Climate velocity, plant dispersal, and ecological surprises at the tree line

Background/Question/Methods

Cold ecosystems such as sub-alpine forests are expected to expand upwards in a warmer world. An integral part of these expected range shifts is the relationship between each species potential velocity (i.e. dispersal capacity) and the rate of climate change (i.e. climate velocity). Due to large gradients in climate over short distances, mountain systems are thought to be safe (refugia) areas where species can potentially track fast climate change, as opposed to large northern plains, where climate velocity becomes fast over very relaxed climatic gradients. Thus, sub-alpine treelines are expected to reach climate equilibrium faster than forest-tundra treelines.

However, subalpine treeline advance might be largely delayed by non-purely climatic mechanisms. Recent work identified slope processes to play a key role in distorting climatic-induced treeline advance in a sub-alpine forest region of the Canadian Rocky Mountains, fundamentally altering the predictions of vegetation response to climate warming in the region (Macias-Fauria & Johnson 2013, PNAS). The significance of these findings at a large spatial scale has not been yet tested.

Results/Conclusions

Here we present a global study covering the major mountain ranges on Earth and thousands of kilometres of treeline which analysed sub-alpine forest advance using moderate resolution (30 meter) satellite imagery (Hansen et al. 2013, Science). Global trends were analysed as a function of climate change velocity, topography, underlying geology, glaciation history, and land conservation status.

We conclude that control of rates of migration (subalpine treeline advance) by factors other than climate in mountain systems can largely modify (reduce) the ability of vegetation to track climate change.