Treeline form - a potential key to understanding treeline dynamics

Aim Treelines occur globally within a narrow range of mean growing season temperatures, suggesting that low-temperature growth limitation determines the position of the treeline. However, treelines also exhibit features that indicate that other mechanisms, such as biomass loss not resulting in mortality (dieback) and mortality, determine treeline position and dynamics. Debate regarding the mechanisms controlling treeline position and dynamics may be resolved by identifying the mechanisms controlling prominent treeline spatial patterns (or 'form') such as the spatial structure of the transition from closed forest to the tree limit. Recent treeline studies world-wide have confirmed a close link between form and dynamics. Location The concepts presented refer to alpine treelines globally. Methods In this review, we describe how varying dominance of three general 'first-level' mechanisms (tree performance: growth limitation, seedling mortality and dieback) result in different treeline forms, what 'second-level' mechanisms (stresses, e.g. freezing damage, photoinhibition) may underlie these general mechanisms, and how they are modulated by interactions with neighbours ('third-level' mechanisms). This hierarchy of mechanisms should facilitate discussions about treeline formation and dynamics. Results We distinguish four primary treeline forms: diffuse, abrupt, island and krummholz. Growth limitation is dominant only at the diffuse treeline, which is the form that has most frequently responded as expected to growing-season warming, whereas the other forms are controlled by dieback and seedling mortality and are relatively unresponsive. Main conclusions Treeline form provides a means for explaining the current variability in treeline position and dynamics and for exploring the general mechanisms controlling the responses of treelines to climatic change. Form indicates the relative dependence of tree performance on various aspects of the external climate (especially summer warmth versus winter stressors) and on internal feedbacks, thus allowing inferences on the type as well as strength of climate-change responses.