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Sequoias: Can they Withstand the California Drought?

Liz Clift

It’s no secret that California is entering Year 6 of a drought-period. And though this rainy season has had one of the wettest starts in 30 years, at least in northern California, that doesn’t guarantee the end of the drought. In fact, the Department of Water Resources made an announcement in November that State Water Project customers can expect to receive 20% of their requested deliveries in 2017 (this is an initial estimate and likely to change).

Part of what impacts this is the overall health, or depth, of the snowpack in the Sierras. Snowpack can store millions of acre-feet of water that refills lakes and reservoirs as it melts—and how empty or full lakes and reservoirs appear provide a quick indicator of overall drought conditions.

However, it’s more difficult to account for some of the other impacts of a prolonged period of drought. Since 2011, more than 102 million trees in California have died, more than half of them (62 million) this year. These trees provide important ecosystem services, including supporting healthy watersheds, providing wildlife habitat, and acting as a “carbon sink”—meaning they capture atmospheric carbon. Even giant sequoias (Sequoiadendron giganteum), which are usually fairly resilient during drought periods, are showing evidence of drought through browning crowns and showing up as standing dead (sequoias are more likely to fall over when they die, because they’ve stretched too far from their stabilizing root systems).

Researchers Wendy Baxter and Anthony Ambrose began studying the impacts of drought to sequoias in 2015, with the support of the National Park Service (NPS). They, along with a team of volunteers, climb the sequoias using jumar ascenders, and collect samples at a couple of different points—including from the crown of the tree. These samples help researchers understand the severity of water stress the sequoias are under, the water content in their needles, and the amount of carbon-13 isotope the tree uses during the photosynthetic process (which provides additional insights into drought stress).

This information will ultimately be used in forest management—and could include selective thinning that would eliminate less resilient trees that are competing for water.

And the competition is tough: giant sequoias can take in 800 gallons of water per day. Water is drawn up through the xylem primarily through the release of water vapor through the leaves’ open stomata (transpiration). This creates a vacuum affect. As leaves conduct photosynthesis, water and sugars flow down the phloem*. These sugars are then either stored for later use when photosynthesis slows or used for the tree’s basic maintenance. As drought conditions continue, the tension created by the transpiration process increases and can eventually snap the xylem. This creates an embolism that prevents additional water from flowing up the trunk. If this happens too much, a tree will shed its leaves and eventually die.

Baxter and Ambrose aren’t the only research team working to figure out what’s happening with California’s forests. Another comes in the form of an airborne observatory that maps what’s happening with trees across the state (the technology is advanced enough that the observatory can pick up images about what’s going to become apparent to the human eye).

This should concern us all because forests act as carbon sinks—and if they are dying, that means the carbon will be released back into the atmosphere. Higher levels of atmospheric carbon are linked to global climate change. In addition, forests have a cooling effect on the surrounding area, in part due to the transpiration process, and provide habitat for many different species.

We’ll have to see what this rainy season in California brings—it is a La Niña year, which comes with the possibility of a relatively dry winter, despite the wet beginnings of the season.

*Like that pneumonic? Flow down à phloem!

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