Tree Ring 101

How tree rings can lead to estimates of past streamflow is not readily obvious. It's natural to think that trees lined right along a river would act as the best gages. As it turns out, those trees are relatively useless - or complacent, as researchers call them. Even in times of drought, riverside trees draw up plenty of water from the moist soil, packing on wide growth rings year after year. The best trees for streamflow reconstruction come from areas where water is limited, making them sensitive to moisture changes.

To collect tree samples, researchers head out to areas within or near the river basin of interest. They collect thin, pencil-sized cores from old trees - usually from Douglas firs, ponderosa pines or pinyon pines. They can also use cross-sections from dead trees that have been well preserved, called remnant wood. After collecting multiple samples of the same species from each site, researchers crossdate the cores. Crossdating involves comparing the samples to one another so that exact years can be assigned to all rings. This technique allows researchers to identify false or missing rings. After all rings have been crossdated, the sample set is called a chronology.

Researchers must then choose a gage record to extend back into the past. Using tree ring chronologies from around the region where the gage is located, researchers calibrate the tree-ring data to the gage measurements to create a mathematical model. Researchers evaluate the model by determining how closely its values match the gage measurements. Then, they test the model either by comparing it to a part of the gage record that wasn't included in the calibration process, or by checking to see if the chronologies used can estimate streamflow in other sections of the data set.

The scientific study of tree rings is not new. The practice was developed by astronomer A.E. Douglass back in the early 1900s. He founded the nation's first tree ring lab at The University of Arizona in 1937. His student, Edmund Schulman, pioneered the efforts to reconstruct the Colorado River streamflow in the 1940s. The first reconstruction was completed by Charles Stockton and Gordon Jacoby in 1976, and dendrochronologists have built on the work ever since.

Source: Western Water Assessment; UA Tree Ring Lab