Over the last year, the central and south-western US has been stuck in drought, a condition that has killed crops and depleted reservoirs. To help manage diminishing water supplies, some utilities and scientists are investigating a surprising source of information: old trees.
More precisely, water managers are looking at tree rings, the concentric layers of tissue that trees produce each year as they grow in diameter. In a wet year, when times are good, trees are able to add a lot of new growth, and produce wide rings. In dry years, trees batten down the hatches and produce much narrower ones. By examining these patterns in a region’s tree trunks, scientists can determine how precipitation has changed over time.
Which trees provide the most useful rings? The ones that have been around the longest, of course. “The most valuable species are trees like redwoods and bristlecone pines, which live hundreds of years”, says tree physiologist Graeme Berlyn. Researchers in California’s Sierra Nevada Mountains have used giant sequoia stumps to reconstruct the region’s drought history as far back as the year 101 BC.
In the South Platte River watershed, which supplies eastern Colorado with most of its water, pinyon and ponderosa pines tell a story in which droughts regularly occur every few decades. By correlating tree ring data from as far back as 1634 with modern stream gauges, Denver Water, a public utility with 1.3 million customers, has been able to determine the severity of Colorado’s worst historic droughts, and conserve resources accordingly. “We plan on having enough water to get through the most severe drought we’ve seen on record, with a bit of a safety on top”, says Steve Schmitzer, Denver Water’s manager of water resources analysis.
As climate change tampers with precipitation patterns, that safety margin could become vital. One of the driest years in recent Colorado history was 2012, and 2013 is shaping up to be just as parched. Is the current dry spell a random fluctuation, or a harbinger of a changing system? “2002 was the worst single year on the historic record, and here we are only 10 years later with very serious droughts”, says Schmitzer. “We’re starting to adjust flows downward based on climate models.”
Not only do tree ring records help water managers prepare for climate change, they may also demonstrate just how much the climate is changing. Scientists in China and Mongolia have used tree rings to calculate rainfall variability, and researchers in India are exploring the technique on the Satluj River. Although climate change will affect precipitation differently in each country, the big picture is plain: rainfall patterns are in serious flux. For Berlyn: “Tree rings give us a valuable baseline against which we can measure the changes that are happening.”– Ben Goldfarb
Photo: John Eastcott & Yva Momatiuk / National Geographic