2 October 2012
Last updated at 14:05 GMT
By Mark Kinver
Environment reporter, BBC News
The samples offer an insight to past meteorological conditions in the Amzon, say researchers
Samples
from eight cedar trees in Bolivia have helped shed light on the
seasonal rainfall in the Amazon basin over the past century, say
researchers.
A study led by UK-based scientists said the data from the
trees provided a key tool to assess the natural variation in the
region's climate system.
It suggested that tree-rings from lowland tropical cedar provided a natural archive of rainfall data.
The findings
appear in the Proceedings of the National Academy of Sciences.
"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," said co-author Manuel Gloor from the
University of Leeds.
"We discovered a very powerful tool to look back into the
past, which allowed us to better understand the magnitude of natural
variability of the system."
The researchers explained that the region's vast size and
position on the equator, the response of the forested area's
hydrological cycle "may significantly affect the magnitude and speed of
climate change for the entire globe".
Dr Gloor added: "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 these species as a natural archive for
precipitation over the Amazon basin."
Wooden signal
The team identified the signal in measurements of two different forms (or isotopes) of oxygen within the wood of
Cedrela odorata.
Seasonal conditions in tropical locations mean many species do not form clear growth rings
Within tropical and sub-tropical evergreen rainforests, trees'
growth rings are less pronounced than in other woodlands - such as
temperate regions - as there is no discernable dry season and
temperature variations are minimal.
But lead author Roel Brienen, from the University's School of
Geography, explained: "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 told us how much it rained not just at that
site but over the entire Amazon catchment," Dr Brienen added.
"The isotopic values recorded in the tree rings were very
closely related to annual variation in the river levels of the Amazon,
and thus the amount of rainfall that flowed into the oceans."
The researchers added that about 17% of the annual discharge from rivers into the world's oceans comes from the Amazon.
Also, they said, the basin's hydrological cycle is closely
tied to the carbon cycle of the rainforest, which is one of the planet's
largest terrestrial biomass carbon pools.
The cedar species used in the study has shallow roots,
therefore they are more dependent on water they are able to gather from
rainfall that gathers in topsoil.
Dr Brienen observed: "The record is so sensitive, we can say what year we are looking at.
"For example, the extreme El Nino year of 1925-26, which caused very low river levels, clearly stands out in the record."
Until now, reliable meteorological data in the region was scarce and only stretched back over the past 50-60 years.
Tree rings show Amazon's rainfall
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