Tree rings tell us many things about how trees grow. Every year a tree adds a new "ring" to the wood which makes up the trunk of the tree, thus making the tree wider. The wider a tree is (depending on the breed) the taller it can grow, the more leaves it can support, and in general the more healthy it is. Trees might be the "missing link" in some of the connection in between how the sun affects the global environment. Due to the connection in between the solar cycle and tree growth this may be one of the underlying causes into the global carbon cycle. The reason for this might be that one of the things trees do very will is turn Carbon Dioxide into Non-Greenhouse gases thus cleaning the environment for us.
Tree rings give us some insight into the local temperature and rainfall for the average from the ring width and density, these things can be used for things such as rebuilding patterns and cycles in weather cycles such as El Nino. The resolution that you can achieve with this type of paleoclimactic data is annual. This is a good method considering the resolution that is achievable. The data that is on file at the National Geophysical Data Center is around 10,000 years of historic data. It has been shown that this tree ring activity does also give insight to the solar activity on an annual resolution.
Considering the fact that we know that tree rings give us insight into local temperature and rainfall, and further considering that tree ring width and density give some insight into solar activity on a decadal scale it only stands to reason that solar activity has some level of effect on temperature and rainfall. The Sun is the driving force for the Earth in as far as temperature goes. It is the only heat source in our solar system. The Earth as it stands does not generate heat, at least not on the scale that temperature on the surface of the Earth varies. If you consider these facts the Sun, and changes in solar activity must not only be the driving force for temperature and rainfall changes but tree ring growth changes as a indirect result.