A Review of the Theories to Explain Arctic and Alpine Treelines Around the World

Abstract

Forest growth is restricted at high latitudes and high elevations, and the limits of tree growth in these environments are dramatically marked by the treeline transition from vertical, erect tree stems to prostrate, stunted shrub forms. However, after 4 centuries of research, there is still debate over the precise mechanism that causes Arctic and alpine treelines. We review the various theories for treeline, including excessive light, low partial pressure of CO2, snow depth, wind exposure, reproductive failure, frost drought, and temperature. Some of these theories are very old and are no longer held in high esteem; while they may help to explain treeline physiognomy or local variation in treeline position, they generally fail as global explanations. Temperature-based theories appear to be the most reasonable, since cold temperature is really the only trait that is universally characteristic of treelines around the world. Temperature may limit a variety of physiological processes, such as carbon fixation, cuticular ripening, or new tissue development, and theories invoking these mechanisms are discussed. The vertical growth habit of trees is unfavorable to growth in this hostile environment: Low-profile vegetation enjoys a far more favorable microenvironment for growth. Microsite enviroment and ecological facilitation have been shown to be essential for successful regeneration, which is a prerequisite for upward advancement of treeline. Recent evidence supports a theory based on “sink limitation,” i.e., that new tissue development is restricted not by carbon availability but by cold treeline temperatures which limit cell division, and that this situation is exacerbated by arborescent growth (aboveground meristems coupled to cold ambient air temperatures) and self-shading (which keeps soil temperatures cold and restricts belowground activity).