A study by the University of Edinburgh found that cosmic rays play a greater role in stimulating tree growth than any other local climate factor, including precipitation and temperature. The researchers examined tree rings of 53-year old trees to look for climatological factors influencing forest growth, but found that the correlation between growth and climatological variables was barely noticeable, while the correlation with cosmic rays was moderately high.
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.