Trees Tell Tales

Oct 13, 2012 By Arati Rao
Arati Rao's picture

Scientists working in the Amazon have found something interesting. Tree rings can tell them how much it rained in which year. In short, trees seem to keep a record of climate fluctuations over the years. Fascinating, right?

How much rainfall?

UK scientists, working in the Amazon forests of Bolivia, have collected data from eight cedar trees. They took samples from these lowland (not growing at a high altitude, but almost at sea level) trees and studied the rings. Why did they choose the cedar trees? That’s because cedar roots are shallow. They therefore depend on rainfall that collects in the topsoil. This means that they could serve as a good barometer for rainfall records.

Now, rainwater contains a heavier isotope of oxygen, Oxygen-18.  Whereas water contains the lighter isotope: Oxygen-16. The scientists studied the ratio of the two isotopes in the rings. What they found is that the changes in the ratios accurately tell you how much rainfall fell in a year. With just eight tree rings, they were able to tell records of rainfall all over the Amazon catchment. Moreover, drought years and El Nino years were very apparent too.

The other interesting thing the scientists found was that the heavier isotope seemed to increase over the years. This showed that the rainfall was increasing over the years. Why this method will prove important in the future is because meteorological data in the Amazon stretches back only 50-60 years.

Dendroecology

Derived from Greek for “tree limb” denron, the use of tree rings to study ecology and climate is much like extracting ice cores in the Arctic to study polar climatology. Tree rings can stretch back over hundreds of years. In fact, they consider 1950 as their “Before Present” or the tree-ring equivalent of “A.D.”

Now, given that this is the Amazon – the tropics, rings may not be very distinct. Why? Tree rings tend to be more distinct where the seasons are distinct. When the weather is more equable (the seasons are not distinct), you cannot tell the rings as clearly. But still, the scientists, by choosing the tree species well, were able to study the isotopic traces in the rings.

What’s next?

In order to extend and confirm this data of rainfall over the years and patterns of change in the Amazon, the scientists want to repeat this study over a larger area.

A video about reading tree rings: