Trees and other plants are more likely to record the availability of CO2, and other fertilisers, than actual temperatures. A summer with moderate temperatures due to high/steady rainfall, which aids food absorbtion from the soil is more likely to produce good growth and wider rings than very hot and dry summers. And of course the dark lines seperating the growth rings tells nothing about the temperature, or anything else for the tree's dormant period. which can, in some cases can last for 6 months or more each year. Having said that there are a lot of data locked up in trees. For examle there are varying amounts of carbon 12, 13 and 14 present and by analysing these amounts we can estimate cloud cover and therefore amounts of sunspots. How this is done is too complicated to explain here in just a few lines but if you are interrested, just type in your question and then click go. Previous estimates, and estimates is all that they are, have been based on sound but, so far, unproven theory. However if the Sun decides to sleep for another decade or to, we may be able to say whether various theories have been proved or disproved.
I was thinking about exactly this question on the weeked, as I was thinning out (cutting down) pines that I planted 20 years ago. Having read much about the Briffa fiasco last week I paid more attention to the tree rings in the fast growing pines I had cut. This year 2009 was a statistically colder growing year than previous. But by the 2009 tree ring you would not know that... it was the largest of them all. Hot droughty years produced smaller rings in the pines in my layman's study. Competition with other trees and summer rainfall frequency, on my particular dry site, have more bearing than temperature on tree rings. And of course the relative winter temperatures are not recorded at all in tree rings here in Canada. My thoughts for what their worth. EJR