I am not an expert on tree rings.I am a layman and so are you. Some of the scientists who participated in writing the paper are experts on tree rings and had written many papers on the subject prior to the Hockey Stick paper. They sorted tree ring data into those that are determined by temperature, vs those that are determined by other climate factors. One way they did this was to use the northernmost location of a tree species, because this would be the most likely to be influence by temperature rather than other factors. They decided not to use NH tree rings after 1960 as proxies. Their calibration period was 1880 to 1980.
Other proxies have been used besides tree rings to create a temperature record. There is a lot of noise in the data, but they all show a Hockey Stick.
It has been pointed out that the Hockey Stick graph is not required to show that GHG’s are currently causing global warming.
Because his graph was used as an icon, Mann has been made a villain by Global Warming Deniers, and has been the subject of legal harassment by politicians.