Patterns in genetic diversity of Trifolium pallescens populations do not reflect chronosequence on alpine glacier forelands.

How does genetic diversity within populations of plants develop during primary succession on alpine glacier forelands? Theory predicts that pioneer populations are characterized by low genetic diversity due to founder effects and that genetic diversity increases within populations as they mature and recurrent gene flow occurs. However, few genetic studies have so far been carried out on plants on glacier forelands. In this study, we analysed the development of genetic diversity with time for populations of Trifolium pallescens along successional series (chronosequences) on three parallel glacier forelands in the European Alps, using neutral amplified fragment length polymorphism. No general trend in the development of genetic diversity was observed with increasing population age: even pioneer populations harboured substantial genetic diversity. Assignment tests showed that the latter consist of a genetic sub-sample from several source areas, and not just from other populations on the glacier forelands. We also detected some long distances-that is, inter-valley gene flow events. However, gene flow was not spatially unrestricted, as shown by a weak isolation by distance pattern within glacier valleys. The actual patterns of genetic diversity along the chronosequences are a result of the combination of factors, such as gene flow and growth rate, influenced by site- and species-specific attributes.

Assessment of climate change effects on mountain ecosystems through a cross-site analysis in the Alps and Apennines.

Mountain ecosystems are sensitive and reliable indicators of climate change. Long-term studies may be extremely useful in assessing the responses of high-elevation ecosystems to climate change and other anthropogenic drivers from a broad ecological perspective. Mountain research sites within the LTER (Long-Term Ecological Research) network are representative of various types of ecosystems and span a wide bioclimatic and elevational range. Here, we present a synthesis and a review of the main results from ecological studies in mountain ecosystems at 20 LTER sites in Italy, Switzerland and Austria covering in most cases more than two decades of observations. We analyzed a set of key climate parameters, such as temperature and snow cover duration, in relation to vascular plant species composition, plant traits, abundance patterns, pedoclimate, nutrient dynamics in soils and water, phenology and composition of freshwater biota. The overall results highlight the rapid response of mountain ecosystems to climate change, with site-specific characteristics and rates. As temperatures increased, vegetation cover in alpine and subalpine summits increased as well. Years with limited snow cover duration caused an increase in soil temperature and microbial biomass during the growing season. Effects on freshwater ecosystems were also observed, in terms of increases in solutes, decreases in nitrates and changes in plankton phenology and benthos communities. This work highlights the importance of comparing and integrating long-term ecological data collected in different ecosystems for a more comprehensive overview of the ecological effects of climate change. Nevertheless, there is a need for (i) adopting co-located monitoring site networks to improve our ability to obtain sound results from cross-site analysis, (ii) carrying out further studies, in particular short-term analyses with fine spatial and temporal resolutions to improve our understanding of responses to extreme events, and (iii) increasing comparability and standardizing protocols across networks to distinguish local patterns from global patterns.

Long-term population dynamics of two Carex curvula species in the Central Alps on native and alien soils.

The demography of two closely related alpine sedges, Carex curvula subsp. curvula and Carex curvula subsp. rosae (=C. curvula and C. rosae) has been investigated on their typical sites in the Central Alps. Both species proliferate vegetatively and develop dense tussocks but they show different dominance behaviours in their respective grasslands. It was hypothesized that this may be caused by different growth abilities. The main aim of the study was to compare the vegetative growth of the species under field conditions, under competition-free conditions and under changed soil conditions. An attempt was also made to clarify whether vegetative growth is density dependent. Permanent plots were established in the respective grasslands of the two species and the ramet density was counted over 3 years. Groups of 10 and of 30 ramets of each species were grown in pots with typical and with alien substrate and their growth was observed for 5 years at the field site. The grassland populations of both species were very stable and the overall ramet growth rate (λ) was close to 1.0. Within the pots, both species reached a high ramet number. Only the group of 30 ramets of C. curvula on alien soil could not recover from the transplantation shock. Within the pots, C. rosae showed a greater ramet turnover and a higher increase in ramets than C. curvula. On their native substrate, both species had a significantly higher ramet increase than on the alien substrate. Ramet growth was found to be density dependent for both species, the increases recorded for the groups of 10 being significantly greater than for the groups of 30. Although C. curvula produced fewer ramets than C. rosae, the aboveground dry weight of the former was significantly higher. This may be decisive for its greater competitive success in closed grasslands.

'Sax-sess'-- genetics of primary succession in a pioneer species on two parallel glacier forelands.

The primary succession on glacier forelands is characterized by a sequence of early and late successional species, but whether there is also a chronosequence at the intraspecific, genetic level is a matter of debate. Two opposing hypotheses differ in their prediction of genetic diversity in colonizing populations due to founder effects and postcolonization gene immigration. The development of genetic diversity in the pioneer Saxifraga aizoides was investigated along a successional gradient on two parallel glacier forelands, in order to test whether populations from older successional stages were less genetically diverse than populations from younger successional stages, and to locate the sources of the propagules that originally colonized new glacier foreland. Genetic diversity was determined with amplified fragment length polymorphisms, and potential sources of colonizing propagules were assessed via assignment tests. Our results indicate considerable postcolonization gene flow among populations on glacier forelands, since population differentiation was low and genetic diversity within populations was significantly higher. Molecular diversity and differentiation of populations did not develop linearly. Dispersal events within the glacier foreland, from the adjacent valley slopes, and from parallel glacier valleys were identified. In summary, it seems that the colonization of glacier forelands in the European Alps is highly dynamic and stochastic.

Genetic introgression as a potential to widen a species' niche: insights from alpine Carex curvula.

Understanding what causes the decreasing abundance of species at the margins of their distributions along environmental gradients has drawn considerable interest, especially because of the recent need to predict shifts in species distribution patterns in response to climatic changes. Here, we address the ecological range limit problem by focusing on the sedge, Carex curvula, a dominant plant of high-elevation grasslands in Europe, for which two ecologically differentiated but crosscompatible taxa have been described in the Alps. Our study heuristically combines an extensive phytoecological survey of alpine plant communities to set the niche attributes of each taxon and a population genetic study to assess the multilocus genotypes of 177 individuals sampled in typical and marginal habitats. We found that ecological variation strongly correlates with genetic differentiation. Our data strongly suggest that ecologically marginal populations of each taxon are mainly composed of individuals with genotypes resulting from introgressive hybridization. Conversely, no hybrids were found in typical habitats, even though the two taxa were close enough to crossbreed. Thus, our results indicate that genotype integrity is maintained in optimal habitats, whereas introgressed individuals are favored in marginal habitats. We conclude that gene flow between closely related taxa might be an important, although underestimated, mechanism shaping species distribution along gradients.