Alpine Lichen Heath Research Articles

Drivers of soil temperature variation in alpine lichen heaths and shrub vegetation during the summer

ABSTRACT Lichen heaths are decreasing in abundance in alpine and Arctic areas because of an increased competition with shrubs. This shift in vegetation might have important consequences for the soil temperature. The aim of this study is to find the drivers of the variation in soil temperature below lichen heaths and shrubs. Moreover, we want to gain more insight in the variability of the soil temperature below lichen heaths. We measured the soil temperature in thirty lichen plots and fifteen shrub plots in an alpine area in southern Norway during July and August 2019. We applied several treatments to study the drivers behind the variation in soil temperature between lichen heaths and shrub vegetation. We found that the average soil temperature was 1.45°C higher below lichen heaths than below shrub vegetation. Moreover, we measured a difference in soil temperature of 1.66°C between north- and south-facing lichen heaths, which contributes to the small-scale spatial variability in soil temperature below lichen heaths. Based on our experiments, we conclude that the buffering capacity of the litter layer below shrubs and shading of the soil by the shrub canopy lead to a lower soil temperature below shrubs compared to lichen heaths during the summer.

Alpine plant communities differ in their seed germination requirements along a snowmelt gradient in the Caucasus

Mesoscale heterogeneity of alpine landscapes generates snowmelt gradients resulting in a distinct vegetation zonation from almost snow-free fellfields to long-lasting snowbeds. Although the vegetative trait variation along such gradients has been intensively studied, little is known about whether and how seed germination is adapted to the variable snowpack duration. Here, we compare the seed germination niches of 18 characteristic plant species occurring in four distinct plant communities (alpine lichen heath—Festuca varia grassland—Geranium-Hedysarum meadow—snowbed) along an alpine snowmelt gradient in the North Caucasus (Russia). In a fully factorial experiment, we tested seed germination responses to temperature (10/2, 14/6, 18/10, 22/14, 26/18 and 30/22 °C) and water potential (0, − 0.2, − 0.4, − 0.6 and − 0.8 MPa) gradients, reflecting the full spectrum of seedling establishment conditions in the study system. Community-specific germination niches were identified by generalised mixed model with Bayesian estimation. Our study revealed that the species from the four focal communities significantly differed in their temperature requirements for germination, whereas soil moisture was found to be a limiting factor for seedling establishment along the entire gradient. The snowbed germination was constrained to comparatively high temperatures above 10 °C, whereas seed germination in alpine lichen heaths occurred under a wide range of experimental conditions. The seed germination patterns of species from Festuca varia grasslands and Geranium-Hedysarum meadows reflected their intermediate position along the snowmelt gradient. We conclude that seed germination niche is affected by the environmental filtering along the snowmelt gradient thus departing from the general alpine germination syndrome.

Shrub encroachment interacts with environmental variation to reduce the albedo of alpine lichen heaths: an experimental study

Global warming causes the replacement of lichens by shrubs in alpine and Arctic ecosystems. Since shrubs have lower albedo than lichens, this shrub encroachment can lead to a positive climatic feedback, resulting in higher temperatures in the surroundings. Therefore, gaining knowledge on the surface albedo of shrubs and lichens is important. Environmental factors also influence the surface albedo, but have often been neglected, potentially leading to biased results. In an experimental setup, we analyzed albedos of the lichen species Cladonia stellaris, Flavocetraria nivalis and Cetraria islandica, and how albedo changes with a stepwise replacement by the dwarf shrub Empetrum nigrum. Albedo was measured with radiometers in a paired set up. By setting certain environmental variables and species composition (monocultures) to be constant, we quantified the impact of environmental factors such as cloud cover, aspect and zenith angle on the surface albedo of two lichen species. Surface albedo (mean values ± SD) differed between C. islandica (0.155 ± 0.015), C. stellaris (0.364 ± 0.019), F. nivalis (0.350 ± 0.022) and E. nigrum (0.154 ± 0.016), and an increase in shrub cover at the expense of lichen cover led to a corresponding decrease in albedo. A 0.6 reduction in clearness index (more clouds) produced a 0.054 albedo decline. On the north‐facing aspect, albedo was 0.023 lower than on the south‐facing aspect. Albedo increased by 0.032 with an increase in zenith angle of 15°. Albedo variations caused by these studied environmental factors significantly affect the radiation budget of alpine and Arctic vegetation. We therefore stress the importance of considering environmental factors when surface albedos are estimated. Likewise, our species‐specific measurements can be a basis for further studies of the impact of climate change on alpine and Arctic vegetation and species‐related feedback mechanisms.

Long-term natural dynamics of an alpine lichen heath in the Teberda State Biosphere Reserve, northwestern Caucasus

The alpine vegetation of Europe and the Caucasus has experienced substantial changes due to climate warming and reduced grazing. Exposed ridge communities, such as alpine heaths, are presumed to be less vulnerable to such processes. Herein, we analyze long-term dynamics (over 37 years) of an alpine lichen heath in the Teberda State Biosphere Reserve, Karachaevo-Cherkessian Republic, Russia. We counted the shoots of all vascular plants present on permanent plots. Autocorrelated linear regressions, a non-metric multidimensional scaling ordination, and a fourth-corner analysis were applied to characterize the relationships between shoot numbers, climate variables (temperatures and precipitation), functional traits, and species strategies. Nine species, including dominant Festuca ovina L. and Antennaria dioica (L.) Gaertn., increased their abundances, whereas Carex spp. were observed to decrease. The overall dynamics were mainly driven by increasing mean temperatures during the growing season (July and August). None of the changes observed in the selected traits were correlated with increasing or decreasing numbers of species; however, some traits (plant height, specific root length, specific leaf area, and leaf carbon content) were potentially associated with climate variables. The observed dynamics suggested an overall increase in the abundance of herbaceous plants. Generally, our results support “greening” effects in tundra and alpine biomes.

Which Plant Strategies Are Related to Dominanсе in Alpine Communities?

The numerical estimation of ecological strategies (according to the classification of J. Grime) can contribute to plant dominance iunder certain environmental conditions. Alpine plants are traditionally considered as stress-tolerators (S) that are adapted to low temperatures and poor soils. We assessed the hypothesis that the C strategy is more expressed in relatively productive alpine meadows, whereas the R strategy is more important in communities with a short growing season and a heavy snow cover. The structure of the aboveground biomass (ratio between the species) and the parameters of leaves of all species of vascular plants were studied in four communities (alpine lichen heath, ALH; Festuca varia grassland, FVG; Geranium–Hedysarum meadow, GHM, and snowbed community, SBC). Based on these values, the contribution of the C, S, R components to the overall strategy of each species (according to S. Peirce et al.) and the weighted mean values ​​of the contribution of strategies for each community were calculated. The a priori hypothesis was confirmed, while the dominant species showed an increase in the role of stress tolerance on ALH and FVG and competitiveness on GHMs, which indicates the adaptability of these strategies in the studied communities. The contribution of C and S is in good agreement with the role of competition in the formation of various communities: it is maximal for GHMs, in which the C strategy is expressed, and minimal for FVGs, in which stress tolerance is most pronounced.

Functional Diversity of Alpine Plant Communities: A Case Study of Plant Height

The parameters of functional diversity are studied on the example of plant height for four alpine communities (alpine lichen heaths, Festuca varia grasslands Hedysarum-Geranium meadows, and alpine snowbed communities) in Teberda Reserve of the northwestern Caucasus. The work tests the hypothesis that, when plants that use different volumes of air, a low volume of the environment is used more evenly than a high volume. It was assumed that the divergence in plant height is more pronounced in meadow communities, where the competitive relationship is stronger and the level of production is higher. For the four main types of phytocenoses (alpine heaths, Festuca varia grasslands Hedysarum-Geranium meadows, alpine snowbeds), data were obtained on the average height of the plant species forming them, the biomass of individual species, and the weighted average plant height in the communities. The indices of functional richness, evenness, and divergence were calculated based on these data. It is shown that taking into account the abundance of species in the community significantly affects the values of average height and indices of functional diversity. Contrary the a priori hypothesis, it was found that the value of evenness is significantly lower at a low air volume occupied by the community (alpine snowbeds) than for a high volume (Festuca varia grasslands). The functional divergence in alpine snowbed communities with low productivity is higher than that in the more productive meadow phytocenoses, e.g. Festuca varia grasslands. The aboveground production positively correlated with functional richness in the aggregate analysis of all data, but this correlation is not detected within communities. Functional evenness is not related to community production. An unimodal relationship between the index of functional-divergence and aboveground biomass was shown only for the Festuca varia grasslands (the maximal divergence is observed on plots with average biomass).

Disappointing Survival Forecast for a Local Population of Androsace albana in a Random Environment

A local population of Andropace albana, a short-lived perennial plant, has been monitored during 10 years on permanent plots laid down in an alpine lichen heath in 2009. We summarize the outcome of monitoring as a non-autonomous matrix model of stage-structured population dynamics. The model originates from a life cycle graph constructed earlier for the stages of ontogenesis and consists of 9 annual “projection” matrices that are calibrated in a unique way from the observation data. Five of the 9 matrices have their dominant eigenvalues greater than 1, i.e., give favorable forecasts for the local population survival, while the rest four have those values less than 1, i.e., give the negative forecasts. To make the resulting prediction, we apply an original concept of the pattern-geometric averaging of given nonnegative matrices and obtain the dominant eigenvalue, λ1(G9), of the average matrix G9 markedly less than 1, indicating the population decline in the long term. The traditional method to forecast the local population is to estimate λS, the stochastic growth rate of the population in a random environment formed by a random choice from the same 9 annual matrices. Assuming the choice to be independent and equiprobable, we obtain the negative result as well, yet with higher quantitative values of λS. We associate these higher values with the artificial assumption of equal choice probability when forming the random sequence of annual matrices, each of which is indirectly reflecting the habitat conditions that have influenced the growth and development of plants during the year prior the calibration moment. This motivates the task to construct a more adequate model for choosing annual matrices, in which the probability of choice would be related to the dynamics of the real habitat factors for a given local population.

Nitrogen Nutrition of Plants in an Alpine Lichen Heath under the Conditions of Soil Enrichment with Biogenic Elements

The results of a long-term experiment on the enrichment of the soil (Umbric Leptosol) of an alpine lichen heath with biogenic elements have shown that calcium addition and reduction of soil acidity do not change the nitrogen regime of the soil and patterns of nitrogen nutrition of plants. The increase in phosphorus availability contributes to the mobilization of soil organic nitrogen and shifts in the role of mycorrhiza in the nitrogen nutrition of plants, which determines their heavier isotopic nitrogen composition. At the same time, this reduces the intensity of nitrogen fixation by a nitrogen-fixing legume species, Oxytropis kubanensis. The increase in the availability of nitrogen leads to its active uptake by all studied plant species except Oxytropis kubanensis, as well as to the heavier isotopic nitrogen composition of plants. However, changes in the nitrogen status differ between plant species.

Surface albedo of alpine lichen heaths and shrub vegetation

ABSTRACT Lichen heaths are declining in abundance while shrubs are increasing their range in alpine and arctic areas due to climate change. This can have a large impact on the surface albedo of these areas. The aim of this article is to quantify the difference in albedo between lichen heaths and shrub-dominated vegetation and the variability within lichen heaths. Several environmental conditions that can influence the albedo measurements are considered. We measured the albedo of twenty lichen- and shrub-dominated plots on an alpine mountain area in southern Norway in the summer of 2018 with two radiometers using a paired plot design. With this design, we ensured similar weather conditions, aspects, and zenith angles between the paired lichen- and shrub-dominated plots. In addition, we collected patches of Cladonia stellaris and Flavocetraria nivalis to measure their albedo. The average difference in albedo between the lichen- and shrub-dominated plots is 0.124. The albedo of the lichen-dominated plots varies between 0.227 and 0.284, and that of the shrub-dominated plots varies between 0.115 and 0.148. This variation in albedo is explained by differences in aspect and vegetation composition. Further studies should focus on the consequences of this decrease in albedo for the microclimate in alpine and arctic areas.

Effects of herbaceous plant functional groups on the dynamics and structure of an alpine lichen heath: the results of a removal experiment

Alpine lichen heaths are polydominant, low-productive communities where lichens and, at some localities, the dwarf shrub Vaccinium vitis-idaea prevail. To analyse the role of herbaceous dominants in the structure of alpine heaths, we established a removal experiment. We tested (1) whether dominant graminoids and forbs differ in their effects on species richness, phytomass, and abundance of individual species; and (2) which remaining species are able to replace the removed dominants. Permanent plots were established in 1996 in the Teberda Reserve (the Northwestern Caucasus, Russia) at 2750 m a.s.l. with four treatments: (1) control, (2) forb removal, (3) graminoid removal, (4) graminoid and forb removal. Target species were clipped every year to avoid regrowth. We counted the shoot numbers of all vascular plants yearly from 1996 to 2009. In 2010, we harvested the aboveground phytomass. We used mixed-effects models to assess the responses of the remaining species and phytomass fractions to the removal. The biomass of the removed dominants was almost compensated for by the remaining species, but due to diffuse competition none of the subordinate species became a new dominant. Graminoid removal reduced community weighted mean leaf dry matter content and litter accumulation. Species number per plot completely recovered after graminoid, not forb removal. Shoot numbers of Antennaria dioica, Arenaria lychnidea, Campanula tridentata, Ranunculus oreophilus, and Trifolium polyphyllum showed positive responses to graminoid removal. Anemone speciosa shoot numbers decreased after graminoid removal. In alpine heath, graminoid dominants contribute more to community structure than forb dominants.

How Long Does a Short-Lived Perennial Live? A Modeling Approach

We have composed a “scale of ontogenesis,” i.e., the sequence of ontogenetic stages, and the life cycle graph (LCG) for Androsace albana, a monocarpic plant species that is considered a short-lived perennial, according to annual observations on permanent sample plots in an alpine lichen heath during the 2009–2016 period. There is only one reproduction event in the LCG, which eliminates any reproductive uncertainty from the data of “identified individuals” type so typical for the former projects with polycarpic species, while the monocarpic cycle excludes any returns from the generative stage to a non-flowering status. The LCG describes the ontogeny through five successive stages: seedlings, juvenile, immature, adult virginal, and generative plants, the generative plants perishing after they have flowered and produced seeds. We have constructed a matrix model of the stage-structured population that corresponds to the LCG; its calibration has reduced to calculating the vital rates directly from the data of one time step, i.e., from the censuses at two successive years of observation. Therefore, the nonautonomous model represents a set of 7 annual matrices, each giving a quantitative measure of how the local population is adapted to its environment as the dominant eigenvalue of the model matrix. Its variations have turned out quite significant from year to year, signifying either a growth or decline of the local population and, in general, its vulnerability to stress factors of the environment. Averaging the annual matrices geometrically over the whole observation period has revealed the tendency to decline and enabled us to extract certain age-specific traits (in years) from the stage-structured model using the technique of virtual absorbing Markov chains and their fundamental matrices. The mean life expectancy at various stages of the A. albana ontogeny has turned out maximal in the adult virginal plants, while the mean age at flowering equal to 13 years has exceeded the horizon of the observation time series, thus proving the technique to be efficient. The model indicates that A. albana plants spend most of their life spans as virginal adults, which characterizes the space holder strategy by Korner (2003), or the delayed-development strategy by Zhukova (1995). The model outcome gives another evidence that the A. albana population is endangered.

Aggregation may or may not eliminate reproductive uncertainty

In matrix population models, there cannot be any ‘reproductive uncertainty’ when the life cycle graph contains only one reproductive stage. Otherwise, it is logical to expect that aggregating all the reproductive stages into a single one would exclude the very basis of uncertainty. However, can the aggregation change principally the model characteristics such as the dominant eigenvalue λ1 of the projection matrix, thus signifying the aggregation failure? I demonstrate that it can with the data mined in a case study on the dynamics of a local stage-structured population of Eritrichium caucasicum, a short-lived perennial plant species inhabiting an alpine lichen heath. Frobenius Theorem for nonnegative matrices specifies the upper and lower bounds for λ1 via the row (or column) sums of matrix elements, and the lower bound, when it exceeds the maximal possible λ1 of the original, disaggregated matrix, does explain why the aggregation may fail to eliminate reproductive uncertainty.

A phylogenetic analysis of plant communities of the Teberda State Biosphere Reserve

A phylogenetic analysis of communities was conducted based on a comparison of the distances on a phylogenetic tree between species of the studied community and the distances in random samples taken from local flora. This made it possible to determine the extent to which community composition is formed by more closely related species or, on the contrary, it is more even and includes species that are less closely related to each other. The first case is usually interpreted as a result of a strong influence caused by abiotic factors, due to which species with a similar ecology (which are a priori more closely related) would remain. In the second case, biotic factors such as competition may lead to community formation among distant clades due to the divergence of their ecological niches. The goal of the study is to explore the phylogenetic structure in communities of the Northwestern Caucasus at two spatial scales, the scale of an area from 4 to 100 m2 and a smaller scale within a community. The list of local flora of the alpine belt has been composed with a database of relevees carried out in Teberda State Biosphere Reserve at altitudes exceeding 1800 m. It includes 585 species of flowering plants belonging to 57 families. Basal groups of flowering plants are not represented in the list. At the scale of communities, three classes, namely Thlaspietea rotundifolii-communities formed on pebbles, Calluno-Ulicetea alpine meadows, and Mulgedio-Aconitetea subalpine meadows, have not demonstrated significant distinction of phylogenetic structure from random model. At the intracommunity level, a larger proportion of closely related species (clustered community) was detected for alpine meadows. Communities developing on rocks (class Asplenietea trichomanis) and the alpine community (class Juncetea trifidi) are closely clustered. At the same time, even alpine lichen heaths proved to have a phylogenetic structure at a small scale. Alpine snowbeds (class Salicetea herbaceae) that develop under the conditions of winter snow accumulation were more even at both scales, i.e., they contained more diverse and distantly related plant species compared with random samples. Scheuchzerio-Caricetea fuscae fen communities in cold springs (Montio-Cardaminetea), sedge springs (Carici rupestris-Kobresietea bellardii), and communities in which shrubs predominate (Juniperus and Rhododendron elfins, class Loiseleurio-Vaccinietea) have been studied only at the larger scale and showed significant uniformity of species composition, i.e., they were phylogenetically more diverse as compared with random samples.

Rate of microsuccessions: Structure and floristic richness recovery after turf transplantation in alpine plant communities

Reciprocal transplantations of sod pieces have been conducted in alpine plant communities of the northwestern Caucasus. During 25 years, the changes in floristic richness and successional rates have been registered. Study objects were chosen to be. plant communities located along the toposequence from ridges to hollows with gradient of snow. cover thickness increase and vegetation period decrease, namely alpine lichen heath (ALH), Festuca varia grasslands (FVG), Geranium-Hedysarum meadows (GHM), and snow bed communities (SBC). The results of the study confirm the hypothesis about floristic richness of transplanted pieces to come closer to that of a background acceptor community. It is shown that during succession the variability reduces if sod pieces from different communities are transplanted into a common one. In particular, this is evident in case of SBC, where floristic richness of sod pieces transplanted from ALH and GHM has reduced noticeably. Also, it is evident from the results that the more different are donor and acceptor communities the higher is the rate of their changing. However, the assumption of higher succession rate in more productive communities has not been affirmed. On the opposite, communities with initially low productivity turned out to change faster than those with high productivity. It is found out that sod pieces transplanted to upper areas of the toposequence have had higher rate of alteration in comparison with those transplanted to lower areas. The reason behind this, as it may be suggested, is a longer growth season, which means a more prolonged period of high functional activity, and, accordingly, more time for the effects of competition, bringing seeds over, etc. In whole, the rate of succession decreases as the time from the moment of transplantation.increases, especially in communities with low productivity.

Determinants of 15N Natural Abundance in Leaves of Co-Occurring Plant Species and Types within an Alpine Lichen Heath in the Northern Caucasus

Several factors may have interactive effects on natural 15N abundance of plant species. Some of these effects could be associated with different plant functional types, including mycorrhizal association type. Due to its high taxonomic and functional diversity, the alpine heath community in the Caucasus is a suitable object for studying 15N natural abundance of plants in relation to different functional/mycorrhizal groups, contrasting with the limited numbers of plant groups or species considered in previous studies of individual communities. The N concentration and δ15N were determined in leaves of 25 plant species from 8 functional/mycorrhizal groups from an alpine lichen heath in the Teberda Reserve, Northern Caucasus, Russia. Functional groups were represented by ericoid mycorrhizal species (ERI), ectomycorrhizal species (ECT), arbuscular mycorrhizal forbs (AM—FORB), arbuscular mycorrhizal grasses (AM—GRA), arbuscular mycorrhizal nodulated legumes (FAB—N), non-mycorrhizal graminoids (sedges and rushes) (NOM—GRA), non-mycorrhizal hemiparasites (NOM—SP), and orchids (ORC). We can summarize our results in two rankings for leaf N concentration (FAB—N > ORC > AM—FORB, ECT > NOM—SP, ERI ≥⃒ NOM—GRA, AM—GRA) and leaf δ15N signature (ORC > NOM—GRA, FAB—N > ECT ≥⃒ ERI ≥⃒ AM—FORB, NOM—SP, AM—GRA) of alpine heath species. We conclude that, within the alpine lichen heath in the Northern Caucasus, the δ15N signature of plant foliage is a relevant indicator of plant functional groups with relatively high 15N content (ORC, FAB-N, NOM-GRA), while the absence of a significant difference between relatively 15N-depleted groups (AM, ERI, and ECT species) isn't clear and may result from both processes, as the increased N isotope fractionation by arbuscular mycorrhizal fungi as the decreased role of ecto- and ericoid mycorrhizal fungi in the flux of N.

Long-term vegetation dynamic in the Northwestern Caucasus: which communities are more affected by upward shifts of plant species?

We studied long-term (25–31 years) dynamics of alpine communities at the Teberda Reserve, NW Caucasus, Russia, to test the following hypotheses: (1) lower altitude species increase and high altitude species decrease their abundance as a consequence of climate warming; (2) such changes in abundance are more significant in communities with short growth season (due to persistent snow cover) compared to exposed communities; (3) species with similar changes in abundance have similar functional traits. Four alpine communities with different positions in relief were considered in order of winter snow cover: alpine lichen heaths (ALH), Festuca varia grasslands (FVG), Geranium-Hedysarum meadows (GHM), and snowbed communities (SBC).

Integral assessment of the regrow capacity of alpine plants

A method for the comparative assessment of the regrow capacity (the ability of plants to the after-mowing recovery of above-ground biomass) of herbs in natural plant communities has been proposed. The method is based on the combined use of biomass parameters and the data on the number of shoots for the cases of the single and double removal of the biomass. The regrow capacity of 42 alpine species from four different plant communities has been assessed. The average weighted regrow capacity score decreases in the following series: Geranium-Hedysarum meadows > alpine lichen heaths ≥ alpine snow-bed communities ≥ Festuca varia meadows.

Experimental comparison of competition and facilitation in alpine communities varying in productivity

AbstractQuestion. Competitive and facilitative interactions among plant species in different abiotic environments potentially link productivity, vegetation structure, species composition and functional diversity. We investigated these interactions among four alpine communities along an environmental productivity gradient in a generally harsh climate. We hypothesised that the importance of competition would be higher in more productive sites.Location. Mt. M. Khatipara (43°27′N, 41°41′E, altitude 2750 m), NW Caucasus, Russia. Communities ranged from low‐productivity alpine lichen heath (ALH) and snowbed communities (SBC), to intermediate productivity Festuca grassland (FVG), and high‐productivity Geranium‐Hedysarum meadow (GHM).Methods. We quantified the relative influence of competition and facilitation on community structure by expressing biomass of target species within each natural community proportionally to biomass of the species in a “null community” with experimental release from interspecific competition by removing all other species (for 6 years). An overall index of change in community composition due to interspecific interactions was calculated as the sum of absolute or proportional differences of the component species.Results. Species responses to neighbour removal ranged from positive to neutral. There was no evidence of facilitation among the selected dominant species. As expected, competition was generally most important in the most productive alpine community (GHM). The intermediate position for low‐productivity communities of stressful environments (ALH, SBC) and the last position of intermediately productive FVG were unexpected.Conclusions. Our results appear to support the Fretwell‐Oksanen hypothesis in that competition in communities of intermediate productivity was less intense than in low‐ or high‐productive communities. However, the zero net effect of competition and facilitation in FVG might be the result of abiotic stress due to strong sun exposure and high soil temperatures after neighbour removal. Thus, non‐linear relationships between soil fertility, productivity and different abiotic stresses may also determine the balance between competition and facilitation.

Experimental Investigation of Fertilization and Irrigation Effects on an Alpine Heath, Northwestern Caucasus, Russia

We investigated the response of an alpine lichen heath plant community to an increase in soil nutrient and water availability. A 5-yr experiment—including additions of calcium, phosphorus, nitrogen, and nitrogen + phosphorus as well as irrigation—was conducted in northwestern Caucasus, Russia, at 2800 m above sea level. Number of plants and generative shoots per species were counted annually. The plant-community composition started to change during the second year of treatments. Plant density and flowering of the community is co-limited by nitrogen and phosphorus. Irrigation and calcium additions caused minimal changes. The total number of forb plants per square meter was not influenced by treatments, whereas the total number of graminoid plants slightly increased in response to P treatment and strongly increased in response to N + P treatment. Forbs responded to N and N + P treatments by an increase in the number of generative shoots. Individual species differed in their response to treatments. Only clonal species responded to experimental treatments, except for one annual nonclonal species, which increased its abundance in response to irrigation. Biodiversity estimated by the Shannon-Wiener index decreased under N + P treatment. Species number was not affected by any of the treatments.

Nitrogen dynamics in alpine ecosystems of the northern Caucasus

Net N mineralization, nitrification, microbial biomass N and 15N natural abundance were studied in a toposequence of representative soils and plant communities in the alpine zone of the northern Caucasus. The toposequence was represented by (1) low-productive alpine lichen heath (ALH) of wind-exposed ridge and upper slope; (2) more productive Festuca varia grassland (FG) of middle slope; (3) most productive Geranium gymnocaulon/Hedusarum caucasicummeadow (GHM) of lower slope; (4) low-productive snowbed community (SBC) of the slope bottom. N availability, net N mineralization and nitrification were higher in soils of alpine grassland and meadow of the middle part of the toposequence compared with soils of lichen heath and snowbed community of extreme habitats in the alpine zone. There was no correlation between intensities of N transformation processes and favorable (low soil acidity, low C/N ratio, long vegetation period, relatively high temperature, absence of hydromorphic features) and unfavorable (opposite) factors, indicating that the intensity of N mineralization and nitrification in the alpine soils is controlled by a complex combination of these factors. Potential net N mineralization and nitrification in alpine soils determined in the short-term laboratory incubation were considerably higher than those determined in the long-term field incubation. The differences of potential nitrification between soils of various plant communities did not correspond to the field determined pattern indicating the importance of on-site climatic conditions for control of nitrification in high mountains. The result of comparison of N transformation potentials in incubated and native soils indicated that nitrification potential was significantly increased after long-term soil incubation. It means that net nitrification determined in the field was probably overestimated, especially in the meadow soils. A soil translocation experiment indicated that low temperature was an important factor limiting net N mineralization and nitrification in alpine soils: net N mineralization and especially nitrification increased when alpine soils were translocated into the subalpine zone and mean annual temperature increased by about 3 °C. Additional N input increased N availability (NH4+-N) and potential nitrification in soils of the lower part of the toposequense (GHM and SBC), and potential net N mineralization in two soils of extreme habitats (ALH and SBC). A positive correlation was found between soil δ15N and net N mineralization and nitrification; the relative 15N enrichment was characteristic of grassland and meadow ecosystems. δ15N of total soil N pool increased during the field mineralization experiment; there was a positive tendency between the change in δ15N and net N mineralization and nitrification, however the relationship was not significant. Foliar δ15N of dominant plant species varied widely within community, however, a tendency of higher foliar δ15N for species growing on the soils with higher net N mineralization, nitrification and δ15N was observed.