Winter Frosts Reduce Flower Bud Survival in High-Mountain Plants

At higher elevations in the European Alps, plants may experience winter temperatures of −30 °C and lower at snow-free sites. Vegetative organs are usually sufficiently frost hardy to survive such low temperatures, but it is largely unknown if this also applies to generative structures. We investigated winter frost effects on flower buds in the cushion plants Saxifraga bryoides L. (subnival-nival) and Saxifraga moschata Wulfen (alpine-nival) growing at differently exposed sites, and the chionophilous cryptophyte Ranunculus glacialis L. (subnival-nival). Potted plants were subjected to short-time (ST) and long-time (LT) freezing between −10 and −30 °C in temperature-controlled freezers. Frost damage, ice nucleation and flowering frequency in summer were determined. Flower bud viability and flowering frequency decreased significantly with decreasing temperature and exposure time in both saxifrages. Already, −10 °C LT-freezing caused the first injuries. Below −20 °C, the mean losses were 47% (ST) and 75% (LT) in S. bryoides, and 19% (ST) and 38% (LT) in S. moschata. Winter buds of both saxifrages did not supercool, suggesting that damages were caused by freeze dehydration. R. glacialis remained largely undamaged down to −30 °C in the ST experiment, but did not survive permanent freezing below −20 °C. Winter snow cover is essential for the survival of flower buds and indirectly for reproductive fitness. This problem gains particular relevance in the context of winter periods with low precipitation and winter warming events leading to the melting of the protective snowpack.