Not every high-latitude or high-elevation forest edge is a treeline

Attempts at identifying climate warming effects on mountain and arctic vegetation caused a recent hype in treeline studies. In this perspectives article, we recall the need of clear-cut definitions, a consistent terminology and a theoretical framework that permits hypothesis testing. Founded in the ecological niche concept, the application of the fundamental niche edge to treeline permits defining the potential climatic limit of tree growth, while the realized niche edge captures all deviations for reasons related to other, more local, abiotic factors, biotic interactions, disturbances and human interventions. An important point is that a globally common phenomenon calls for a common abiotic driver which is the temperature at the low temperature edge of the niche of the life form tree. We explain why other abiotic factors that may affect the local range limits, such as microclimate, moisture and wind do not devaluate the classical isotherm concept. Our key message is that applying a clearly defined concept of potential treeline, also allows defining deviations from it and explaining the deviations within a reproducible theoretical framework.