The tree-line marks the frontier, usually at a certain latitude or elevation, where cold temperatures restrict tree growth. Scientists have speculated that climate change could cause tree-lines to advance, but new research reveals that their response to warming is more complex than predicted.
Melanie Harsch of Lincoln University, New Zealand, and colleagues used data from 166 forested sites across the globe, together with temperature data from nearby climate stations, to look at whether tree-line advance during the twentieth century was linked to local warming. While tree-lines advanced in 87 of the sites studied, they receded at two of the sites and remained stable in the rest. The researchers found that the tendency to advance was highly dependent upon the type of tree-line. Diffuse tree-lines — where the density of trees decreases gradually — were the most responsive to warming, with 80 per cent advancing since 1900. In contrast, only 22 per cent of abrupt tree lines and those dominated by crooked trees advanced over the same time period.
Seasonal temperatures also strongly influenced the response, with abrupt and crooked tree-lines likely to advance when warming occurred during winter and diffuse treelines more commonly affected when warming occurred during summer.