Intraspecific functional differences of subalpine plant species growing in low-altitude microrefugia and high-altitude habitats

Mountain ice holes are microrefugia at low altitudes characterized by colder temperatures compared to the surrounding areas. Ice holes harbor extra-zonal plant communities composed of plant species that generally grow at higher altitudes in subalpine habitats. Understanding how subalpine species adjust their functional traits to extra-zonal environmental conditions in ice holes can improve our knowledge of plants’ ability to cope with environmental changes. We compared the intraspecific trait variability of four species growing in ice holes and in subalpine areas in the south-eastern Alps of Italy. Plant functional traits presented different patterns of variation between the two habitats. Leaf area, leaf nitrogen and phosphorus content and stomatal traits (stomatal density and stomatal length) differed consistently between habitats in all species. Conversely, specific leaf area and leaf dry matter content varied differently between habitats depending on species. The four species developed differing strategies for adapting leaf functional traits to the peculiar environment of ice holes. Two species, Vaccinium myrtillus and Rhododendron ferrugineum, took advantage of extra-zonal location by developing a more competitive ecological strategy. Conversely, Vaccinium vitis-idaea and Homogyne alpina adopted a more conservative strategy by optimizing adaptation of leaf traits to cold temperature. Our study represents a first attempt to understand the role of marginal populations within the total functional variation of a species. We highlighted the importance of protecting ice holes as important reservoirs of functional diversity for subalpine plant species.