The treeline describes the elevation (or latitude) at which the climate becomes unsuitable for tree growth. This line does not depend on the actual presence of trees and is thus a hypothetical line, which can be modelled at the global scale based on mean growing-season air temperature and a minimum growing-season length (Paulsen & Körner 2014). The problem for trees appears to be a lack of warmth. Trees experience this before low-stature vegetation does, because trees stick out into the free air and thus cannot warm up in the sun as much as more compact life-forms closer to the ground can.
A treeline ecotone is a transition zone between closed forest and low-stature alpine vegetation. It thus describes the realised elevational (or latitudinal) limit in the presence of trees. Not any transition between forest and non-forest is a treeline ecotone. A treeline ecotone should be the last such transition along an elevational gradient and should connect upper-montane / subalpine forest from alpine or replacement alpine-style vegetation.
A tree is an upright single-stemmed woody plant above a certain size. This size is arbitrary, with 2 or 3 m being most often used. In the context of alpine treeline, we suggest 2 m as a useful convention, because a 2-m tall plant will already be coupled to the atmosphere and will thus be limited by air temperatures.
Krummholz is German for crooked or bent wood, but has been widely adopted in the English-language treeline literature. The term is used, on the one hand, for species genetically programmed to form layered shrubs (e.g. Pinus mugo in central European mountains, Pinus pumila in north-eastern Asia, or Rhododendron species in the Himalayas), which would not grow into upright trees even under optimal conditions. On the other hand it is used for environmentally stunted individuals of tree species. We suggest to reserve the term for the latter, preferably referring to environmental krummholz upon first use to avoid confusion, while referring to genetic krummholz as krummholz species or simply shrubs.
Paulsen, J. & C. Körner, 2014. A climate-based model to predict potential treeline position around the globe. Alpine Botany 124(1): 1-12. DOI 10.1007/s00035-014-0124-0.