On the Limits of Alpine Plants: A Systematic Review of the Factors Behind Species' Elevational Range Limits.

Understanding the factors behind species' range limits is a fundamental objective in ecology. Recent research in alpine plant ecology has moved beyond the classical view that distributions are chiefly shaped by climate and competition. Specifically, broader sets of factors have been taken into account, comprising both biotic factors such as facilitation and herbivory as well as additional abiotic factors such as soil properties. However, an overview of the factors that have been identified and studied as important for elevational range limits of alpine plant species is lacking. In this systematic literature review, we synthesize evidence derived from 107 empirical studies on 226 vascular plant species occurring beyond elevational and latitudinal treelines. We find a persistent research focus on the upper elevational range limit (73% of the studies) and on the role of abiotic factors (54% of the studies), particularly temperature (36% of the studies), whereas research on inter- and intraspecific factors (40% and 25%, respectively), such as herbivory or phenology, remained comparatively rare. While temperature was clearly identified as a major factor influencing the upper range limit (29% of the studies), water availability (15% of the studies) was most commonly studied at the lower range limit. Even though a broad set of factors has been investigated, many potentially important factors remain poorly researched, such as the influence of gene flow and connectivity between populations, phenology and light (each only one or two studies). Our findings highlight the need to move beyond temperature and plant-plant interactions as factors influencing the elevational range limits of alpine plants and to integrate intraspecific (such as gene flow and adaptations) and edaphic factors more fully into future research. Improved methodological standardization and transparency and increased attention on lower range limits will be essential for explaining and predicting alpine plant responses under accelerating environmental change.