The Causes of Treeline

Abstract

The word generally evokes images of alpine or arctic ecotones, but treelines appear under a much wider variety of circumstances. Wet treelines occur along the margins of bogs or swamps (42, 43, 64, 74, 99, 100, 101). Dry treelines mark the transitions between forest and grassland habitats (18, 89, 103, 104). Cold treelines include the familiar arctic and alpine (31, 65, 71), but also maritime (31) and mountain meadow (25, 58) ecotones. While all treeline studies focus on the replacement of trees by nontrees along an environmental gradient, rarely is this shared theme explicitly developed to make comparisons between different treeline types. Our goal is to develop general principles of treeline ecology based on the ecological correlates of differences in plant growth form (35, 90). To do this, we use a review of the current explanations for treelines to highlight those features of the tree growth form that appear to limit tree distribution. Our suggestions for future research arise from the gaps in knowledge that appear once the literature is reorganized in this fashion.