Principal components analysis among the 17 study sites, based on 27 environmental variables (24 climate variables, elevation, slope angle and aspect). PCA axis 1 explained 80% of the variation and PCA axis 2 explained 13% of the variation. Shrinking sites in each year are dark grey, while stable/growing sites in each year are light grey. Ellipses are standard deviations centred on the mean of each group.