150 million trees died in California’s drought, and worse is to come

The drought that recently ravaged California for six years
crushed salmon runs, torched hundreds of houses, and sent lawn-care
professionals into a deep depression. It was the worst droughts in
hundreds of years, and by the time it was finished in 2017, it had
killed some 150 million trees in the Sierra Nevada Mountains. Many
groves lost nearly all of their mature pine trees, turning green
forests brown across swathes of the state.

But they didn’t die everywhere. Scientists studied one spot
where nearly 80 percent of trees died, and another where just 2
percent did. A new
study
, just published in Nature Geoscience, reveals an elegant
formula to explain why some trees died and others didn’t — and
it suggests more suffering is in store for forests as the climate
heats up.

This matters because California’s ambitious plan to slash its
greenhouse gas emissions
relies on trees soaking up a lot of carbon
. But many scientists

now think
that all the dead and burning trees have turned
California’s forests from a carbon sink to a carbon source.

There were a lot of problems ganging up on the forests in the
last decade. But this study by Michael Goulden at the University of
California at Irvine, and Roger Bales at the University of
California at Merced, one thing driving it all: How much water came
into the forest and how much water evaporated out.

“Where there was less precipitation than the trees’ demand
for water, that’s where trees died,” Bales said.

Because trees need more water when it’s hot, the scientists
estimate that tree deaths will increase 15 to 20 percent in the
Sierra Nevada Mountains for every increase of 1 degree Celsius.

So why didn’t California didn’t have a similar die-off of
trees during the last major drought, which started in 1987?

“The difference with this drought is that it was hotter,”
Bales said. “The drought in the late ‘80s was longer, but it
wasn’t so hot.”

The number and density of trees also stoked the competition for
water. Each tree wound up fighting with the others for the last
drops.

“There’s just too many stems in the ground sucking up water
because of the fire suppression of the last century,” Bales
said.

And in places where trees had exhausted their water supply, they
were particularly vulnerable to bark beetles.

California’s climate and forest officials are now trying to
figure out what to do with all the dead trees — tinder for
wildfires. This research suggests that we could prevent future
forest massacres by allowing fires or humans to thin vulnerable
areas. We could also, you know, try to stop heating up the
place.

This story was originally published by Grist with the headline
150 million trees died in California’s drought, and worse is to
come
on Jul 3, 2019.

Source: FS – All – Science – News
150 million trees died in California’s drought, and worse is to come