Physiological explanations of current and future treeline positions require a stringent treeline definition

Background/Question/Methods

The absence of forests at any point on the planet can be related to either stochastic phenomena (disturbance) or to the gradual action of environmental drivers such as moisture shortage or low temperature. The biological low temperature limit for tree growth is commonly termed arctic or alpine treeline, which separates the treeless arctic or alpine life zone from the naturally forested boreal or montane life zone. Historically, attempts at a mechanistic explanation of this most prominent biogeographic demarcation did not arrive at a testable theoretical framework, because stochastic and non-stochastic phenomena were not clearly separated, and because trees pass through different life stages during which the action of low temperature differs. In this presentation we will offer a framework of definitions and terminology, and present testable hypothesis for the advance of this field of research. One specific reason for seemingly inconsistent observations in the past was a regional emphasis at scales, at which stochastic phenomena overshadow the underlying biological principles. Hence, our approach is global and based on physiological principles that operate across taxa.

Results/Conclusions

We will recall the evidence for the competing carbon centric and developmental approaches at treeline explanations (carbon limitation versus growth limitation). Given that non-stochastically determined treeline positions across the globe match a common isotherm, our focus will be on temperature related processes. We advise against applying the term treeline to high elevation forest limits that are related to stochastic phenomena, or to neither high elevation nor high latitude specific climatic factors such as drought. When such drivers come into play, they may cause absence of trees from the potential treeline, but the biological treeline as such is not suppressed. To our understanding, these are the criteria needed to proceed with mechanistic modeling treeline phenomenon and to predict the future development of the treeline ecotone on a global scale.