From the BBC:
Smaller trees at the vanguard at Lee Ridge, in the Glacier National Park of Montana may be more than 50 years old.
Trees around the world are colonising new territories in response to higher temperatures.
From the US west coast to northern Siberia and south-east Asia, trees are growing at higher elevations, and at higher latitudes as the climate warms.
Of 166 sites studied, trees are advancing at more than half, while they are receding at just two sites.
The shift is revealed by the first global analysis of treelines published in the journal Ecology Letters.
One of the significant things here I think is that it highlights the kind of self-correcting feedbacks that operate in the climate system that we either still don't understand properly or even know nothing about.
It also highlights the folly of the assumption made by the IPCC and climate modellers that climate feedbacks in response to rising carbon dioxide levels must be positive, that is, they'll accentuate any warming effect of the extra CO2.
Maybe they wont. The reality is that we just don't know.