27 October 2009

Cosmic rays speed up tree growth

Cosmos Online
Cosmic rays, which constantly strike the Earth and are regulated by the solar wind, may influence how fast trees grow, according to British research.
Spruce forest

The researchers studied the growth of Sitka spruce forests in Scotland. Credit: iStockphoto

SYDNEY: Cosmic rays, which constantly strike the Earth and are regulated by the solar wind, may influence how fast trees grow, according to British research.

The study, published in the journal New Phytologist looked at the factors that influence the growth of Sitka spruce trees (Picea sitchensis) felled in the Forest of Ae in Dumfriesshire, Scotland.

Trees grow faster during summer when there is more light. But other factors, such as cloud cover and the amount of carbon dioxide in the atmosphere, can also influence tree growth.

Tree rings

The researchers measured the width of tree rings in 30 slices of spruce preserved by freezing. While looking for climatic factors that might influence the growth of the trees, they made the surprising discovery that the trees grew faster in a pattern that matched with cycles of galactic cosmic rays; high energy particles (mostly protons) that stream from space.

The intensity of cosmic rays hitting the Earth at any time is modulated by the strength of the solar wind, which in turn is linked to solar activity. For example, when there are few sunspots, during the solar minimum, there are more galactic cosmic rays hitting the Earth.

Since solar activity varies on a roughly 11-year cycle, there are periods when the Sun is relatively quiescent and the intensity of galactic cosmic rays is high.

Puzzling correlation

These periods between 1961 and 2005 match growth spurts recorded in the tree rings, report the researchers, led by Sigrid Dengel from the Institute of Atmospheric and Environmental Science at the University of Edinburgh in Scotland.

But so far no-one knows why the link – which is small, but significant statistically – should exist. “These cyclic phenomena have been previously been identified in tree ring records but rarely explained,” Dengel told Cosmos Online.

One mechanism being considered by Dengel and her colleagues is the tendency of cosmic rays to create particles that ‘seed’ clouds. Research in the U.S. journal Science in 2002 suggested this effect may explain why solar variation can have a relatively big effect on climate by changing the amount of cloud cover on Earth.

Sceptics say the creation of cloud cover by cosmic rays is too small to play a role in climate variation. If the model is correct, however, then the researchers say that cloud cover might increase the amount of ‘diffused’ rather than direct sunlight hitting the forests.

Diffuse sunlight is better at penetrating the forest canopy than direct sunlight, said Dengel, and would promote photosynthesis and growth.

The other hypothesis put forward by the team is that cosmic rays somehow directly influence tree growth, which has been suggested as a possibility in studies of organic material exposed to cosmic rays in experiments in space.

“Intriguing phenomenon”

Plant physiologist and tree growth expert David Ellsworth, at the University of Western Sydney, in Australia, said it was an “intriguing phenomenon”, and that the hypothesis that the growth spurts were caused by diffuse radiation driven by galactic cosmic rays, was “reasonable”.

However he added that there were aspects of the research that were speculative and, as a commentary in the same journal by Finnish authors noted, more data from different locations would be needed to substantiate the effect.


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