On a global scale, climate-driven high-altitude treelines occur at surprisingly similar growing-season temperatures (means from 5.5 to 7.5 °C), whereas season length varies between 2.5 and 12 months, and many other climatic constraints which have been suggested as contributing to alpine treeline formation show large regional variations. In order to resolve this apparent discrepancy, which has hindered attempts to explain treelines, I suggest separating "modulative" (regional) from "fundamental" (global) forcing factors. Most of the literature relates to modulative factors on treeline position (i.e. the fine tuning — such as by winter drought damage on saplings, which is absent in the tropics). As the underlying and unifying global determinant (with less precise predictive potential at the local scale), I suggest direct impacts of low temperature on meristem activity. Hence, I hypothesize a sink, rather than a carbon source (photosynthesis) limitation of tree growth at high elevations. I suggest upright trees become thermally constrained by their own life form, because of the close thermal coupling of their shoots to the atmosphere and through self-shading of their root zone. Alpine vegetation (including small tree seedlings) escapes close thermal coupling and cold soil by adopting compact life forms, by radiant canopy warming and by a high soil heat flux. The physiological limitations are likely to be the same.