The use of dendrochronology, or tree ring chronologies has been used to replicate past climates and the occurrence of El Niño. A study was done where 65 tree ring chronologies were taken from western North America and studied. These rings could depict climate change by the size of their annual growth bands. The tree rings studied covered a time span between 1600 and 1963. Warm events of El Niño were associated with increased tree growth in Colorado, Utah, Arizona, New Mexico, and Mexico. A slightly reduced tree growth was recorded in Montana and south-western Canada. This pattern, as seen in the tree rings, was compared to other data from the 20th century, where a similar growth pattern was seen.

(Taken from www.duke.edu)