Subalpine Grasslands Research Articles

Temperature sensitivity of soil respiration declines with climate warming in subalpine and alpine grassland soils

AbstractWarming as a climate change phenomenon affects soil organic matter dynamics, especially in high elevation ecosystems. However, our understanding of the controls of soil organic matter mineralization and dynamics remains limited, particularly in alpine (above treeline) and subalpine (below treeline) grassland ecosystems. Here, we investigated how downslope (warming) and upslope (cooling) translocations, in a 5-years reciprocal transplanting experiment, affects soil respiration and its temperature sensitivity (Q10), soil aggregation, and soil organic matter carbon (C) and nitrogen (N) composition (C/N ratio). Downslope translocation of the alpine (2440 m a.s.l.) and subalpine (1850 m a.s.l.) to the lowland site (350 m a.s.l.) resulted in a temperature change during the growing seasons of + 4.4K and + 3.3K, respectively. Warming of alpine soils (+ 4.4K) reduced soil organic carbon (SOC) content by 32%, which was accompanied by a significant decrease of soil macroaggregates. Macroaggregate breakdown induced an increased respiration quotient (qCO2) by 27% following warming of alpine soils. The increase in qCO2 respiration was associated with a significant decrease (from 2.84 ± 0.05 to 2.46 ± 0.05) in Q10, and a change in soil organic matter composition (lower C/N ratios). Cooling did not show the opposite patterns to warming, implying that other mechanisms, such as plant and microbial community shifts and adaptation, were involved. This study highlights the important role of SOC degradability in regulating the temperature response of soil organic matter mineralization. To predict the adverse effect of warming on soil CO2 release and, consequently, its negative feedback on climate change, a comprehensive understanding of the mechanisms of C storage and turnover is needed, especially at high elevations in the Alps that are particularly affected by rising temperatures.

Agroforestry and the Climate Crisis: Prioritizing Biodiversity Restoration for Resilient and Productive Mediterranean Landscapes

This study deals with the future of the traditional agroforestry systems (TAFSs) in the northern Mediterranean Basin. Important productive systems, such as ancient non-irrigated olive groves and extensive silvopastoral systems, from subalpine grasslands to coastal landscapes, are being irreversibly degraded, threatening the bio-cultural heritage of the Mediterranean region, an area recognized as a global biodiversity hotspot. In the midst of the global biodiversity and climate crisis, immediate actions are proposed for the protection and conservation management of TAFSs, within a new EU policy framework. Having the TAFSs of Greece in the spotlight of this analysis, a four-step approach was used in order to review the value of agroforestry in terms of (a) biodiversity (birds, mammals, invertebrates and soil biota), (b) agroforestry landscapes, such as traditional olive groves on terraces and valonia silvopastoral systems, (c) ecosystem services, especially concerning cultural values, and (d) the modern threats to traditional agroforestry. Through this research, problems are highlighted and European policy priorities are defined. Our conclusion is that there is an immediate need to revise the European Union rural, forestry, and environmental policies in the Mediterranean region, following the provisions of the new EU Nature Restoration Law, in order to revive agroforestry landscapes and make them productive and sustainable again for the benefit of rural economies, local communities, and biodiversity, especially in marginal Mediterranean mountainous and island areas, where depopulation and susceptibility to wildfires are major threats.

Climate Changes and its Effect on Yartsagunbu Collectors in the Himalayan Region of North-Western Nepal

This paper explores the effects of climate change on the distribution of Yartsagunbu based on local people’s experiences. Yartsagunbu (Ophiocordyceps Sinensis) is a high-valued fungus that is understood as a stamina enhancer and endoparasitic complex of the parasitic fungus. It is found in the alpine and subalpine grasslands of the Himalayas and the Tibetan Plateau, at 3,000–5,200 meters above sea level. Yartsagunbu has been a significant livelihood resource and generates significant income for the collectors in rural Dolpa, Mugu, Jumla, Bajhang, and Manang of Nepal. This paper has explored climate change and its effects on the Yartsagunbu collectionin the Dolpa district. This brings out the Tarali people’s local experiences and perceptions of climate change and its effect on Yartsagunbu. Data and information for the study were collected from multiple fieldworks during the period 2019, 2020, and 2021. Furthermore, it is supported by personal experience of Yartsagunbu harvesting from the years 2001 to 2005. Present and past trends of Yartsagunbu harvesting experience and perception are collected by interviews with Yartsagunbu collectors, traders, and senior citizens. The finding of the study reveals that climate change is affecting the Yartsaguubu abundance and livelihood of Yartsagunbu collectors.

Mosaic coexistence of two subalpine grassland types as a consequence of soil nutrient heterogeneity

High-mountain areas often exhibit high soil heterogeneity, which allows for the close coexistence of plant species and communities with contrasting resource requirements. This study investigated the nutritional factors driving the mosaic distribution of Nardus stricta L. grasslands and chalk grasslands dominated by forbs in the subalpine southern Pyrenees (Spain). The concentrations of C, N, P, S, K, Ca and fiber fractions in herbage were analyzed in relation to soil nutrient availability; soil β-glucosidase, urease, phosphatase and arylsulfatase activities; and plant species and functional type compositions. The chalk grassland showed higher N:P ratios in herbage and higher enzyme demand for P relative to N in the soil, which indicates a greater limitation of P versus N compared to Nardus grassland. This limitation was related to the higher soil and plant Ca levels in the chalk grassland, where the calcareous bedrock lies close to the soil surface. In the Nardus grasslands, the alleviation of P limitation translated into increased productivity and the replacement of forbs with taller graminoids rich in structural carbohydrates, which was mirrored by greater β-D-glucosidase activity. The plant N:K and P:K ratios indicated potential K deficiency in both grasslands, which resulted from decreased uptake of K in competition with Ca, as indicated by the correlation between plant K and the soil K+:Ca2+ ratio. Our results highlight the effect of the heterogeneity of soil nutrient constraints, mediated by stoichiometry and dependent on microtopography, on the biodiversity of high-mountain ecosystems.

Constraints on Organic Matter Stability in Pyrenean Subalpine Grassland Soils: Physical Protection, Biochemical Quality, and the Role of Free Iron Forms

Constraints on Organic Matter Stability in Pyrenean Subalpine Grassland Soils: Physical Protection, Biochemical Quality, and the Role of Free Iron Forms

Vegetation responses to snow cover removal and rainfall reduction in subalpine grasslands: Insights from a 4-year field experiment in the Spanish Pyrenees

ABSTRACT Reduced precipitation as well as warming may result in less snow accumulation in seasonally snow-covered areas, leading to lower minimum soil temperatures and more frequent and severe soil frosts. Therefore, plant stress is increased not only by drought in warmer months, but also by increased exposure to frost in cooler periods. We conducted a 4-year field experiment to evaluate the effects of diminished snowpack accumulation (through snow removal) and rainfall reduction (through rain-out shelters) on aboveground plant productivity, diversity and species composition of two subalpine grassland plant communities from the central region of the Spanish Pyrenees. We found that the snow removal treatment decreased minimum soil temperature by 0.5°C. Plant diversity decreased by 16 percent, although this effect was only observed in one of the grasslands studied. Aboveground primary productivity seemed to be unaffected. In contrast, we found that the rainfall reduction treatment negatively affected aboveground productivity of leguminous forb species, yet no effect on plant diversity was observed. Both treatments were important drivers of changes in plant species composition. Overall, our results suggest that the resilience of subalpine grasslands to snow cover removal and rainfall reduction treatments may depend on the specific community composition and dominant plant groups.

Hawaiian Treeline Ecotones: Implications for Plant Community Conservation under Climate Change.

Species within tropical alpine treeline ecotones are predicted to be especially sensitive to climate variability because this zone represents tree species' altitudinal limits. Hawaiian volcanoes have distinct treeline ecotones driven by trade wind inversions. The local climate is changing, but little is known about how this influences treeline vegetation. To predict future impacts of climate variability on treelines, we must define the range of variation in treeline ecotone characteristics. Previous studies highlighted an abrupt transition between subalpine grasslands and wet forest on windward Haleakalā, but this site does not represent the diversity of treeline ecotones among volcanoes, lava substrates, and local climatic conditions. To capture this diversity, we used data from 225 plots spanning treelines (1500-2500 m) on Haleakalā and Mauna Loa to characterize ecotonal plant communities. Treeline indicator species differ by moisture and temperature, with common native species important for wet forest, subalpine woodland, and subalpine shrubland. The frequency or abundance of community indicator species may be better predictors of shifting local climates than the presence or absence of tree life forms per se. This study further supports the hypothesis that changes in available moisture, rather than temperature, will dictate the future trajectory of Hawaiian treeline ecotone communities.

Assessing plant trait diversity as an indicators of species α‐ and β‐diversity in a subalpine grassland of the Italian Alps

AbstractAs the need for ecosystem biodiversity assessment increases within the climate crisis framework, more and more studies using spectral variation hypothesis (SVH) are proposed to assess biodiversity at various scales. The SVH implies optical diversity (also called spectral diversity) is driven by light absorption dynamics associated with plant traits (PTs) variability (which is an indicator of functional diversity) which is, in turn, determined by biodiversity. In this study, we examined the relationship between PTs variability, optical diversity and α‐ and β‐diversity at different taxonomic ranks at the Monte Bondone grasslands, Trentino province, Italy. The results of the study showed that the PTs variability, at the α scale, was not correlated with biodiversity. On the other hand, the results observed at the community scale (β‐diversity) showed that the variation of some of the investigated biochemical and biophysical PTs was associated with the β‐diversity. We used the Mantel test to analyse the relationship between the PTs variability and species β‐diversity. The results showed a correlation coefficient of up to 0.50 between PTs variability and species β‐diversity. For higher taxonomic ranks such as family and functional groups, a slightly higher Spearman's correlation coefficient of up to 0.64 and 0.61 was observed, respectively. The SVH approach was also tested to estimate β‐diversity and we found that spectral diversity calculated by Spectral Angle Mapper showed to be a better proxy of biodiversity in the same ecosystem where the spectral diversity approach failed to estimate α‐diversity. These findings suggest that optical and PTs diversity approaches can be used to predict species diversity in the grasslands ecosystem where the species turnover is high.

Effectiveness of two mechanical shrub removal treatments for restoring sub-alpine grasslands colonized by re-sprouting woody vegetation

The extent of European sub-alpine grasslands and their associated ecosystem services are decreasing due to woody plant encroachment. Commonly used methods of woody vegetation suppression like prescribed burning or clearcutting usually cause little damage to belowground bud-banks, offering poor results against re-sprouting shrubs. In this study, we assessed the effects on vegetation and soil properties of two mechanical shrub removal methods for restoring sub-alpine grasslands colonized by the re-sprouting shrub Rosa sp. in the Central Spanish Pyrenees: a commonly used method based on clearcutting (Clearcutting); and a non-previously assessed method based on pulling shrubs off the soil to remove both the aerial and belowground bud-banks (Uprooting). We set a parallel experiment to test whether or not clustering Rosa sp. debris generated in Uprooting (which held many mature fruits) at certain grassland locations may promote colonization of new grassland spots by Rosa sp. seedlings. By the end of the study period, vegetation composition and structure was more similar to the reference grassland in Uprooting than in Clearcutting. Indeed, woody vegetation cover was 71 % smaller in Uprooting than in Clearcutting three years after shrub removal. Nevertheless, by the end of the study period, chemical and microbiological soil properties were slightly more similar to the reference grassland in Clearcutting than in Uprooting. Additionally, the results of our study showed that clustering unusually high number of mature fruits of Rosa sp. at certain grassland locations increased shrub seedling colonization in comparison with other areas of the reference grassland, indicating that operational planning needs to take into account shrub phenology. In conclusion, our work showed that Uprooting may be a useful tool for land managers aiming to restore sub-alpine grasslands colonized by re-sprouting shrubs, though it is advisable using it for scatter shrub patches to prevent significant medium to long-term soil disturbance at landscape scale.

Increases in functional diversity of mountain plant communities is mainly driven by species turnover under climate change

Warming in mountain regions is projected to be three times faster than the global average. Pronounced climate change will likely lead to species reshuffling in mountain plant communities and consequently change ecosystem resilience and functioning. Yet, little is known about the role of inter‐ versus intraspecific changes of plant traits and their consequences for functional richness and evenness of mountain plant communities under climate change. We performed a downslope translocation experiment of intact plant‐soil mesocosms from an alpine pasture and a subalpine grassland in the Swiss and Austrian Alps to simulate an abrupt shift in climate and removal of dispersal barriers. Translocated plant communities experienced warmer and dryer climatic conditions. We found a considerable shift from resource conservative to resource acquisitive leaf‐economy in the two climate change scenarios. However, shifts in leaf‐economy were mainly attributable to species turnover, namely colonization by novel lowland species with trait expressions for a wider range of resource use. We also found an increase in vegetative height of the warmed and drought‐affected alpine plant community, while trait plasticity to warming and drought was limited to few graminoid species of the subalpine plant community. Our results highlight the contrast between the strong competitive potential of novel lowland species in quickly occupying available niche space and native species' lack of both the intraspecific trait variability and the plant functional trait expressions needed to increase functional richness under warming and drought. This is particularly important for the trailing range of many mountain species (i.e. subalpine zone) where upward moving lowland species are becoming more abundant and abiotic climate stressors are likely to become more frequent in the near future. Our study emphasizes mountain plant communities' vulnerability to novel climates and biotic interactions under climate change and highlights graminoid species as potential winners of a warmer and dryer future.Keywords: alpine grassland, functional diversity, invasion, species turnover, traitspace, translocation

Subalpine Grassland Vegetation of Yardimli and Astara Districts (Azerbaijan)

Subalpine grasslands of Talish highlands are common in the subalpine zone of Yardimli and Astara highlands at lawny mountain-meadow soils at the height of 1750 m to 2400 m above sea level. During the carried out ecological-geobotanical research, it has been determined 3 formation classes, 5 formation groups and 11 associations. In parallel with the study of type composition and structure of phytocenosis found in the area of investigation, it has been also determined endemic species, productivity of formations, and intensification of degradation at soil-plant coverage of some lawny subalpine grasslands, decrease in abundance and productivity of forage crop. Currently, for the purpose of the improvement of natural phytocenosis productivity, crop quality, protection of their genetic reserve and landscapes, as well as the vegetation study for solution of protection problems on the base of scientific means is of a great importance.

Effect of Grazing Management on Predator Soil Mite Communities (Acari: Mesotigmata) in Some Subalpine Grasslands from the Făgăraş Mountains-Romania.

For the first time in Romania, a complex study was conducted on soil mite communities from two types of managed grasslands: ungrazed and intensively grazed. The study was accomplished in August 2018, in the Făgăraş Mountains. Within the soil mite communities (Mesostigmata), 30 species were identified, from 80 soil samples. The following population parameters were investigated: species richness, numerical abundance, dominance, Shannon index of diversity, evenness and equitability. Eight environmental variables were also measured: soil and air humidity; soil and air temperature; soil pH; resistance of soil to penetration; soil electrical conductivity; and vegetation coverage. The results revealed that species richness, Shannon index of diversity, evenness and equitability indices had higher values in ungrazed grasslands, whereas in intensively grazed areas, the numerical abundance and dominance index had significantly higher values. The species Alliphis halleri was dominant in the ungrazed grasslands. Each type of managed grassland was characterised by specific environmental conditions, which had an important influence, even at the species level.

Contrasting effects of nitrogen enrichment on ecosystem temporal stability before and after nitrogen saturation in a subalpine grassland

Increased atmospheric nitrogen deposition generally increases plant biomass production until reaching soil N saturation, which could increase the uncertainty of changes in ecosystem temporal stability and their mechanisms. Yet, the response of ecosystem stability to N enrichment and their underlying mechanisms are uncertain, especially when N saturation reached. Here, we conducted a multi-level N addition (0, 2, 5, 10, 15, 25, and 50 g N m−2 year−1; high added rates reached N saturation) experiment from 2018 to 2022 to estimate the effect of simulated N deposition on ecosystem biomass stability in a subalpine grassland located on the Qilian mountain of north-eastern Tibetan Plateau. Our results show that community biomass production increased with an increase in N addition in the first N addition experiment year, but decreased with increase in N addition rates after N saturation in subsequent years. We first find a negative quadratic relationship between biomass temporal stability and added N rate, whereby above N saturation threshold (5 g N m−2 year−1 at this site), increases in N addition reduces biomass temporal stability. The changes in biomass temporal stability are largely determined by dominant species stability, species asynchronous, and species richness. These results provide a better understanding of N-induced effect on ecosystem stability and their underlying mechanisms, which is important to evaluate functioning and services of ecological systems under global change scenarios.

Early spring snowmelt and summer droughts strongly impair the resilience of bacterial community and N cycling functions in a subalpine grassland ecosystem

Subalpine grasslands support biodiversity, agriculture, and tourism but their resilience to extreme climatic events is challenged accelerating their vulnerability to tipping points. Microbial communities, central in ecosystem functioning, are usually considered more resistant and highly resilient to extreme events albeit their functional redundancy and strong selection by local harsh climatic conditions. This study explored the soil microbial responses upon recurrent spring‐summer droughts associated with early snowmelt in subalpine grasslands mesocosms set‐up at the Lautaret Pass (French Alps). Potential soil microbial respiration, nitrification and denitrification activities were monitored over a period of two growing seasons along with quantification of related gene abundances. Impacts of simulated spring‐summer drought and early snowmelt were quantified to assess their resistance and recovery. Results revealed that droughts had a low and short‐term adverse impact on bacterial total respiration supporting their hypothesized high resilience, i.e. resistance and ability to recover. Nitrification and abundances of the corresponding functional guilds showed relatively strong resistance to summer droughts but declined in response to early snowmelt. This resistance of nitrification was paralleled by the recovery of denitrification and abundances of denitrifying communities from all climatic extremes, except from the summer droughts where nitrifiers were collapsed. Denitrification and respective functional groups faced high impact of applied stresses with strong reduction in abundance and activity. Although, consequently lower denitrifiers' competition for nitrate may be positive for plant biomass production, warnings exist when considering the potential nitrate leaching as well as risks of greenhouses gases emission such as N2O from these ecosystems.

Concurrent and lagged effects of drought on grassland net primary productivity: a case study in Xinjiang, China

Xinjiang grasslands play a crucial role in regulating the regional carbon cycle and maintaining ecosystem stability, and grassland net primary productivity (NPP) is highly vulnerable to drought. Drought events are frequent in Xinjiang due to the impact of global warming. However, there is a lack of more systematic research results on how Xinjiang grassland NPP responds to drought and how its heterogeneity is characterized. In this study, the CASA (Carnegie Ames Stanford Application) model was used to simulate the 1982–2020 grassland NPP in Xinjiang, and the standardized Precipitation Evapotranspiration Index (SPEI) was calculated using meteorological station data to characterize drought. The spatial and temporal variability of NPP and drought in Xinjiang grasslands from 1982 to 2020 were analyzed by the Sen trend method and the Mann-Kendall test, and the response characteristics of NPP to drought in Xinjiang grasslands were investigated by the correlation analysis method. The results showed that (1) the overall trend of NPP in Xinjiang grassland was increasing, and its value was growing season > summer > spring > autumn. Mild drought occurred most frequently in the growing season and autumn, and moderate drought occurred most frequently in spring. (2) A total of 64.63% of grassland NPP had a mainly concurrent effect on drought, and these grasslands were primarily located in the northern region of Xinjiang. The concurrent effect of drought on NPP was strongest in plain grassland and weakest in alpine subalpine grassland. (3) The lagged effect is mainly in the southern grasslands, the NPP of alpine subalpine meadows, meadows, and alpine subalpine grasslands showed mainly a 1-month time lag effect to drought, and desert grassland NPP showed mainly a 3-month time lag effect to drought. This research can contribute to a reliable theoretical basis for regional sustainable development.

Effects of Short-Term Nitrogen and Phosphorus Addition on Soil Respiration Components in a Subalpine Grassland of Qilian Mountains

In order to investigate the effects of short-term nitrogen and phosphorus addition on soil respiration and its components in a subalpine grassland located on the Qilian Mountains, a random block design of nitrogen[10 g·(m2·a)-1, N], phosphorus[5 g·(m2·a)-1, P], nitrogen and phosphorus addition[10 g·(m2·a)-1N and 5 g·(m2·a)-1P, NP], the control (CK), and complete control (CK') was conducted from June to August 2019, and total soil respiration and its component respiration rates were measured. The results showed that nitrogen addition reduced soil total respiration and heterotrophic respiration rates at a lower rate than P addition[-16.71% vs. -19.20%; -4.41% vs. -13.05%], but the rate of decrease in autotrophic respiration was higher than that of P addition (-25.03% vs. -23.36%); N and P mixed application had no significant effect on soil total respiration rate. The total soil respiration rate and its components were significantly exponentially correlated with soil temperature, and the temperature sensitivity of soil respiration rate was decreased by nitrogen addition (Q10:-5.64%-0.00%). P increased Q10 (3.38%-6.98%), and N and P reduced autotrophic respiration rate but increased heterotrophic respiration rate Q10 (16.86%) and decreased total soil respiration rate Q10 (-2.63%- -2.02%). Soil pH, soil total nitrogen, and root phosphorus content were significantly correlated with autotrophic respiration rate (P<0.05) but not with heterotrophic respiration rate, and root nitrogen content was significantly negatively correlated with heterotrophic respiration rate (P<0.05). In general, autotrophic respiration rate was more sensitive to N addition, whereas heterotrophic respiration rate was more sensitive to P addition. Both N and P addition significantly reduced soil total respiration rate, whereas N and P mixture did not significantly affect soil total respiration rate. These results can provide a scientific basis for the accurate assessment of soil carbon emission in subalpine grassland.

Probabilistic assessment of drought stress vulnerability in grasslands of Xinjiang, China.

In the process of climate warming, drought has increased the vulnerability of ecosystems. Due to the extreme sensitivity of grasslands to drought, grassland drought stress vulnerability assessment has become a current issue to be addressed. First, correlation analysis was used to determine the characteristics of the normalized precipitation evapotranspiration index (SPEI) response of the grassland normalized difference vegetation index (NDVI) to multiscale drought stress (SPEI-1 ~ SPEI-24) in the study area. Then, the response of grassland vegetation to drought stress at different growth periods was modeled using conjugate function analysis. Conditional probabilities were used to explore the probability of NDVI decline to the lower percentile in grasslands under different levels of drought stress (moderate, severe and extreme drought) and to further analyze the differences in drought vulnerability across climate zones and grassland types. Finally, the main influencing factors of drought stress in grassland at different periods were identified. The results of the study showed that the spatial pattern of drought response time of grassland in Xinjiang had obvious seasonality, with an increasing trend from January to March and November to December in the nongrowing season and a decreasing trend from June to October in the growing season. August was the most vulnerable period for grassland drought stress, with the highest probability of grassland loss. When the grasslands experience a certain degree of loss, they develop strategies to mitigate the effects of drought stress, thereby decreasing the probability of falling into the lower percentile. Among them, the highest probability of drought vulnerability was found in semiarid grasslands, as well as in plains grasslands and alpine subalpine grasslands. In addition, the primary drivers of April and August were temperature, whereas for September, the most significant influencing factor was evapotranspiration. The results of the study will not only deepen our understanding of the dynamics of drought stress in grasslands under climate change but also provide a scientific basis for the management of grassland ecosystems in response to drought and the allocation of water in the future.

Net Ecosystem CO2 Exchange in Mountain Grasslands Is Seriously Endangered by the Temperature Increase in the Eastern Pyrenees

Mediterranean mountain grasslands, including the Pyrenees, are highly vulnerable to climate change, due to the increasing temperatures and heat weaves frequency, among other factors. However, the effects of the increased temperatures on CO2 fluxes in those ecosystems have been barley explored. To address this gap of knowledge, we established the FLUXPYR-ECOFUN micrometeorological flux network, which included three eddy covariance flux stations in grasslands along a management and a climatic gradient (montane to subalpine) at the Pyrenees; we aimed at assessing interactions among environmental and phenological drivers on CO2 fluxes, with special attention at the role of temperature as CO2 flux driver under the different climatic and management conditions across the studied gradient. Our results showed that temperature drove CO2 dynamics along the studied gradient in different ways. At the subalpine grassland net CO2 uptake was linearly enhanced by temperature and CO2 fluxes had not reached a temperature shifting point yet (according to the segmented linear models) at which the net uptake would become CO2 emissions. This suggests that in the short term, and under the incoming enhanced temperatures, sub-alpine grasslands in the Pyrenees might increase their net CO2 uptake, although the mid long-term uptake may be compromised. On the contrary, the montane grasslands already presented CO2 emissions at the highest temperatures, most likely driven by a decrease in the greenness and photosynthesis, which suggests that montane grasslands are expected to reduce their CO2 sink capacity under the increasing temperatures. Overall, mountain grasslands in the mid- to long-term in the Pyrenees may experience a reduction in their net CO2 uptake capacity under the current climate change scenario.

Soil organic matter changes under experimental pedoclimatic modifications in mountain grasslands of the French Alps

Mountain grasslands contain large stocks of soil organic carbon (SOC), of which a good part is in labile particulate form. This labile SOC may be protected by cold climate that limits microbial activity. Strong climate change in mountain regions threatens to destabilize these SOC stocks. However, so far the climate response of SOC stocks in mountain grasslands remains highly uncertain, under either warming or cooling conditions. To overcome this knowledge gap, we studied the effect of pedoclimatic regime changes on topsoil (0–10 cm) SOC in two complementary experiments: 3 °C of warming or cooling by reciprocal transplanting to an alpine (2450 m a.s.l.) and a subalpine (1950 m a.s.l.) grassland and 1 °C of warming by open-top chambers in the same grasslands.Topsoil SOC stocks were higher at the alpine site than at the subalpine site, and the biogeochemical signature of the soil organic matter (SOM) also differed between the two study sites. SOM was O-enriched, H-depleted, and more thermally stable at the warmer subalpine site. After three years, abrupt warming by transplanting tended to decrease topsoil SOC content. The remaining SOC was characterized by a more thermostable signature. This result suggests the preferential depletion of labile SOC upon experimental topsoil warming. Cooling did not modify overall SOC content but uphill transplanted topsoils showed a more thermolabile biogeochemical signature. In contrast, open-top chamber warming of alpine and subalpine topsoils caused limited changes to SOC stocks and SOM biogeochemical signature, possibly because the induced pedoclimatic change was more limited and more gradual compared to the warming by transplantating which reduced the annual snow cover period by around 60 days and increased cumulative degree days by a factor of ten as compared to the OTC-induced warming. Gradual temperature changes may take longer to become effective than a shock transplant treatment. We conclude that SOC in mountain grassland topsoils can be highly reactive to climate shocks.

Higher facilitation for stress‐intolerant ecotypes along a metal pollution gradient are due to a decrease in performance in absence of neighbours

The study of variation in plant–plant interactions along metal‐pollution gradient is in its infancy, although this is worth to be assessed for both restoration and theoretical perspectives. Additionally, the mechanisms of facilitation at stake in these particular stressed conditions are poorly known. We aim at understanding the importance of species and ecotypes stress‐tolerance in competitive and facilitative responses to neighbours along metal‐pollution gradients. We addressed this goal in a field experiment conducted in a former mining area located in a subalpine grassland of the French pyrenees. Two ecotypes of Armeria muelleri (a highly tolerant species to metal stress) and Agrostis capillaris (a less stress‐tolerant species) were harvested at the highest and lowest levels of pollution within their distribution range and transplanted with and without neighbours along a strong metal‐pollution gradient. The relative interaction index (RII) was used to assess both the effect of neighbours at different stress levels and the effect of metallic stress with and without neighbours. With increasing pollution, plant–plant interactions shifted from negative to positive for Agrostis, but did not vary significantly for Armeria. At high pollution level, the unpolluted Agrostis ecotype was more facilitated than the polluted one, and in benign habitats the polluted Armeria ecotype was more sensitive to competition than the less polluted ecotype. Interestingly, the increase in facilitation with increasing metal stress for the stress‐intolerant Agrostis ecotype was primarily due to a decrease in performance without neighbours, whereas the increase in competition with decreasing metal stress for the stress‐tolerant Agrostis ecotype was primarily due to an increase in performance with neighbours. More generally, these results suggest that the high size‐plasticity of competitive (and stress‐intolerant) species or ecotypes may explain both their competitive effects in benign habitats through neighbour‐trait effects and facilitative responses in stressed habitats in the context of environmental‐severity effects.