How endangered is sexual reproduction of high-mountain plants by summer frosts? Frost resistance, frequency of frost events and risk assessment

Abstract

In temperate-zone mountains, summer frosts usually occur during unpredictable cold spells with snow-falls. Earlier studies have shown that vegetative aboveground organs of most high-mountain plants tolerate extracellular ice in the active state. However, little is known about the impact of frost on reproductive development and reproductive success. In common plant species from the European Alps (Cerastium uniflorum, Loiseleuria procumbens, Ranunculus glacialis, Rhododendron ferrugineum, Saxifraga bryoides, S. moschata, S. caesia), differing in growth form, altitudinal distribution and phenology, frost resistance of reproductive and vegetative shoots was assessed in different reproductive stages. Intact plants were exposed to simulated night frosts between −2 and −14 °C in temperature-controlled freezers. Nucleation temperatures, freezing damage and subsequent reproductive success (fruit and seed set, seed germination) were determined. During all reproductive stages, reproductive shoots were significantly less frost resistant than vegetative shoots (mean difference for LT50 −4.2 ± 2.7 K). In most species, reproductive shoots were ice tolerant before bolting and during fruiting (mean LT50 −7 and −5.7 °C), but were ice sensitive during bolting and anthesis (mean LT50 around −4 °C). Only R. glacialis remained ice tolerant during all reproductive stages. Frost injury in reproductive shoots usually led to full fruit loss. Reproductive success of frost-treated but undamaged shoots did not differ significantly from control values. Assessing the frost damage risk on the basis of summer frost frequency and frost resistance shows that, in the alpine zone, low-statured species are rarely endangered as long as they are protected by snow. The situation is different in the subnival and nival zone, where frost-sensitive reproductive shoots may become frost damaged even when covered by snow. Unprotected individuals are at high risk of suffering from frost damage, particularly at higher elevations. It appears that ice tolerance in reproductive structures is an advantage but not an absolute precondition for colonizing high altitudes with frequent frost events.

Vascular plants at extreme elevations in eastern Ladakh, northwest Himalayas

“…Another question is to what degree the seed development is slowed down by unfavourable events like cold spells or snowfall, as was usual in this study region. The frequency of such periods, when plants stop growing or even get damaged, seems to be of importance, as is the total time needed for seed development and also for reproductive success (Ladinig and Wagner 2007;Wagner et al 2012;Ladinig et al 2013).…”

Frost resistance in alpine woody plants

“…Important factors contributing to of the degree of thermal insulation are the thickness of the snow pack ( Figure 2 ) and its consistency (Kuhn, 2012). While protective effects of snow cover have long been recognized (Sakai and Larcher, 1987), thermal insulation by snow may also be crucially important for protecting plants from low freezing temperatures during episodes of freezing temperatures in summer (Larcher and Wagner, 2009; Ladinig et al, 2013). A certain degree of thermal insulation is possible by snow coverage for dwarf shrubs and young trees in the summer months but will be lost at a certain stage of tree development when a specific tree height is reached.…”

“… Frost resistance (LT i /LT 10 ; °C) of different organs of woody alpine plant species listed in Table 1 assessed during summer [data compiled from: Sakai and Larcher (1987), Taschler and Neuner (2004), Ladinig et al (2013), and Neuner et al (unpublished)]. Air temperature minima in June (blue horizontal bar) and July and August (light blue horizontal bar) as recorded at the upper distribution boundary of the species (2100–2800 m) in the years 2003–2013 (see legend to Figure 1 ).…”

“…To date, the only alpine plant species found to actually exhibit freezing tolerance in its reproductive shoots is the nival herb Ranunculus glacialis (Ladinig et al, 2013). All other species have been reported to be freezing sensitive, i.e.…”

Past selection explains differentiation in flowering phenology of nearby populations of a common alpine plant

“…Such correlations could strengthen the case for past and current adaptive evolution of flowering time and inform about selective agents. An emerging key environmental determinant of plant distributions in alpine habitats is spring frost (Bannister et al 2005;Ladinig et al 2013;Lenz et al 2013;Briceño et al 2014;Wheeler et al 2014). The likelihood of spring frost at any elevation is largely determined by the date of snowmelt, which in turn is a function of winter precipitation, and topography.…”