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Dr. Andrea Lloyd says:

Last year we went to 3 sites and collected age structure data and what those data suggested to us that treeline is advancing at 2 of our 3 sites. And what we found is that if you go to spruce populations at treeline, at current treeline, there's been really steady recruitment of spruce for the last 300 years. So there are really old individuals, and middle aged individuals, and young individuals. If you go up above treeline, there are a couple of really old individuals, and then nothing established until about 1960, at which point at 2 of our 3 sites, there's a big pulse of establishment, and lots and lots of young trees at above treeline which suggests to us that treeline is advancing.