Treelines are temperature sensitive transition zones that are expected to respond to climate warming by advancing beyond their current position. Response to climate warming over the last century, however, has been mixed, with some treelines showing evidence of recruitment at higher altitudes and/or latitudes (advance) whereas others reveal no marked change in the upper limit of tree establishment. To explore this variation, we analysed a global dataset of 166 sites for which treeline dynamics had been recorded since 1900 AD. Advance was recorded at 52% of sites with only 1% reporting treeline recession. Treelines that experienced strong winter warming were more likely to have advanced, and treelines with a diffuse form were more likely to have advanced than those with an abrupt or krummholz form. Diffuse treelines may be more responsive to warming because they are more strongly growth limited, whereas other treeline forms may be subject to additional constraints.

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Biotic interactions present a challenge in determining whether species distributions will track climate change. Interactions with competitors, consumers, mutualists, and facilitators can strongly influence local species distributions, but few studies assess how and whether these interactions will impede or accelerate climate change–induced range shifts. In this paper, we explore how ecologists might move forward on this question. We first outline the conditions under which biotic interactions can result in range shifts that proceed faster or slower than climate velocity and result in ecological surprises. Next, we use our own work to demonstrate that experimental studies documenting the strength of biotic interactions across large environmental gradients are a critical first step for understanding whether they will influence climate change–induced range shifts. Further progress could be made by integrating results from these studies into modeling frameworks to predict how or generalize when biotic interactions mediate how changing climates influence range shifts. Finally, we argue that many more case studies like those described here are needed to explore the importance of biotic interactions during climate change–induced range shifts.

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. MethodsIn 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. ResultsWe 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 conclusionsTreeline 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.

Life cycles can limit the abilities of species to track changing climatic conditions. We combined age or stage structure and a moving-habitat model to explore the effects of life history on the persistence of populations in the presence of climate change. We studied four dissimilar plant species in moving patches and found that (1) population growth rates, (2) elasticities with respect to the survival (stasis and shrinkage) components of the projection matrix, and (3) the evenness of the elasticities with respect to the components of the projection matrix all decreased as we increased the translational speeds of the patches. In addition, the value of long-distance dispersal increased with patch speed for three of the four species. Our analyses confirm that rapid growth, high fecundity, and long-distance dispersal can benefit species in moving patches. Thus, species with long generation times and limited dispersal ability are especially vulnerable to habitat movement. Stage-structured moving-habitat models can easily incorporate spatial complexity and can help us predict the effects of shifting climatic conditions.