Alpine Glacier Foreland Research Articles

Structural shifts in plant functional diversity during biogeomorphic succession: Moving beyond taxonomic investigations in an alpine glacier foreland

AbstractThe complex interrelation between plants and geomorphic processes is described in the concept of biogeomorphic succession. While ecological research on succession and community assembly has transitioned towards functional approaches, studies on functional diversity in biogeomorphic settings, particularly in glacier forelands, remain limited. In this study, we investigated abundance of vascular plant species and functional traits in an alpine glacier foreland using data from 199 plots. Our objective was to unravel the development of functional diversity during biogeomorphic succession. Specifically, the study determined whether structural shifts in functional diversity are associated with stability thresholds related to plant cover, geomorphic influence, and examined trait spectra for stages of biogeomorphic succession. Our findings revealed a nonlinear trajectory of functional diversity along the plant cover gradient, marked by two distinct structural shifts at 30% and 74% cover, corresponding to established stability thresholds. Along the gradient of geomorphic influence, we observed an increase in functional diversity until 54% of the plot area was affected, beyond which functional diversity declined below the initial level. The analysis of community‐weighted means of traits across four stages of biogeomorphic succession determined by plant cover and absence and presence of geomorphic influence revealed significant differences in trait values. In the transition to the biogeomorphic stage, associated with the identified initial structural shift, there is a shift from a prevalence of above‐ground adaptation and reproductive traits, such as leaf longevity, structure, growth form and mixed reproductive strategies, to an increased dominance of competitor species and traits related to below‐ground structures, including root type and structures, as well as vegetative reproduction. Our results contribute to understanding the relationship between vegetation succession and geomorphic influence by linking them to plant functional traits. This study advances beyond traditional taxonomic investigations by emphasizing functional approaches to biogeomorphic succession. Moreover, the functional trait data used in this study, easily downloadable from a public repository, can serve as a valuable template for future research in (bio)geomorphology, along with the employed methodologies.

Catchment-scale patterns of geomorphic activity and vegetation distribution in an alpine glacier foreland (Kaunertal Valley, Austria)

The interaction between geomorphological and ecological processes plays a significant role in determining landscape patterns in glacier forelands. However, the spatial organization of this biogeomorphic mosaic remains unclear due to limited catchment-scale data. To address this gap, we used a multi-proxy analysis to map potential geomorphic activity related to surface changes induced by sediment transport on drift-mantled slopes and a glaciofluvial plain. High-resolution vegetation data were used to generate a catchment-scale map delineating vegetation cover and stability thresholds. The two maps were integrated, and an exploratory regression analysis was conducted to investigate the influence of geomorphic activity on vegetation colonization. The multi-proxy analysis resulted in an accurate mapping of catchment-wide geomorphic activity, with a validation accuracy ranging from 75.3% through field mapping to 85.9% through plot sampling. Through vegetation cover mapping, we identified biogeomorphic stability thresholds, revealing a mosaic of vegetation distribution. Distinct colonization patterns emerged across different geomorphic process groups, influenced by process magnitude and the time since the last disturbance event. The exploratory regression analysis showed that vegetation distribution is significantly affected by geomorphic processes. Based on the overlay of results regarding geomorphic activity and vegetation distribution, we suggest an age-independent framework that indicates four potential situations of biogeomorphic succession.

Heterogeneous changes of soil microclimate in high mountains and glacier forelands

Landscapes nearby glaciers are disproportionally affected by climate change, but we lack detailed information on microclimate variations that can modulate the impacts of global warming on proglacial ecosystems and their biodiversity. Here, we use near-subsurface soil temperatures in 175 stations from polar, equatorial and alpine glacier forelands to generate high-resolution temperature reconstructions, assess spatial variability in microclimate change from 2001 to 2020, and estimate whether microclimate heterogeneity might buffer the severity of warming trends. Temporal changes in microclimate are tightly linked to broad-scale conditions, but the rate of local warming shows great spatial heterogeneity, with faster warming nearby glaciers and during the warm season, and an extension of the snow-free season. Still, most of the fine-scale spatial variability of microclimate is one-to-ten times larger than the temporal change experienced during the past 20 years, indicating the potential for microclimate to buffer climate change, possibly allowing organisms to withstand, at least temporarily, the effects of warming.

Glacial biodiversity of the southernmost glaciers of the European Alps (Clapier and Peirabroc, Italy)

We applied a multi-taxa approach integrating the co-occurrence of plants, ground beetles, spiders and springtails with soil parameters (temperatures and chemical characteristics) in order to describe the primary succession along two glacier forelands in the Maritime Alps (Italy), a hotspot of Mediterranean biodiversity. We compared these successions to those from Central Alps: Maritime glacier forelands markedly differ for their higher values of species richness and species turnover. Contrary to our expectation, Maritime glacier forelands follow a ‘replacement change model’, like continental succession of Inner Alps and differently from other peripheral successions. We propose that the temperatures along these Mediterranean glacier forelands are warmer than those along other Alpine glacier forelands, which promote the faster species turnover. Furthermore, we found that early and mid successional stages of the investigated glaciers are richer in cold-adapted and endemic species than the later ones: we confirmed that the ‘replacement change’ model disadvantages pioneer, cold-adapted species. Given the overall correspondence among cold-adapted and endemic species, the most threatened in this climate phase, our results raise new concerns about the extinction risk of these species. We also describe supraglacial habitat of Maritime glaciers demonstrating that supraglacial debris represents an environment decoupled from the regional climate and may have an important role as refugium for coldadapted and hygrophilous plant and animal species, whose survival can be threatened by climate change and by a rapid ecological succession in the adjacent forelands.

Catchment-Scale Stability and Disturbance in Biogeomorphic Succession in an Alpine Glacier Foreland (Kaunertal Valley, Austria)

Catchment-Scale Stability and Disturbance in Biogeomorphic Succession in an Alpine Glacier Foreland (Kaunertal Valley, Austria)

Vertical root distribution and biomass allocation along proglacial chronosequences in Central Switzerland

ABSTRACT Investigating changes in belowground functional plant traits is an important step toward a better understanding of vegetation dynamics during primary succession. However, in alpine glacier forelands, we still lack an accurate assessment of plant rooting patterns. In this study, we established two proglacial chronosequences with contrasting bedrocks to investigate changes in rooting patterns and biomass allocation with terrain age. We extracted soil cores up to 1 m depth and measured root traits every 10 cm of each drilled core. Furthermore, we sampled aboveground biomass determining the contributions of functional groups to total aboveground biomass. We found that root traits associated with the root economics spectrum varied significantly along the chronosequences. Vertical root distribution coefficients revealed that early successional communities had more evenly distributed root systems compared to late successional communities. Biomass allocation showed diverging patterns. We found evidence for both the isometric allocation and optimal partitioning hypotheses. In addition, we observed a significant correlation between rooting parameters and plant community composition, suggesting that the dominance of distinct plant functional groups was one important factor explaining the observed rooting patterns. Our results shed light on the often neglected belowground compartments during plant succession and contribute to a better understanding of hillslope functioning.

Common Patterns and Diverging Trajectories in Primary Succession of Plants in Eastern Alpine Glacier Forelands

This paper deals with the vegetation development in four glacier forelands, aligned along a distance of 250 km from West to East in the siliceous Eastern Central Alps. The study employs a chronosequence approach, which assumes a temporal sequence in vegetation development by spatially different sites regarding time since deglaciation. The chronosequences cover the area between Little Ice Age (LIA) maximum glacier extent around 1850, and the current glacier terminus. Despite some shortcomings, chronosequences allow the identification of general patterns of primary succession of plants as a function of site age and local environmental conditions, e.g., changes in species richness, ground cover, plant functional traits, and community structure. While there is no shortage of chronosequence studies in glacier forelands of the Alps, a straightforward comparison aimed at the deduction of general successional trajectories is tricky, due to different procedures of vegetation sampling and data analyses. The comparative examination by a standardized sampling and analyzing protocol of four glacier forelands in the Eastern Central Alps presented here proves the existence of several common patterns in primary succession, but also diverging successional trajectories from West to East. While the pioneer stage in all glacier forelands is similar both floristically and structurally, from the early successional stage onwards, differences increase, leading to different phases in the late successional stage, which is shrub dominated throughout in the westernmost study site, herb–grass–dwarfshrub dominated throughout in the easternmost study site, and divided into an earlier herb–grass–dwarfshrub phase and a later shrub phase in the two study sites in between.

Vegetation dynamics in Alpine glacier forelands tackled from space

Monitoring of plant succession in glacier forelands has so far been restricted to field sampling. In this study, in situ vegetation sampling along a chronosequence between Little Ice Age (LIA) maximum extent and the recent glacier terminus at Jamtalferner in the Austrian Alps is compared to time series of the Normalized Difference Vegetation Index (NDVI) calculated from 13 Landsat scenes (1985–2016). The glacier terminus positions at 16 dates between the LIA maximum and 2015 were analysed from historical maps, orthophotos and LiDAR images. We sampled plots of different ages since deglaciation, from very recent to approx. 150 years: after 100 years, roughly 80% of the ground is covered by plants and ground cover does not increase significantly thereafter. The number of species increases from 10–20 species on young sites to 40–50 species after 100 years. The NDVI increases with the time of exposure from a mean of 0.11 for 1985–1991 to 0.20 in 2009 and 0.27 in 2016. As the increase in ground cover is clearly reproduced by the NDVI (R² ground cover/NDVI 0.84) – even for sparsely vegetated areas –, we see a great potential of satellite-borne NDVI to perform regional characterizations of glacier forelands for hydrological, ecological and hazard management-related applications.

Shift from facilitative to neutral interactions by the cushion plant Silene acaulis along a primary succession gradient

AbstractBackgroundThe stress‐gradient hypothesis predicts a shift from facilitative to competitive plant interactions with decreasing abiotic stress. This has been supported by studies along elevation and temperature gradients, but also challenged by the hypothesis of a facilitation collapse at extremely harsh sites. Although facilitation is known to be important in primary succession, few studies have examined these hypotheses along primary succession gradients.AimTo examine whether there is a relationship between the presence of the circumpolar cushion plant Silene acaulis and other species, and if so, whether there is a shift between positive and negative interactions along a primary succession gradient in a glacier foreland.LocationFinse, southern Norway.MethodsWe examined the performance of the common alpine forb Bistorta vivipara, species richness of vascular plants, bryophytes and lichens, and the number of seedlings and fertile vascular plants in S. acaulis cushions, and control plots without S. acaulis, along a succession gradient with increasing distance from a glacier front, and thus decreasing abiotic stress. To examine if S. acaulis cushions modify the abiotic environment, we recorded soil temperature, moisture, organic content and pH in cushions and control plots.ResultsBistorta vivipara performed better, as shown by bigger leaves in S. acaulis cushions compared to control plots in the harshest part of the gradient close to the glacier. There were few differences in B. vivipara performance between cushion and control plots in the more benign environment further away from the glacier. This suggests a shift from facilitative to mainly neutral interactions by S. acaulis on the performance of B. vivipara with decreasing abiotic stress. A trend, although not significant, of higher vascular species richness and fertility inside S. acaulis cushions along the whole gradient, suggests that S. acaulis also facilitates community‐level species richness. The causal mechanism of this facilitation is likely that the cushions buffer extreme temperatures.ConclusionsOur results support the stress‐gradient hypothesis for the relationship between the cushion plant S. acaulis and the performance of a single species along a primary succession gradient in a glacier foreland. S. acaulis also tended to increase vascular plant species richness and fertility regardless of stress level along the gradient, suggesting facilitation at the community level. We found no collapse of facilitation at the most stressful end of the gradient in this alpine glacier foreland.

Assembly patterns of soil-dwelling lichens after glacier retreat in the European Alps.

AimTo assess the spatial‐temporal dynamics of primary succession following deglaciation in soil‐dwelling lichen communities.LocationEuropean Alps (Austria, Switzerland and Italy).MethodsFive glacier forelands subjected to relevant glacier retreat during the last century were investigated. In each glacier foreland, three successional stages were selected at increasing distance from the glacier, corresponding to a gradient of time since deglaciation between 25 and 160 years. In each successional stage, soil‐dwelling lichens were surveyed within five 1 × 1 m plots. In addition to a classical ecological framework, based on species richness and composition, we applied a functional approach to better elucidate community assembly mechanisms.ResultsA positive relationship was found between species richness and time since deglaciation indicating that richer lichen communities can be found at increasing terrain ageing. This pattern was associated with compositional shifts, suggesting that different community assemblages can be found along the successional stages. The analysis of β‐diversity revealed a significant nested pattern of species assemblages along the gradient (i.e. earlier successional stages hosted a subset of the species already established in older successional stages), while the turnover component was less relevant. Considering functional groups, we found contrasting patterns in relation to time since deglaciation: the incidence of species with a cyanobacterial photobiont and those reproducing by spores decreased, while that of species reproducing by vegetative propagules increased.Main conclusionsThis study reveals that community assembly patterns of soil‐dwelling lichens in alpine glacier forelands are ruled by mechanisms of directional species accumulation and trait selection that involve a trade‐off between different functional strategies. Functional traits that reflect the dispersal and adaptation capability of the species underpin the colonization success of soil‐dwelling lichens in glacier forelands.

Patterns of spatial autocorrelation in the distribution and diversity of carabid beetles and spiders along Alpine glacier forelands

Spatial autocorrelation is a common feature of ecological data and can be found in the distribution pattern of many species or in the diversity of many species assemblages. The presence of spatial autocorrelation in species distribution along primary successions on recently deglaciated terrains has been largely overlooked until now, despite its potential consequences for comparisons between glacier forelands. Here, we investigated the occurrence of spatial autocorrelation at different spatial scales in the distribution and diversity of ground beetles and spiders along glacier forelands. We found positive spatial autocorrelation in patterns of occurrence of most species and in patterns of species assemblages diversity at spatial scales representing single (< 3 km) or adjacent valleys (< 7 km), whereas at a larger spatial scales (0–30 km) such positive spatial autocorrelation disappeared. We concluded that spatial autocorrelation could be a key issue for studies investigating distribution of arthropods, including in glacier forelands, in order to perform more robust analysis and to avoid misinterpretation of species distribution in relation to the glacier retreat and other environmental characteristics.

Temporal patterns of prokaryotic abundance, community structure and microbial activity in glacier foreland soils.

To reveal temporal variability of archaeal and bacterial abundance, community structure, as well as microbial biomass and activity, soils of different ages (young, intermediate, mature) were sampled along a glacier foreland in the Austrian Central Alps, at the beginning (summer) and at the end (autumn) of the plant growing season. No significant changes of 16S rRNA gene copy numbers of bacteria or archaea occurred in the young, recently de-glaciated soil. However, in intermediate and mature soils, bacteria were more abundant in autumn than in summer, whereas archaea decreased at the end of the growing season. Bacterial and archaeal community structures were differentially affected by the sampling date. Both soil microbial biomass and microbial activities in intermediate and mature soils significantly increased in autumn. On the contrary, the overall abiotic parameters did not undergo changes during summer. Bacterial communities at young, intermediate, and mature sites formed different clusters, thus reflecting changes according to soil age. Archaeal communities, on the other hand, were mainly influenced by the growing season, at least in young and intermediate soils. Our results indicate that temporal variations of microbial activities, biomass, and abundance in alpine glacier foreland soils distinctly increased along with the age of the soils and highlight the importance of sampling date for ecological studies.

Application of the mean individual biomass of ground beetles (Coleoptera: Carabidae) to assess the assemblage successions along areas of recent glacier retreats

Glacial retreat creates new habitat which is colonized by plants and animals during the process of primary succession. Tra­ ditionally, invertebrate succession along the glacier forelands has been spatially and temporally described by the following predictors: species richness, diversity/similarity indices and species traits. In this paper another approach is used and applied to carabid beetle assemblages in two alpine glacier forelands. It utilizes mean individual biomass (MIB), which is considered by several authors to be a good indicator of habitat successional stages. MIB increased with time since deglaciation and this is related to species turnover along the glacier forelands. Specifically, MIB is able to separate early from late successional stages. The former are characterized by carabid assemblages with low MIB values and species typical of pioneer habitats, while the latter are characterized by carabid assemblages with high MIB values and species typical of stable and mature habitats. Therefore, MIB seems to be a sensitive indicator of carabid beetle successional stages also along the glacier forelands.

Molecular identification of adult and juvenile linyphiid and theridiid spiders in Alpine glacier foreland communities.

In glacier forelands spiders constitute a large proportion of the invertebrate community. Therefore, it is important to be able to determine the species that can be found in these areas. Linyphiid and theridiid spider identification is currently not possible in juvenile specimens using traditional morphological based methods, however, a large proportion of the population in these areas are usually juveniles. Molecular methods permit identification of species at different life stages, making juvenile identification possible. In this study we tested a molecular tool to identify the 10 most common species of Linyphiidae and Theridiidae found in three glacier foreland communities of the Austrian Alps. Two multiplex PCR systems were developed and over 90% of the 753 field-collected spiders were identified successfully. The species targeted were found to be common in all three valleys during the summer of 2010. A comparison between the molecular and morphological data showed that although there was a slight difference in the results, the overall outcome was the same independently of the identification method used. We believe the quick and reliable identification of the spiders via the multiplex PCR assays developed here will aid the study of these families in Alpine habitats.

Intraguild predation in pioneer predator communities of alpine glacier forelands

Pioneer communities establishing themselves in the barren terrain in front of glacier forelands consist principally of predator species such as carabid beetles and lycosid spiders. The fact that so many different predators can co-inhabit an area with no apparent primary production was initially explained by allochthonous material deposited in these forelands. However, whether these populations can be sustained on allochthonous material alone is questionable and recent studies point towards this assumption to be flawed. Intraguild predation (IGP) might play an important role in these pioneer predator assemblages, especially in the very early successional stages where other prey is scarce. Here, we investigated IGP between the main predator species and their consumption of Collembola, an important autochthonous alternative prey, within a glacier foreland in the Ötztal (Austrian Alps). Multiplex PCR and stable isotope analysis were used to characterize the trophic niches in an early and late pioneer stage over 2 years. Results showed that intraguild prey was consumed by all invertebrate predators, particularly the larger carabid species. Contrary to our initial hypothesis, the DNA detection frequency of IGP prey was not significantly higher in early than in late pioneer stage, which was corroborated by the stable isotope analysis. Collembola were the most frequently detected prey in all of the predators, and the overall prey DNA detection patterns were consistent between years. Our findings show that IGP appears as a constant in these pioneer predator communities and that it remains unaffected by successional changes.

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

AbstractQuestionsHow 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?LocationRotmoos glacier foreland, Austrian Central Alps (Obergurgl, Tyrol, Austria).MethodsSeeds 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 (LT 50, i.e. 50% of samples being lethally frost damaged).ResultsDry seeds had the highest frost resistance (LT 50: −19 °C), followed by wet seeds after imbibition (LT 50: −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 (LT 50: −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.ConclusionsThe low frost resistance during and after germination may not suffice to survive summer frosts and may at least in certain years explain the high seedling mortality rates recognized in the glacier foreland.

Primary Succession of Surface Active Beetles and Spiders in an Alpine Glacier Foreland, Central South Norway

Spiders and beetles were pitfall-trapped in the foreland of the receding Hardangerjøkulen glacier in central south Norway. At each of six sampling sites, ages 3 to 205 years, twenty traps covered the local variation in moisture and plant communities. Thirty-three spider species and forty beetle species were collected. The species composition was correlated to time since glaciation and vegetation cover. A characteristic pioneer community of spiders and mainly predatory beetles had several open-ground species, and some species or genera were common to forelands in Svalbard or the Alps. While the number of spider species increased relatively constant with age, the number of beetle species seemed to level off after about 80 years. Half of the beetle species were Staphylinidae, and contrary to Carabidae, most of these were rather late colonizers. Most herbivore beetles colonized after more than 40 years, but the moss-eating Byrrhidae species Simplocaria metallica and also certain Chironomidae larvae developed in pioneer moss colonies after 4 years. The large Collembola Bourletiella hortensis, a potential prey, fed on in-blown moss fragments after 3 years. In the present foreland, chlorophyll-based food chains may start very early. Two pioneer Amara species (Carabidae) could probably feed partly on seeds, either in-blown or produced by scattered pioneer grasses.

Can successional species groups be discriminated based on their life history traits? A study from a glacier foreland in the Central Alps

Background: Retreating glaciers expose mineral substratum for colonisation and, as such, offer unique opportunities to study primary succession processes and life history traits of the colonisers. Aims: Results of published and unpublished case studies in a central Alpine glacier foreland were used to clarify if the species groups involved in primary succession could be discriminated by their life history traits. Methods: The case studies were carried out in the glacier foreland of the Rotmoosferner, Obergurgl site (Ötztal, Tyrol, Austria) using standard methods for life history trait determination and population analyses. Results: Pioneer species could be discriminated by their low seed masses and their high seed numbers from all other successional stage species. On the moraine exposed in 1971, seed rain and seed bank composition was consistent with that of the standing vegetation. Divergent dynamics were detected on the moraines of 1923 and of 1858. The highest seedling mortality rates were recorded for the pioneer Saxifraga aizoides (86–94%); the other species studied showed survival rates of 40–76%. Conclusions: Life history traits such as seed mass, number, dispersal, persistence, and seedling recruitment govern the colonisation process and the progress of plant succession. However, seed availability appears to be the bottleneck. Succession success therefore highly depends on input of diaspores in the form of plant fragment from adjacent slopes.

Initial Colonization of New Terrain in an Alpine Glacier Foreland by Carabid Beetles (Carabidae, Coleoptera)

The melting of glaciers in the European Alps has exposed new terrain in the last decades. Ground beetles (Carabidae, Coleoptera) are among the first colonizers of this new alpine land. Since 1999, we have studied the ground beetle assemblages of the recently deglaciated areas in the Hornkees glacier foreland (Zillertal Alps, Austria). Data were collected in July 1999, 2001, 2007, and 2009. Two species of Carabidae, Nebria jockischii and Nebria germari, were dominant in the areas immediately below the glacier in all sampling periods. The occurrence of Nebria jockischii larvae very close to the edge of the glacier strongly suggests that reproduction occurred in the area which had been ice-free for only one year. Comparison of the carabid beetle assemblages from the recently deglaciated areas with those from neighboring and longer exposed areas indicates that the first colonizers continuously expanded their range following the path of the glacial retreat. We hypothesize that Nebria jockischii populated the new alpine terrain by initial migration from the banks of the glacial stream, whereas Nebria germari probably colonized the recently deglaciated area by migration from the lateral moraines. These carabid beetles represent the first predatory insects in the initial phase of colonization of the Hornkees foreland.

Distribution of High Bacterial Taxa Across the Chronosequence of Two Alpine Glacier Forelands

Little is known about the changes in abundance of microbial taxa in relation to the chronosequence of receding glaciers. This study investigated how the abundances of ten bacterial phyla or classes varied along successional gradients in two glaciers, Ödenwinkelkees and Rotmoosferner, in the central Alps. Quantitative PCR was used to estimate the abundance of the different bacterial taxa in extended glacier chronosequences, including 10- to 160-year-old successional stages, the surface of the glacier, and a fully established soil. Actinobacteria (15-30%) was the dominant group within the chronosequences. Several taxa showed significant differences in the number of taxa-specific 16S rRNA gene copies per nanogram of DNA and/or in the ratio of taxa-specific to the total bacterial 16S rRNA gene copies (i.e., the relative abundance of the different taxa within the bacterial community) between the established soils or the glacier surface and the 10- to 160-year-old successional stages. A significantly higher proportion of Βetaproteobacteria (20%) was observed on the surface of both glaciers. However, no differences were observed between the 10- to 160-year-old successional stages in the number of taxa-specific 16S rRNA gene copies per nanogram of DNA or in the ratio of taxa-specific to the total bacterial 16S rRNA gene copies for the different taxa. Nevertheless, when the relative abundance data from all the studied taxa were combined and analyzed altogether, most of the sites could be distinguished from one other. This indicates that the overall composition of the bacterial community was more affected than the abundance of the targeted taxa by changes in environmental conditions along the chronosequences.