Mountain Ferns: What Determines Their Elevational Ranges and How Will They Respond to Climate Change?

Abstract. Mountains are the global centers of fern diversity and at the same time strongly affected by climate change, raising the question of how fern species and communities will respond to these changes. In the present review, which also includes our own unpublished data, we first outline the challenges of identifying distributional boundaries in ferns. We suggest that the elevational ranges of many fern species are determined by geographical constraints such as low mountain tops and sea level, as well as habitat availability rather than by climate. We then show that climate-range limits of ferns are driven by numerous physiological processes, not only involving the effects of cold and drought stress at high elevations, but also of drought stress coupled with high temperatures at low elevations, and possibly even of such poorly considered factors as low frost tolerance at low elevations in the absence of snow cover. Finally, there is also some evidence for biotic limitations, such as interspecific competition and the negative influence of leaf litter, especially in species-rich assemblages without extreme climatic factors. Overall, we find that elevational distributions of ferns are determined by a broad suite of factors, many of which do not involve physiological tolerance to climate or only indirectly so, and therefore that reactions of ferns to climate change will likely be species- and context-specific. We also emphasize the paucity of studies focusing both on the physiological limitations for fern growth and reproduction, and on biotic interactions affecting fern distributions. To overcome these knowledge gaps, we advocate a range of further studies, including resampling of old vegetation plots, lab experiments, and transplantation experiments, on both gametophytes and sporophytes.