Frost as a limiting factor for recruitment and establishment of early development stages in an alpine glacier foreland?

Questions: How frost resistant are the early development stages (seeds, seedlings, plantlets and juveniles) of alpine plant species? Do summer frosts impair establishment of plant species typical of different successional stages on a central alpine glacier foreland? Location: Rotmoos glacier foreland, Austrian Central Alps (Obergurgl, Tyrol, Austria). Methods: Seeds of 12 species typical of different successional stages were collected in the glacier foreland and either sown directly in the field or in a growth chamber (25/10 °C, 16/8 h) and grown to the investigated development stages. Frost resistance of the early development and adult stages was determined by exposing them to a set of freezing temperatures and assessing viability with the tetrazolium test (LT50, i.e. 50% of samples being lethally frost damaged). Results: Dry seeds had the highest frost resistance (LT50: 19 °C), followed by wet seeds after imbibition (LT50: 8 °C). With the onset of germination, frost resistance decreased rapidly. While germinated seeds tolerated a mean of 3.2 ° C, seedlings and juveniles were less frost resistant (LT50: 2.5 °C). Along the primary succession, seedlings of pioneer species were significantly less frost resistant than early- and late-successional species. However, field grown seedlings, mainly of pioneer species, showed higher frost resistance (mean: 5 °C) than the growth chamber seedlings (mean: 3 °C), indicating that frost hardening (transition from a lower to a higher level of frost resistance) is already possible during these early stages of development.