Every 11.2 years there is slight change in the degree of solar radiation that reaches Earth. This is caused by spots forming on the sun and known as the sunspot cycle. We know that the cycle shapes plant growth today but have little evidence of what solar cycles once were. Botanists are perfectly capable of looking at recently felled trees to infer solar cycle information but applying these methods to fossil plants has been really hard. To make matters worse, trees with the relevant structures preserved in them are almost never fossilised in a good enough state to be used in this manner. Thus it is all the more remarkable that a team of researchers are revealing the discovery of spectacularly well preserved fossil trees which show evidence of sun spot cycles that took place 290 million years ago. You can read more in The Economist article that I wrote on this here.