Treeline in the Arctic

As promised, I'm going to consider the Arctic Treeline for this next post. Just as a warning, one of the papers I use in this post includes some sites from below the 66 degrees latitude level, so I hope you'll forgive the slight digression from our brief.

An alpine treeline in the background. Although not an Arctic treeline,
the clear change illustrates the concept well.

Firstly, what is the Arctic treeline? As an intuitive general definition, a treeline separates regions where tree-growth is sustainable, and areas where climatic factors such as low temperatures prohibit the establishment
of trees. This can happen with altitude ie on the side of a Mountain (Alpine treeline), at the desert margin, and at the edge of Polar regions. In the Arctic the treeline separates the tundra and boreal forest biomes. It's not always easy to determine the point of the line, however, as there isn't an exact point where dense forest just stops, so it's more accurate to include an 'ecotone' or a region where the two blend into each other.

Due to this sensitivity to temperature, it is likely that the location(s) of the treeline is strongly related to climate. A number of studies have looked at treeline changes in the distant past to try and learn more about these links and any potential feedback mechanisms or lags between climate and tree extent. One such study by Macdonald et al (2000) who carbon dated fossilised tree-stumps (macrofossils) they found in the north of Russia. This method is more appropriate than counting pollen through lake cores, as pollen is found a long distance away from the source and wouldn't give the spatial accuracy needed for the small changes expected during the Holocene. Using tree stumps in 'root position' ie unmoved, we can be pretty sure there was vegetation there!

Results of MacDonald et al (2000) showing number of dated macrofossils found through time
at sites above the current treeline.

As you can see above, they found trees far north of the current tree line dating back to ~9000 cal yr BP, suggesting a treeline far north of it's current position. Sea Surface Temperature (SST) reconstructions of the North Atlantic show significant warming at around 9000 years ago, reducing at around 4000 years likely due to lower summer insolation; showing correlation with the macrofossil data. This strongly suggests an influence of climate upon the location of the Arctic treeline. As you've probably noticed though, the temporal resolution is pretty low; 1000 year intervals. This reflects the nature of carbon dating and the slow growth of trees.

Despite the slow change of the treeline, some authors have tried to document changes to the treeline during recent climate warming. For example Lloyd and Fastie (2003) found spruce north of their 1800 limits in Alaska at all but one site they sampled, a change they assign to recent climate warming. They also reported spatial variations in advancement rates, with some advancing in the 19th and others in the 20th century, showing that the response of the treeline to Climate Change is unlikely to be uniform and predictable.

The big question is though: Will treeline advance northward in the future? The answer is 'probably'.
Factors controlling this expansion are debated are still not clear however, but its likely that CO2 levels and temperature are dominant factors. I think trying to find recent changes in the treeline is a bit of an inexact and inaccurate science to be conclusive. It simply takes too long for trees to grow and develop for us to be seeing significant changes happening now. If warming continues, however, it is quite certain that the trees will be 'advancing' northward. Next time for a change that's likely to be much faster; the melting of permafrost.