Future climate change is expected to cause many species to shift their ranges. Alpine tree lines have already been observed to shift their upper limits to higher elevations in response to the warming temperatures, and this trend will likely continue in the coming decades. However, not all elevation shifts are due to climate. In Europe, increasing woody encroachment into alpine pastures and meadows over the last 40 years has largely been attributed to increasing land abandonment and changing management practices. Thus, predicting how the tree line will shift under future climate change scenarios requires an explicit consideration for pasture management and land use history.
To improve our understanding of how future climate change and changing management will influence alpine tree lines, we used a process-based, dynamic forest landscape model that included grazing by cattle and pasture management. The model was used to simulate a high-elevation pasture-woodland landscape in the Swiss Alps. The current tree line is located approximately halfway up the pasture, with alpine meadows above. We simulated current management and current climate, as well as all combinations of 10 climate change scenarios and pasture management scenarios (0 – 10 times the current grazing pressure), and compared woody encroachment rates.
Simulation results using current climate and management identified a disequilibrium between observed vegetation patterns and current stocking densities (i.e., observed vegetation patterns are consistent with higher historical grazing pressure). Thus even without climate change, the tree line was simulated to shift ~ 1.4 m per year. Climate warming increased woody encroachment and shifted the tree line to higher elevations, up to a 400 m shift under the most extreme climate scenarios. Increased temperatures also caused an increase in forage production, which resulted in a greater proportion of the pasture that remained un-grazed. Therefore, an even higher grazing pressure was required to maintain the same degree of pasture openness under various climate change scenarios. These results increase our understanding of the processes which shape current vegetation patterns at the tree line and improve our ability to predict how these landscapes will respond to future changes.