Changing Mountain Forests Threaten California Water Supply

Mountain forests are growing increasingly vulnerable to climate change, and as a result changes in their ecology are threatening California's water supply, according to new research.

As temperatures warm, the lush greenery more prevalent at lower elevations on mountains like California's Sierra Nevada is moving higher and higher uphill. That is, according to findings published Monday in the journal Proceedings of the National Academy of Sciences.

Normally these lands are too cold for trees to survive, but as temperature gradients shift uphill, thirsty forests are going the distance, so to speak, in search for water. Dense forests typically make up a mountain's middle region - where water supply is ample and temperatures are optimal for plant growth - and now this "sweet spot" is on the move, and so trees are moving with it.

Such landscape changes, seen in the Sierra Nevada, could have major repercussions for California's water supply and economy - water that flows down the mountain's western rivers could be reduced by a quarter by the end of this century. That's bad news considering draining rivers on the Sierra Nevada account for 60 percent of residents' water needs.

"It's a study that goes in there and says, 'Could this be a big effect?'" co-author Michael Goulden, an associate professor at the University of California at Irvine, told Climate Central. "Yeah, this could definitely be a big effect."

A team of researchers studied the upper Kings River basin in California's Sierra Nevada to determine if water runoff was increasing with warming. Specifically, they measured evapotranspiration - the amount of water sucked up by roots and breathed out by leaves.

"The first thing we saw was that evapotranspiration was highest at mid-elevation, and it was considerably lower at the highest site that we looked at," Goulden explained.

After subtracting evapotranspiration rates from snowfall and rainfall figures, they found that most of the mountain's runoff is being produced in areas above 7,000 feet. And as thirstier forests move higher up, this vital region could soon be invaded.

Despite the fact that this new study only took measurements from one water basin in the Sierra Nevada, and didn't account for effects of expected precipitation changes or carbon dioxide levels, researchers say the findings could have implications for water runoff at other mountains around the world as Earth's climate continues to change.