Shrubland expansion - photographic evidence

To start off the posts on terrestrial ecosystem change, I'd like to share a short (one page!) correspondance article from Nature by Sturm et al (2001). It's pretty simple and is completely qualitative based on only two 'observations', but it's interesting nonetheless. The authors found high quality photographs, taken during oil reserves exploration in 1949 at around 68° latitude in the Alaskan Arctic. By taking 66 modern photographs at some of the same locations, they could compare the vegetation cover at the same sites over the 50 year period. Here's one of the comparisons:


A hillside in the Alaskan Arctic in 1949 and 2000. A and B show identical locations.

Although a crude assessment, it is clear that there is a difference between the two photographs. Abundance
 and cover of shrub are significantly higher in the recent photograph, and it appears that areas with no cover in 1949, to the left of the shot, now have patches of shrub growth. A similar difference was found in 36 of the 66 sites which were compared, with some showing signs of significant colonisation of bare land.

The limitations for such a study are obvious. Taking two snapshots in time with little knowledge of the patterns in between can lead to a misinterpretation. For example, if there is a growth and die-back cyclical pattern, the snapshots could capture photographs at either end of the cycle. Also, errors in shot framing and the fact that sites are lined up by eye all add to the uncertainty.

However, despite these limitations the study, in my opinion, still indicates a change. Firstly, due to cold and short seasons, growth in the Arctic is relatively slow. Also, the gap between the photographs (50yrs) is short, reducing the likelihood that the photographs are just snapshots of two points in a natural cycle. Also, if there were no regional forcing, it would be expected that the sites, when plotted, would span between a reduction in shrub extent, and an expansion. Although this plot is not available (due to the qualitative nature of the study) the authors wording would indicate a large bias towards expansion. Of course, although these changes could be due to any widespread environmental change, ecological theory and the presence of peat bogs in the region (indicating significant past vegetation growth during warmer periods) would suggest climatic warming to be the cause for this expansion.