The BBC published an interesting article summarizing some recent scientific research that suggests that the tree growth rate is an increasing function of the cosmic rays flux.
This relationship between the tree growth and the cosmic rays flux actually seems to be stronger than the influence of both temperature and precipitation on the tree growth (in Scotland)! Well, after all, wood may grow much like tumors when a lot of ionizing radiation is around. ;-)
I think that if true, such a relationship could also help to explain the divergence problem.
Hat tip: Benny Peiser