Posted by: Saving Water SA (Cape Town, South Africa) – partnered with Water Rhapsody conservation systems – 19 April 2011

A new study of the oldest trees in Mexico provides the first ever detailed, year-by-year look at the climate of Mesoamerica over a thousand-year span. The data, gathered from the annual growth rings in trees, supplies precise dates for the duration of three historically important droughts, including one that was previously unknown.

Scientists calibrate tree-rings by comparing their width to soil moisture and growth data

Droughts are an important factor in the rise and fall of civilizations. One such drought, dated now by tree rings to the years A.D. 897-927, is thought to have hastened the decline of Mayan culture.

David Stahle, a geoscientist at the University of Arkansas, refers to this as the “Terminal Classic Drought,” since it is perhaps one of the most significant milestones in the decline of what historians refer to as the Classic period in Mesoamerica — a region that includes Mexico and Central America.

Tree rings are a sort of time capsule for climate since they reveal information about wetness for each year. Trees grow by adding new wood in a tiny outside layer each year. The width of each layer will depend on available moisture and soil nutrients. A dry year often results in a thin layer of growth, while a wet year results in a thicker layer. The cross section of a tree is in effect an album of past growth. The ring pattern –with the oldest layers at the center and newer layers at the outside edge — tells the story of past wetness.

To make conclusions from the data, however, it’s not enough to measure the width of the rings. Scientists calibrate the ancient tree-rings by comparing their width to soil moisture and growth data measurements from recent decades. This science of dating things using tree rings is called dendrochronology. Continue reading Oldest trees provide records of historical droughts