Ranunculus glacialis L.: successful reproduction at the altitudinal limits of higher plant life
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- Wagner, J., Steinacher, G. & Ladinig, U. Protoplasma (2010) 243: 117. doi:10.1007/s00709-009-0104-1
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Biodiversity decreases with increasing altitude, mainly because of the increasingly adverse climate. In the European Alps, only a few plant species occur above 4,000 m a.s.l., among these is Ranunculus glacialis L. Current studies have shown that R. glacialis has a highly conservative growth strategy and low developmental plasticity in response to different dates of snowmelt. Therefore, it was of particular interest to observe whether this strategy is maintained at higher altitudes and to reveal the reproductive limits. We examined the effect of the date of snowmelt on reproductive development and reproductive success in R. glacialis over several years at two subnival sites (2,650 and 2,880 m a.s.l.) and at a nival site (3,440 m a.s.l.) in the Austrian Alps. At the subnival sites, reproductive performance was relatively stable (prefloration period, i.e. snowmelt to onset of anthesis, 2–3 weeks; postfloration period, i.e. onset of anthesis until fruit maturity, 4–5 weeks). Depending on the date of flowering, the mean seed/ovule (S/O) ratio was 0.5–0.8. The temporal safety margin between seed maturation and the onset of winter conditions was at least 1 month. The situation was quite different in the nival zone: the prefloration period usually lasted 1 month, anthesis up to 2 weeks, and seed development 6–7 weeks; when seeds matured in time, the S/O ratio was 0.4–0.6. Overall, R. glacialis shows a high developmental plasticity. At higher altitudes, R. glacialis can double the time taken for seed development but runs a high risk of seeds not maturing in time.
KeywordsRanunculus glacialisDevelopmental plasticityEmbryologyMountain plantPrefloration periodReproductive successSeed development
- S/O ratio
Days after onset of anthesis
Day of year
In mountain systems, species richness diminishes with altitude. In the European Alps, most alpine plant species occur between 2,000 and 3,000 m a.s.l. (Grabherr et al. 1995). As altitude increases, fewer species are adapted to the prevailing environmental extremes. Along the alpine–nival ecotone (Pauli et al. 1999), species richness decreases from more than 200 species in the alpine zone to about 30–40 species in the nival zone (Grabherr et al. 1995). Only about a dozen species still occur above 4,000 m. There, isolated individuals inhabit microhabitats where the thermal regime is probably not very different to that some hundred metres lower (Körner 2003). Among the highest ascending vascular plants in the Alps is Ranunculus glacialis L. along with some saxifrages such as Saxifraga biflora, Saxifraga bryoides, Saxifraga moschata, and species of the genus Androsace, Gentiana, Achillea, and Draba (Heer 1885; Anchisi 1985; Ozenda 1988; Grabherr et al. 1995; Körner 2003).
The main constraints at higher altitudes are large temperature fluctuations, an increased number of days with snow and regular night frosts during the growing season, which lasts about 10 weeks (Körner 2003; Larcher and Wagner 2009). Therefore, plants adapted to such environments must be snow resistant and to a certain extent frost resistant in their active state (Larcher and Wagner 1976; Taschler and Neuner 2004; Larcher et al. 2010). Additionally, they must exhibit a high developmental flexibility in this climatically stochastic environment. In spite of a certain degree of adaptation, it is a life at the margin (Crawford 2008), permanently at risk of suffering damage and loss of the seed crop in climatically adverse periods (Kudo 1991; Ladinig and Wagner 2007).The same situation holds true for plants growing in arctic environments (Molau 1993, 1997). Unfavourable weather conditions additionally reduce the abundance of pollinators and restrict their activity to short favourable periods (Arroyo et al. 1985; McCall and Primack 1992). This can lead to pollen limitation and a reduced seed set in predominant outcrossing species (Muñoz and Arroyo 2006; García-Camacho and Totland 2009).
Short vegetation periods are not only restricted to higher altitudes but are also characteristic of snow-beds in lower mountain regions. In hollows, huge snow packs can accumulate during winter, which melt 1–2 months later than in the surrounding terrain. These late-melting microhabitats are colonised by snow-bed specialists, which tolerate the constraints of late flowering: growth and reproduction take place at a decreasing day length and at decreasing temperatures in a mostly wet and therefore already cool environment. Snow-bed species, as a rule, show a high degree of phenological plasticity (e.g. Kudo 1991; Galen and Stanton 1991, 1995; Kudo and Suzuki 1999; Inouye et al. 2002; Totland and Alatalo 2002; Huelber et al. 2006) and may condense phenophases when emerging later (Billings and Bliss 1959; Inouye and Wielgolaski 2003, and citations therein). However, late-emerging individuals usually set fewer seeds or may even fail to mature fruits when winter conditions set in too early (Kudo 1991; Wagner and Reichegger 1997). Galen and Stanton (1991) demonstrated fitness differences in Ranunculus adoneus individuals flowering at different times along a snowmelt gradient: early-flowering individuals set larger seeds than late-flowering individuals, and seed size positively affected the survival rate of seedlings.
The arctic–alpine pioneer species R. glacialis L. grows in snow-beds and sparsely vegetated alpine fellfields at higher altitudes in the European mountains and in the Arctic. In the European Alps, R. glacialis occurs in genetically different groups (Schönswetter et al. 2004). The main distribution range is between 2,400 and 3,200 m a.s.l. (Landolt 1992). In climatically favourable microhabitats, the species ascends over 4,000 m a.s.l.; the highest locality in the Swiss Alps is documented at 4,275 m a.s.l. (Zimmermann 1975). In the High Arctic, the species is still found in the far north of Greenland up to 80°N (Meusel et al. 1965) and in Spitsbergen, where it grows at sea level (Rønning 1996). Like many alpine Ranunculaceae, this species prefers moist sites; sprouting and flowering occur when the meltwater runoff at the growing sites is highest. Vegetative and reproductive development takes only a few weeks (Moser et al. 1977). These traits are most advantageous for colonising cool and wet sites where the growing season is short.
Previous studies have shown that R. glacialis has a highly conservative growth strategy in that the number of phytomers in a shoot sympodium remains unaffected by changing environmental conditions (Järvinen 1989; Prock and Körner 1996; Totland and Alatalo 2002). Moreover, Totland and Alatalo (2002) observed, in an open top experiment in the alpine zone of south Norway, that neither an increase in temperature nor a year-to-year variation in melting dates significantly affected the speed of phenological development or the reproductive output. These findings are consistent with our observations in the main distribution range in the Alps. Therefore, we were particularly interested in finding out whether the seemingly low developmental plasticity is maintained under suboptimal climatic conditions at higher altitudes. We further aimed to reveal where the limits of reproductive performance of this species are.
Over several years, we examined the effect of the date of snowmelt on reproductive development and reproductive success in R. glacialis at two subnival sites and at a nival site in the Austrian Alps. The specific objectives were to explore at the different altitudes: (1) the variation in the length of the prefloration period with respect to the date of snowmelt, (2) the species-specific pattern of seed development and the plasticity of this developmental phase in response to the date of flowering, (3) the relationship between the date of flowering and reproductive success, and (4) the safety margin between seed maturity and the end of the growing season. In addition, we assessed the seed mass for subnival and nival individuals and speculated about consequences for offspring fitness.
Materials and methods
Morphology of R. glacialis
R. glacialis forms a rhizome system, consisting of individually rooted, loosely connected ramets. During the short active period of a few weeks, new ramets form belowground, developing four to six leaves, and depending on the ramet size, a uni- to multiflorous inflorescence (Totland and Alatalo 2002; J. Wagner, personal observations). These ramets become functional in the following growing season. The bowl-shaped flowers contain up to 180 extrorsely opening anthers and 108 ± 47 standard deviation (SD) carpels (Steinacher and Wagner 2010). Carpels of a flower mature into an aggregate fruit consisting of single-seeded achenes, which disperse independently. Soon after fruit dispersal, aboveground tissues (leaves, reproductive shoots) senesce.
Study areas and sites
The investigations were carried out in the Tyrolean Central Alps at two subnival sites and at one nival site. The subnival sites were on the northeast-facing forelands of the Hintertux Glacier (designated as subnival low, SN-L, 2,650 m a.s.l., 47°04′N, 11°39′E, Zillertal Alps) and of the Stubai Glacier (designated as subnival high, SN-H, 2,880 m a.s.l., 46°59′N, 11°07′E, Stubai Alps). The coverage of vascular plants is medium to low and shows typical elements of a subnival association (Androsace alpina, Cerastium uniflorum, R. glacialis, and Saxifraga bryoides, and in the Hintertux Glacier foreland, also Geum reptans and Oxyria digyna). The study areas were about 100–200 m below the rims of the glaciers in disturbed scree habitats interspersed with larger pieces of rock. Most investigations were performed on individuals growing naturally in this area. At the Stubai Glacier, in addition, 15 randomly selected individuals were excavated with root balls at the end of the 2003 growing season and immediately replanted in a plain area of about 1.5 x 1.0 m (referred to as subnival Ranunculus plot, SN-H plot). This was to facilitate the determination of the prefloration period (see below). Investigations in the nival zone were carried out on Mt. Brunnenkogel in the Ötztal Alps (3,440 m a.s.l., 46°55′N, 10°52′E). This mountain rises from the Pitztal Glacier area and is fully glaciated on its northern side. A small ridge on the summit and the steep southern flank are ice free. R. glacialis individuals were distributed along the crest, with an additional 20 individuals found growing in a flat area of about 2 x 1.5 m (referred to as nival plot).
Plant temperatures were recorded at all sites at hourly intervals throughout the investigation period, using small data loggers (Tidbit, Onset, Bourne, MA, USA). Loggers were installed on the soil surface and shaded by leaves or white plastic grids to avoid overheating due to high radiation. Periods with snow cover and the exact melting dates could be derived from the logger data. To reveal the impact of temperature on the speed of development, the length of the respective developmental phase was correlated with the frequency of days with mean temperatures at ≤2°C (which is, according to Körner (2003), the lower temperature limit for effective growth in mountain plants).
The prefloration period (time span from snowmelt until first flowering; Cleve 1901) was determined in plants for which the date of snowmelt was known. At the SN-L site, 10 individuals were marked with colour-coded skewers as they emerged from the winter snow along snow fronts. At the SN-H site and at the nival site, prefloration periods were studied in the individuals of the Ranunculus plots; temperature loggers in the plots indicated the exact melting dates. Plants were regularly monitored, and for each individual, the day of first flowering was assessed.
To investigate the duration of seed development, about 30 flowers from different, randomly chosen individuals at the subnival sites, and about 15 flowers in the nival Ranunculus plot were labelled with coloured plastic rings at the beginning of anthesis in each study year. For those flowers that attained fruit maturation, the postfloration period, i.e. the period between onset of anthesis and onset of seed dispersal (Molau 1993; in the present study: achenes start to fall out) was recorded.
Additionally, the dynamics of seed development was investigated at the SN-H site during the 2003 growing season. From the date of labelling (n = about 200 randomly chosen flowers in at least 120 individuals) until fruit maturity, 10 labelled flowers or fruits from different individuals were sampled at 2–4 day intervals. Samples were fixed in Carnoy solution (96% ethanol, acetic acid, 3:1) and stored in the fixative until further processing. Ovules and developing seeds were dissected from the ovaries, mounted in the clearing solution according to Herr (1971), and microscopically examined using differential interference contrast optics. The length (longest axis between chalaza and micropyle) of the seed, endosperm, and embryo were measured with image analysing software (Optimas 6.5, Optimas Corp., Seattle, WA, USA). On average, 60 seeds from 10 flowers were investigated per sampling date. From the embryological state on each date, the duration of the following developmental phases was determined: onset of anthesis until fertilisation, histogenesis (fertilisation until formation of cellular endosperm), and maturation (end of histogenesis until release of achenes).
Fruit mass was determined at the SN-H site and at the nival site during four study years. For each site, mature achenes from at least 10 randomly collected flowers from different individuals were pooled and stored in paper bags at 5°C. The hundred grain weight was assessed with an analytical balance.
To determine the relationship between reproductive success and flowering time, flowers from 20 to 30 randomly chosen individuals per subnival site (SN-L, SN-H) and year were labelled at onset of anthesis, harvested shortly before fruit maturity, and fixed in Carnoy solution. Under a dissecting microscope, the number of intact and undeveloped (unfertilized or aborted) seeds per flower was counted, and the seed/ovule ratio (S/O ratio, i.e. the proportion of ovules that successfully developed into seeds; Wiens 1984) was calculated. At the nival site, it was not possible to label a sufficient number of flowers on a certain day. Therefore, 44–50 maturing fruits of 24–25 individuals were harvested along the whole summit ridge and fixed. The S/O ratio and additionally the seed size, the seed consistency, and the embryological state were determined. From these data, the approximate onset of flowering for each aggregate fruit could be estimated.
Overview of investigations at the different sites in the different study years
Differences in the length of the prefloration period and in the duration of seed development among sites were analysed with one-way analysis of variance (ANOVA) followed by Bonferroni post hoc comparison. As the sample size varied greatly and to avoid pseudoreplication, we used sample means of each variable. The t test was applied to detect site differences (subnival versus nival) in the seed mass. To study relationships between the duration of different developmental phases and the frequency of days with snow and frost, a correlation analysis (Pearson) was carried out. All analyses were made with the statistical package SPSS (SPSS Inc., Chicago, IL, USA). The critical level of significance was α = 0.05.
Climatic parameters for the different study sites during the investigation period 2001–2008
Onset of the growing season (doy)
End of the growing season (doy)
Length of the growing season (days)
% days with snow cover
% days with minimum temperature <0°C
Mean temperature during the growing season (°C)
Hintertux Glacier (SN-L) 2,650 m a.s.l. mean (min–max)
Stubai Glacier (SN-H) 2,880 m a.s.l. mean (min–max)
Mt. Brunnnenkogel (nival) 3,440 m a.s.l. mean (min–max)
Anthesis and seed development
Our investigations over several years have shown that sexual reproduction of R. glacialis is at its optimum in the subnival zone. At the subnival study sites, the snow-free period (period between snowmelt and the onset of winter snow cover) lasted about 3 months. During this period, mean plant temperature was about 8°C, temperatures fell below zero on only an average of 16% of days, and plants were temporarily covered by snow on an average of 8% of days (see Table 2). Under these conditions, the duration of phenological stages of R. glacialis was rather stable: first individual flowering started 2–3 weeks after snowmelt, anthesis lasted about 1 week, and seed development took 4 to 5 weeks. In total, propagation and vegetative growth proceeded over 7–8 weeks, usually leaving a safety margin between seed maturation and the onset of winter conditions of at least 1 month. The high developmental stability coincided with a high phenological flexibility in that the date of emergence in June and July had hardly any influence on the temporal sequence of reproductive phenophases, but it has to be added that vegetative and reproductive growth in R. glacialis is under the control of photoperiod, as Prock and Körner (1996) could show in a cross-continental comparison study. Ecotypes in the Alps are adjusted to the photoperiodic regime in Central Europe and undergo only one development cycle per growing season. When transplanted to the Arctic, however, plants show a considerable phenorhythmic disorder and may flower several times per growing season. Obviously, extended long-day conditions provoke a cascade of sympodial shoots on which flowers open proleptically. This would mean that an earlier start to the growing season in a warmer climate would not necessarily be beneficial for R. glacialis, but on the contrary, could cause developmental disorder. Evidence for such disturbances comes from our observations in the particularly long, warm growing season in 2003: R. glacialis in the subnival zone had already flowered by the end of May, and we found several Ranunculus individuals flowering a second time in August.
R. glacialis finishes its yearly reproductive cycle faster than other nival plant species studied to date. The remainder of species mostly show both a longer prefloration and postfloration period. For example, Saxifraga bryoides needs about 6 weeks for prefloration, about 10 days for anthesis, and another month for seed development, giving a total of around 12 weeks (Ladinig and Wagner 2007). A similar reproductive period has been observed for S. moschata (Ladinig and Wagner 2005). Androsace alpina requires 3–4 weeks for prefloration and 7–8 weeks for seed formation; and Cerastium uniflorum needs 6 weeks to form new shoots with terminal flowers and another 5 weeks for anthesis and seed formation (J. Wagner, G. Steinacher, S. Erler, and S. Widmann, unpublished results). The comparatively short reproductive period in R. glacialis became particularly evident at sites where individuals of the different species grew side-by-side and thus had the same melting day. We observed that R. glacialis, as a rule, had already dispersed its seeds by the time S. bryoides and C. uniflorum started to flower and A. alpina was in late anthesis.
Two weeks of prefloration and 4 weeks postfloration seems to be the shortest period for reproductive development of R. glacialis with no ability to further accelerate the development. This also became apparent in the open top experiment at an alpine site in Southern Norway (Totland and Alatalo 2002): warming had no significant effect on vegetative growth and reproductive success. Despite large differences in the dates of snowmelt among years, average reproductive output and ramet size differed little among years. Obviously, the given ambient temperature conditions did not constrain growth (Totland and Alatalo 2002), which is consistent with our findings in the subnival zone. The only possible constraint is a lack of time in late melting sites when a permanent snow cover forms too early in autumn. But this situation is not the rule and arose only once within the study years.
The situation was quite different in the nival zone. Here, the snow-free period lasted only about 85 days with mean plant temperatures of around 4°C and about 50% of days with snow and frost. Days with snow cover were more frequent, and summer snow cover lasted longer than in the subnival zone. Particularly, the longer periods with snow produced a stop–start situation, markedly retarding reproductive growth in R. glacialis. The prefloration period usually dragged on for more than 1 month, anthesis lasted up to 2 weeks, and seed development 6–7 weeks—giving a total of at least 3 months. The particularly long anthesis period at lower temperatures can be attributed not only to a slower progamic flower development (anther dehiscence, stigma expansion) but might also be the effect of low pollinator visitation rates. Low pollinator activity retards pollen removal from stamens and pollen deposition on the stigma, and thus prolongs flowering (Arroyo et al. 1985; Robertson and Lloyd 1993; Itagaki and Sakai 2006).
Thus, with increasing elevation, not only the shorter growing season becomes limiting but also the delay in reproductive development. This further signifies that, at higher altitudes, seed formation in R. glacialis is only safe at early melting sites. For melting dates later than mid-June, there is a high risk of the plants not attaining seed maturity. Nevertheless, the reproductive development of R. glacialis appears more efficient than in other nival species which take as long to mature seeds in the subnival zone (see above) as R. glacialis in the nival zone.
In the subnival zone, seeds matured each year. Early- and midflowering individuals attained mean S/O ratios of around 0.8. In late-flowering individuals, the S/O ratio decreased to about 0.5, which was in the range usually found at the nival site (S/O ratio 0.4–0.6) and reported by Totland and Alatalo (2002) for the alpine population in Southern Norway. Remarkably, Järvinen (1989) found S/O ratios of between 0.6 and 0.8 in Finnish Lapland (69°N) where the growing season is even shorter than at the nival site in our study. One reason could be that under extended long-day conditions, pollinator visits are more frequent. Most arctic insects are active whenever it is warm enough, independent of the time of day (Danks 2004). One might further speculate that, at higher latitudes, ecotypes of R. glacialis have evolved that under continuous daylight grow and reproduce especially quickly and thus compensate for the short growing season. Ecotypic variation in response to different lengths of the growing season has been reported for a number of species growing in the Arctic (McGraw and Antonovics 1983; Crawford et al. 1993; Crawford and Smith 1997; Crawford 2008). Ecotypes adapted to short growing seasons in a low temperature regime, inter alia, show higher rates of photosynthesis and metabolism than ecotypes that grow at warmer and earlier thawing sites (Crawford and Smith 1997).
A high seed set is typical for inbreeding plant species and those with a mixed mating system (Wiens 1984; Molau 1993; Richards 1997; Totland and Schulte-Herbrüggen 2003). According to this, the high S/O ratios of R. glacialis would suggest a high selfing capacity. However, pollinator exclusion experiments at the subnival site yielded only a low selfing rate (S/O ratio 0.08 for spontaneous selfing, and 0.12 for selfing by hand; G. Steinacher, unpublished). Emasculated and bagged flowers showed an S/O ratio of 0.05 ± 7.4 SD, which suggests a small contribution from apomixis, but this still has to be investigated in more detail. These data thus indicate predominant outcrossing as was reported for the R. glacialis population in Scandinavia (Totland and Alatalo 2002). There are only a few studies of the breeding systems of Ranunculus species growing in cold climates. Species investigated so far propagate sexually (Philipp et al. 1990, Totland 1994, Pickering 1997a, b) and/or asexually (Hörandl et al. 2005; Paun et al. 2005; Hörandl and Paun 2007, Cosendai and Hörandl, 2010). Most sexual species are facultatively xenogamous but they are self-compatible to varying degrees; e.g. five Ranunculus species cooccurring in the Australian alpine region promote outcrossing by incomplete protogyny, though most of them are fully self-compatible (Pickering 1997a,b). Self-compatibility can provide reproductive assurance when due to a harsh and stochastic climate, pollinators are sparse or uncertain. Similarly, those species which have evolved pseudogamous apomicts are self-compatible and thus independent of pollinators. By contrast, most sexual ancestors are largely self-incompatible (Hörandl 2008). It is remarkable that R. glacialis in spite of its extreme habitat has only a low potential for uniparental reproduction. Obviously, xenogamy is favoured to maintain genetic diversity, particularly since cross-pollination seems not to be a limiting factor in this species.
In our study, the high reproductive success in early- to midflowering subnival individuals points to sufficient pollinator activity. The main pollinators of R. glacialis are Muscid and Syrphid flies whose pollen carry over is not very effective (Escaravage and Wagner 2004). However, a high frequency of visiting insects, which can be regularly observed on bright days (G. Steinacher, J. Wagner, personal observations) and an extended flower longevity (Steinacher and Wagner 2010) may compensate for the low pollination effectiveness of a single insect. At the nival site, pollinator activity seems to become more limiting as shown by generally lower S/O ratios. Similarly, the lower reproductive success of late-flowering individuals at the subnival sites might be due to a decrease in flight activity of insect pollinators later in the season as has been reported for Scandinavian mountain habitats (Totland 1993, 2001). Even if the seed output of R. glacialis is reduced later in the season or at higher elevations, an average of between 40 and 70 seeds per flower mature when the growing season is long enough.
Achenes of R. glacialis—as in many Ranunculus species—are released from the receptacle in a premature state when the pericarp and seed coat are still green. The photosynthetically active tissues enable seed maturation to be completed independently of the mother plant (Boesewinkel and Bouman 1995). Maturing achenes are protected by the bowl-shaped perianth, which can be seen as an adaptation to the harsh conditions in cold climates (Hörandl et al. 2005).
An early independence of the offspring may be advantageous in short growing seasons; however, in the case of R. glacialis, it is combined with a low degree of embryo differentiation. Mature seeds contain a firm endosperm and a morphologically undifferentiated embryo with only one poorly developed cotyledon which is common in basal eudicots (Förster 1997; Engell 1995). Thorough investigation of the germination physiology of R. glacialis is lacking, but seeds may be dormant and need afterripening to become germinable, as is the case in several Ranunculus species (Stanton and Galen 1997; Baskin and Baskin 1998). Preliminary investigations (J. Wagner, unpublished results) suggest a morphophysiological dormancy similar to that of Caltha leptosepala (Forbis and Diggle 2001). It is possible that the rudimentary embryo is dormant during winter, and then undergoes further development during the following growing season, becoming ready to germinate in the second year after dispersal.
The germination capacity of seeds seems to be high in R. glacialis—at least in the main distribution zone. In a study on the population dynamics in an upper alpine habitat, Diemer (1992) found a high seedling density particularly on south-facing slopes (more than 200 seedlings per square metre on average). However, the high seedling density did not lead to a corresponding increase in the abundance of juvenile stages, suggesting that seedling mortality is substantial. There are no investigations of the seedling dynamics of R. glacialis in the nival zone. Given the lower reproductive success, a lower seedling density would be expected. Additionally, nival individuals produce significantly lighter seeds with less storage reserve than subnival individuals in most years. A decrease in seed weight with altitude has been reported for different high mountain species and points to limiting conditions for producing and filling seeds (Pluess et al. 2005; Zoller et al. 2005). Smaller seeds might negatively affect fitness of the offspring as demonstrated by Galen and Stanton (1991) for Ranunculus adoneus. However, we could also show that nival individuals of R. glacialis are capable of producing normally sized seeds in climatically favourable periods. Possibly, these seeds play a crucial role in population turnover at higher altitudes; in that, offspring is mainly recruited from this pool of more vigorous seeds.
The reproductive performance of R. glacialis is quite stable and largely independent of the emergence date within the main habitat in the subnival zone. Because of the short reproductive period, there is usually no risk of losing the seed crop. Though flowers are mainly pollinated by small flies which are weak pollinators, a long flower lifetime increases the frequency of pollinator visits and thus assures reproductive success. The stable performance found in earlier studies suggested a lack of developmental plasticity. Our investigations have shown that the response of R. glacialis is plastic only at higher altitudes, where climatic conditions become increasingly limiting. Under the far lower temperature regime in the nival zone, Ranunculus adoneus is capable of markedly prolonging all phases of reproductive development. The total reproductive period can be twice as long as in the subnival habitat. This also illustrates that growth in R. glacialis is rather insensitive to permanently low temperatures as long as tissues do not suffer frost damage (first damage between −6°C and −8°C; Larcher and Wagner 1976; Taschler and Neuner 2004). The adaption to cooler growth temperatures is also apparent in the pronounced heat sensitivity of R. glacialis (Larcher et al. 1997) and its poor ability to adjust dark respiration when temperatures increase (Cooper 2004). Thus, high respiration rates and the inability to thermally acclimate seem to be a fundamental reason why R. glacialis is restricted to cold environments. At higher elevations, however, time for growth and reproduction may become limiting as a longer development time coincides with a markedly shorter growing season. This signifies that, in the nival zone, only early-flowering cohorts have the chance to mature seeds. For late-flowering individuals, this becomes increasingly unlikely.
This study was funded by the Austrian Science Foundation (FWF) as part of the project “Diversity of sexual reproduction in high mountain plants” (P15595-B3). We thank Stefanie Erler, Daniela Hosp, and Stephanie Widmann for their help with fieldwork and for providing data. We thank E. Hörandl and M. Akhalkatsi for valuable suggestions on the manuscript. Thanks also to the Stubaier Gletscherbahn and the Zillertaler Gletscherbahn for free transportation by cable car.
Conflict of interest
The authors declare that they have no conflict of interest.