Saxifraga Bryoides Research Articles

Pollen limitation is not the rule in nival plants: A study from the European Central Alps.

Seed output of high-mountain plants in the uppermost life zones is highly variable. One possible reason might be pollen limitation due to inadequate pollinator services. We tested this hypothesis for the insect-pollinated species Cerastium uniflorum, Ranunculus glacialis, and Saxifraga bryoides, which have their distribution center in the subnival and nival zone of the European Central Alps. We recorded insect visitors and determined their impact as pollinators. By analyzing pollination success and seed set following natural and saturating hand pollination, the magnitude of quantitative and qualitative pollen limitation was assessed. Anthomyiid and muscid flies had the highest pollination impact for R. glacialis and S. bryoides and syrphids for C. uniflorum. Natural stigma pollen load was highly variable in individual flowers of all species, but in most cases the number of conspecific pollen grains clearly exceeded the number of ovules to be fertilized. There was also a surplus in germinated pollen grains, whereas the pollen tube to ovule ratio was only sufficient in R. glacialis (2.6 on average) and S. bryoides (1.3), but not in C. uniflorum (0.6). Accordingly, seed to ovule ratio was around 0.8 in R. glacialis, 0.7 in S. bryoides but 0.4 in C. uniflorum. In C. uniflorum, saturating pollination slightly increased seed set. Regression analyses revealed that natural pollination success was more frequently limited by quality than by quantity. Our results do not support the idea of chronic, widespread pollen limitation in the subnival but rather fit into the concept of parental optimism by overinvesting in the number of ovules as an adaptation to variable resource availability.

Structure-dynamic relationship of plant–insect networks along a primary succession gradient on a glacier foreland

There is a growing interest in understanding the structure–dynamic relationship of ecological networks. Ecological network changes along primary successions are poorly known: to address such topic, gradient of primary succession on glacier forelands is an ideal model, as sites of different age since deglaciation stand for different ecosystem developmental stages. We aimed to investigate the assembly processes of plant–insect networks and to elucidate its functional implications for ecosystem stability along this time sequence succession. We collected data on the functional role of anthophilous insect groups and performed network analysis to evaluate their relative importance in the structure of plant–insect interaction networks with increasing time since deglaciation along the primary succession of a debris-covered glacier foreland. We sampled anthophilous insects visiting the flowers of two models plant species, Leucanthemopsis alpina and Saxifraga bryoides. Insects were identified and trophic roles were attributed to each taxon (detritivores, parasitoids, phytophagous, pollinators, predators, and opportunists) at five sites representing the primary succession gradient. Plant–insect interactions were visually represented by a bipartite network for each successional stage. For each plant species and insect group, centrality indices were computed quantifying their community importance. For the whole network, centralization and link density were calculated. Pollinators dominated pioneer communities on the debris-covered glacier and in recently deglaciated areas, while parasitoids, predators and opportunists characterised late-succession stages. Plant species centrality varied along the succession. Pollinators showed initially higher but then decreasing centrality, while the centrality of predators and parasitoids increased with time since deglaciation. Along the same gradient link density showed an increasing trend while network centralization tended to decrease. The present study provides new insight into the initial steps of plant–insect network assembly and sheds light on the relationship between structure and dynamic in ecological networks. In particular, during the succession process, more links are formed and plant–anthophilous insect interactions change from a network dominated by pollinators to a functionally more diversified food web. We conclude that applying network theory to the study of primary succession provides a useful framework to investigate the relationship between community structure and ecosystem stability.

The Progamic Phase in High-Mountain Plants: From Pollination to Fertilization in the Cold.

In high-mountains, cold spells can occur at any time during the growing season and plants may be covered with snow for several days. This raises the question to what extent sexual processes are impaired by low temperatures. We tested pollen performance and fertilization capacity of high-mountain species with different elevational distribution in the European Alps (Cerastium uniflorum, Gentianella germanica, Ranunculus glacialis, R. alpestris, Saxifraga bryoides, S. caesia, S. moschata) during simulated cold snaps in the laboratory. Plants were exposed to 0 °C (the temperature below the snow) for 12, 36, 60 and 84 h. In S. caesia, the experiment was verified in situ during a cold snap. Sexual processes coped well with large temperature differences and remained functional at near-freezing temperatures for a few days. During the cooling-down phase a high percentage (67–97%) of pollen grains germinated and grew tubes into the style. At zero degrees, tube growth continued slowly both in the laboratory and in situ below the snow. Fertilization occurred in up to 100% of flowers in the nival species and in G. germanica, but was strongly delayed or absent in the alpine species. During rewarming, fertilization continued. Overall, progamic processes in high-mountain plants appear fairly robust toward weather extremes increasing the probability of successful reproduction.

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

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.

Effect of temperature on the progamic phase in high‐mountain plants

Progamic processes are particularly temperature-sensitive and, in lowland plants, are usually drastically reduced below 10 °C and above 30 °C. Little is known about how effectively sexual processes of mountain plants function under the large temperature fluctuations at higher altitudes. The present study examines duration and thermal thresholds for progamic processes in six common plant species (Cerastium uniflorum, Gentianella germanica, Ranunculus alpestris, R. glacialis, Saxifraga bryoides, S. caesia) from different altitudinal zones in the European Alps. Whole plants were collected from natural sites shortly before anthesis and kept in a climate chamber until further processing. Flowers with receptive stigmas were hand-pollinated with allopollen and exposed to controlled temperatures between -2 and 40 °C. Pollen performance (adhesion to the stigma, germination, tube growth, fertilisation) was quantitatively analysed, using the aniline blue fluorescence method. Pollen adhesion was possible from -2 to 40 °C. Pollen germination and tube growth occurred from around 0 to 35 °C in most species. Fertilisation was observed from 5 to 30-32 °C (0-35 °C in G. germanica). The progamic phase was shortest in G. germanica (2 h at 30 °C, 12 h at 5 °C, 24 h at 0 °C), followed by R. glacialis (first fertilisation after 2 h at 30 °C, 18 h at 5 °C). In the remaining species, first fertilisation usually occurred after 4-6 h at 30 °C and after 24-30 h at 5 °C. Thus, mountain plants show remarkably flexible pollen performance over a wide temperature range and a short progamic phase, which may be essential for successful reproduction in the stochastic high-mountain climate.

Apomixis is not prevalent in subnival to nival plants of the European Alps.

Background and AimsHigh alpine environments are characterized by short growing seasons, stochastic climatic conditions and fluctuating pollinator visits. These conditions are rather unfavourable for sexual reproduction of flowering plants. Apomixis, asexual reproduction via seed, provides reproductive assurance without the need of pollinators and potentially accelerates seed development. Therefore, apomixis is expected to provide selective advantages in high-alpine biota. Indeed, apomictic species occur frequently in the subalpine to alpine grassland zone of the European Alps, but the mode of reproduction of the subnival to nival flora was largely unknown.MethodsThe mode of reproduction in 14 species belonging to seven families was investigated via flow cytometric seed screen. The sampling comprised 12 species typical for nival to subnival plant communities of the European Alps without any previous information on apomixis (Achillea atrata, Androsace alpina, Arabis caerulea, Erigeron uniflorus, Gnaphalium hoppeanum, Leucanthemopsis alpina, Oxyria digyna, Potentilla frigida, Ranunculus alpestris, R. glacialis, R. pygmaeus and Saxifraga bryoides), and two high-alpine species with apomixis reported from other geographical areas (Leontopodium alpinum and Potentilla crantzii).Key ResultsFlow cytometric data were clearly interpretable for all 46 population samples, confirming the utility of the method for broad screenings on non-model organisms. Formation of endosperm in all species of Asteraceae was documented. Ratios of endosperm : embryo showed pseudogamous apomixis for Potentilla crantzii (ratio approx. 3), but sexual reproduction for all other species (ratios approx. 1·5).ConclusionsThe occurrence of apomixis is not correlated to high altitudes, and cannot be readily explained by selective forces due to environmental conditions. The investigated species have probably other adaptations to high altitudes to maintain reproductive assurance via sexuality. We hypothesize that shifts to apomixis are rather connected to frequencies of polyploidization than to ecological conditions.

Flower longevity and duration of pistil receptivity in high mountain plants

Flowers of high mountain plants are generally long-lived to compensate for the low frequency of pollinator visits at higher altitudes. However, nothing is known about floral plasticity when pollinators are absent because of unsuitable weather conditions. We investigated both, actual longevity of naturally pollinated flowers and potential flower longevity, the capacity to prolong corolla life-time and pistil receptivity when pollination is retarded. Seven high mountain plant species from different elevations in the European Alps were tested: Gentianella germanica, Ranunculus alpestris and Saxifraga caesia are restricted to the alpine zone, Cerastium uniflorum, Ranunculus glacialis, Saxifraga bryoides and Saxifraga moschata occur from the higher alpine to the glacier zone. Flower longevity differed greatly among species, according to their taxonomic background and their gender sequence type. Actual flower longevity ranged from a few days ( G. germanica, C. uniflorum, R. alpestris) to more than two weeks (saxifrages, R. glacialis). Unpollinated flowers generally increased longevity, but the plasticity of single floral functions was quite different. Stigma receptivity could be maintained longest. Pollen adhesion was enabled during the full stigma life-time. High levels of pollen germination and pollen tube growth were still possible 29 days after anthesis (DAA) in R. glacialis, and 23 DAA in S. bryoides. In the remaining species this ability was gradually lost or, in C. uniflorum, abruptly ended around 10 DAA. The overall pistil receptivity primarily depended on the duration of ovule receptivity, which in most species ended between 16 and 20 DAA. Corolla life-time was significantly prolonged in S. bryoides, S. moschata and G. germanica. In the remainder of species mean corolla longevity did not differ from the actual longevity, and thus any prolongation of female functions over the corolla life-time would not contribute to reproductive success. The maximal potential longevities of individual flowers with fresh corolla and receptive pistils were 21 d in S. moschata followed by S. caesia (19 d), S. bryoides, R. glacialis (18 d) and G. germanica (14 d). R. alpestris (8 d) and C. uniflorum (8 d) exhibited the shortest potential overall flower longevities. Contrary to our expectations, flower longevity did not significantly differ between the alpine and the high-elevation species.

Dynamics of flower development and vegetative shoot growth in the high mountain plant Saxifraga bryoides L.

Saxifraga bryoides L. is an abundant species in the subnival and nival zone of the European mountains. First flowering occurs, at the earliest, 6 weeks after snowmelt. This is a remarkably long prefloration period in an environment with a short growing season. To gain more information about the developmental strategies of this species, the timing and the dynamics of flower bud formation and vegetative shoot growth were studied at sites with growing seasons of different lengths at two subnival locations (2650 and 2880 m a.s.l.) in the Tyrolean Alps. At an early, mid and late thawing site, individuals emerging from the winter snow were labelled. Reproductive and vegetative shoots were sampled at regular intervals throughout the growing season and analysed, using different microscopic techniques. Flower buds of S. bryoides develop in three cohorts. Provided the growing season is long enough, cohorts 1 and 2 come into flower, whereas cohort 3 buds remain primordial and continue to develop after winter. New flower primordia appear as day-length decreases from August on, which suggests a short-day requirement for floral initiation. At the end of the growing season, flower buds of different stages are present, but only primordial stages survive winter. Thus, flower buds of S. bryoides develop largely or even completely in the year of anthesis. Developmental dynamics were quite similar at the different sites. Time from flower initiation until anthesis took about 2 months, independently of whether flowers were formed within one or two seasons. All of the leaves on vegetative short-stem shoots turnover within a growing season. Leaves having passed winter continuously decline and are replaced by newly formed ones (21±3 at the mid-thawing site and 18±1 leaves at the short-season site). An individual leaf functions therefore, on average, about 12 months. In most years the seed crop of S. bryoides results mainly from the first cohort of flowers in an individual. In a changing climate with a prolonged growing season, the chance of two cohorts to develop mature seeds from flower cohorts 1 and 2 would increase.

Timing of Sexual Reproduction and Reproductive Success in the High‐Mountain Plant Saxifraga bryoides L.

SAXIFRAGA BRYOIDES L. is one of the plant species reaching the upper limits of distribution for flowering plants in the European Alps. Because of its abundance in the subnival and nival zones, we expected S. BRYOIDES to reproduce efficiently in the highly stochastic climate at higher altitudes. Investigations were carried out at two subnival sites (2650 m and 2880 m a.s.l.) in the Austrian Alps. We studied flowering phenology, dynamics of seed development, and reproductive success in the climatically different years from 2001 - 2004. For a nival plant species, S. BRYOIDES showed a particularly long prefloration period (6 - 9 weeks). From onset of anthesis until seed maturity took an individual flower another 6 - 7 weeks and all individuals at a site 9 - 10 weeks. The length of the prefloration period and seed histogenesis was temperature-dependent, whereas seed maturation seemed to be endogenously controlled. Only in the exceptionally long and warm growing season of 2003 did all fruits mature at a site. In the other years, the onset of winter conditions halted development in many fruits before maturity. The seed/ovule ratio of mature fruits was around 0.7 in all years. The relative reproductive success (RRS) ranged from zero to 0.7, depending on the site and year. In conclusion, S. BRYOIDES needs an unexpectedly long time to undergo reproductive development. Though fruit maturation is uncertain, the high S/O ratio of single intact fruits results in at least a small seed crop in most years. This seems to be sufficient to assure the spread and maintenance of S. BRYOIDES at higher altitudes. As a seed-risk strategist (Molau,1993), S. BRYOIDES would clearly benefit from a prolonged growing season, which might occur more often if climate warming continues.