A new study from the University of Edinburgh provides evidence that cosmic rays play a greater role in stimulating tree growth than any other local climate factor -- including precipitation and temperature.

Professor John Grace of the university's Institute of Atmospheric and Environmental Science and his graduate students, Sigrid Dengel and Dominik Aeby, made the intriguing discovery when studying rings on freshly harvested spruce trees.  The trees were specially harvested in 2006 from the Forest of Ae in Dumfriesshire, Scotland, by Forest Research, the research branch of the UK's Forestry Commission.  The trees sections were immediately frozen after harvest to prevent shrinkage.

Scientists then scanned the 53-year old trees, which were planted in 1953.  What they found was that like most trees there were variations in growth rates by year.  What frustrated scientists was that yearly temperature and precipitation showed minor correlations, but not to the extent they were seeing.  There was some other factor also inducing growth.

That factor, it turns out, appears to be Galactic Cosmic Rays (GCRs).  NASA describes GSRs as follows:

Galactic cosmic rays (GCRs) come from outside the solar system but generally from within our Milky Way galaxy. GCRs are atomic nuclei from which all of the surrounding electrons have been stripped away during their high-speed passage through the galaxy. They have probably been accelerated within the last few million years, and have traveled many times across the galaxy, trapped by the galactic magnetic field. GCRs have been accelerated to nearly the speed of light, probably by supernova remnants. As they travel through the very thin gas of interstellar space, some of the GCRs interact and emit gamma rays, which is how we know that they pass through the Milky Way and other galaxies.

Apparently they also make for great tree growth.  In years which received the most rays, the trees grew the most.  Describes Ms. Dengel, "We were originally interested in a different topic, the climatological factors influencing forest growth.  The correlation between growth and cosmic rays was moderately high, but the correlation with the climatological variables was barely visible."

The researchers were initially reticent to touch the topic as rays are influenced by solar cycles, a common topic of discussion among global warming skeptics, and thus unfortunately a subject of some scientific stigma.  Now they're thrilled about the unusual findings. 

It is unclear how exactly the radiation benefits plant growth.  It may operate either through an indirect environmental affect, such as raising cloud cover ever so slightly, which leads to better rates of photosynthesis (counterintuitively, diffuse light is better for trees).  Another possibility is a direct mechanism, such a radiation hormesis, a theory that low levels of radiation help protect organisms against harmful mutations.  Both theories are quite controversial.

The new research is reported in the New Phytologist.