October 2, 2012

Study Finds Rings On Trees Reveal More Than Just Age

Alan McStravick for - Your Universe Online

As a child, we all learned about how the rings of a tree could tell us how old the tree was. Did you know that now we can know how much rainfall occurred year-to-year through this same tried and true method? Well, with a twist.

A research study led by a team from the University of Leeds has used tree rings from eight cedar trees in Bolivia to determine and document a 100-year history of rainfall across the Amazon River Basin, the world´s largest river system.

Researchers have shown that the rings of these lowland tropical cedars are a natural archive of data closely related to historic rainfall levels.

By measuring two different oxygen isotopes (oxygen-16 and the heavier oxygen-18) that are trapped within the tree rings, researchers were able to determine the pattern of rainfall from year to year over the entirety of the past century.

The lead author of the study, Dr Roel Brienen from the School of Geography at the University of Leeds, said: "We already knew that some tropical tree species form annual rings and we also anticipated that the isotopic signature in these rings might record changes in the climate.”

"What surprised us, however, is that just eight trees from one single site actually tell us how much it rained not just at that site but over the entire Amazon catchment. That is an area about 25 times the size of the UK. The isotope values recorded in tree rings were very closely related to annual variation in the river levels of the Amazon, and thus of the amount of rainfall that flows into the ocean."

The purpose of the study was to observe the hydrological cycle of the Amazon River Basin, which is widely regarded as the richest natural ecosystem in the world. Here you can find approximately one-tenth of the planet´s total biodiversity and a full one-fifth of the globe´s carbon, which is stored in plant biomass. Additionally, the Amazon basin is one of the wettest places on Earth with one fifth of global land precipitation falling in this region where it drains into the Atlantic Ocean via the world´s largest and second-longest river.

With the basin´s location along and around the equator, the hydrological cycle of the basin may have an important impact response in relation to climate change. Understanding this cycle may eventually help researchers to recognize the magnitude and speed of climate change across the globe.

Co-author Dr Manuel Gloor, also at the University of Leeds, said: "Climate models vary widely in their predictions for the Amazon, and we still do not know whether the Amazon will become wetter or dryer in a warmer world. We discovered a very powerful tool to look back into the past, which allows us to better understand the magnitude of natural variability of the system."

"In a similar way that annual layers in polar ice sheets have been used to study past temperatures, we are now able to use tree rings of this species as a natural archive for precipitation over the Amazon basin. If we find older trees with similar signal strength then this will greatly help us to advance our knowledge of the system."

Dr Brienen added: "The record is so sensitive that simply from the isotope values we can say which year we are looking at. For example, the extreme El Niño year of 1925-26 which caused very low river levels, clearly stands out in the record."

Although the century-long record provided by the trees is relatively short, some interesting trends are evident.

"The oxygen isotope series shows an increase over time, which may be due to an intensification of the hydrological cycle," said Dr Gloor. "That could also explain the observed long-term trend in river discharge. We need however to replicate this research at different places in the Amazon to really be able to say more."

This research project was a collaborative effort between researchers from the University of Leeds, the Helmholtz Centre in Potsdam, Germany, the University of Utrecht and the Institut de Recherche pour le Developpement (IRD). The findings were published in the journal Proceedings of National Academy of Sciences.