Mountain treelines climb slowly despite rapid climate warming

Aim: To better understand how climate change drives altitudinal treeline dynamics at large spatial scales. Location: Northern Hemisphere. Time period: 1901–2018. Major taxa studied: Tree species that constitute alpine treelines. Methods: We conducted a meta-analysis of annual treeline shift rates at 143 sites from 38 published studies. For each site, we calculated current change rates in annual or seasonal temperatures, precipitation and drought (standardized precipitation evapotranspiration index). We then evaluated relationships between treeline shift rates, and site characteristics and climate using standard and partial correlation analyses, as well as generalized linear models. These analyses were conducted at the hemispheric scale and for two geographic subsets of subarctic (north of 60° N) and temperate regions (south of 60° N). Results: Treelines ascended at 88.8%, remained stable at 10.5% and descended at 0.7% of the sites. The mean hemispheric shift rate (0.354 m/year) was half of what would be expected from climate warming alone. Treeline shifts were significantly faster in subarctic than temperate regions. The precipitation effect was more important than temperature for predicting treeline shift rate. In the subarctic regions, autumn precipitation mostly determined treeline shift rates. In the temperate region, warmer temperatures and higher autumn precipitation accelerated shift rates, whereas wetter springs reduced them. Autumn precipitation and winter mean minimum temperature best explained the treeline shift rates across the Northern Hemisphere. Main conclusions: A combination of thermal and hydrological factors drives treeline shift rates across the Northern Hemisphere, with precipitation assuming an important modifying role of the general temperature-driven treeline ascent. Regional treeline shift rates, therefore, co-depend on drying and wetting trends, which should be considered in future estimates of global change impacts on alpine ecosystems.