Highly Stressed By Drought [Study]

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Tree roots can only go down so far. Unfortunately, the record-breaking multi-year drought in California has led to the water table falling so low in many areas that millions of trees in California forests are dying and tens of millions more are at risk.

According to a new study just published in the Proceedings of the National Academy of Sciences, the severe drought across the Western U.S. is having an especially damaging effect on California forests. The new study used an airborne imaging technology to produce a series of eye-opening maps that visually depict dead, dying or endangered trees.



New technology measures water in tree canopy

The research involved the use of a new, high-tech airborne sensing technology together with satellite imagery to produce detailed maps illustrating the condition of state’s trees. The data shows that almost every California forest has been affected in some way, noted lead author Greg Asner, from the Carnegie Institution for Science at Stanford University.

Asner commented that he and his colleagues were stunned by the “sheer degree of loss and mortality” in Southern California forests as they flew over the forests making their scans.

“There was a lot of silence in the lab in the back of the plane because it was just so bad to see,” he said.

Earlier this summer, the team flew a plane (the Carnegie Airborne Observatory) over forests across California and aimed a laser sensor at the trees to measure the water content in the canopies.

The new sensor detects specific molecules such as water. The primary data from the study was the water content of the canopies and then they matched up that data with 3-D forest imagery. This enables the team to estimate the water content of 180 million acres of trees. They also used satellite data from 2011, 2013 and 2014 to model how the progression of the drought has impacted trees.

Canopy water, the liquid water in the leaves of a tree, gives researchers insight into a tree’s overall health and important data about how the drought is stressing trees. The canopy water content of trees can also be used to project the flammability of a forest.

Asner and colleagues determined that 58 million trees had lost more than 30% of their canopy water content since 2011, keeping in mind that  30% is considered the threshold for severe stressed.

The study also allowed modeling of how California forests have changed since the beginning of the ongoing historic Western drought.

More than 400 million acres of lower-elevation forests and woodlands in the Golden State’s Central Valley saw a measurable loss of water from 2011 to 2013. By 2014, the water loss moved to lowland and foothill forests covering 600 million acres (including the Santa Cruz Mountains), with around 507 million large trees being negatively impacted by the drought.

Of interest, the maps make it clear that the drought does not impact all forests (or even trees in a forest) equally. Keep in mind that blue shows a higher water content and red-orange represents highly stressed trees with a low water content.

Of note, the maps do not include areas burned by forest fires to avoid confusing water loss and fire loss.

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