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1. In many ombrotrophic bog areas the invasion of grass (e.g. Molinia caerulea) and tree (e.g. Betula pubescens) species has become a major problem. We investigated whether the invasion of such species is due to high atmospheric nitrogen (N) deposition by conducting a fertilization experiment. 2. The effects of experimentally increased N input on Molinia, Betula and Eriophorum vaginatum were studied in desiccated bog vegetation in Ireland, where there is relatively low background N deposition. Four different N treatments were applied for 3 years: 0 (control), 2, 4 and 8 g m-1year-1. 3. Ammonium and nitrate concentrations in the peat moisture increased at high N addition rates, leading to significantly higher carbon: nitrogen (C: N) and nitrogen: phosphorus (N: P) ratios in the top layer of the peat. The potential CO2production rate of the peat was not stimulated at high N addition rates due to severe acidification of the peat. 4. Despite high tissue N: P ratios (above 40), above-ground biomass production by Molinia was stimulated at high N addition rates, and foliar nutrient concentrations were unaffected. In contrast to Molinia, Betula and Eriophorum were unable to increase their above-ground biomass, probably due to P limitation. Regrowth of the lichen Cladonia portentosa was suppressed at high N addition rates. 5. Synthesis and applications. We conclude that the invasion of bogs by Molinia and Betula is likely to be less affected by desiccation than by increased N availability. Apparently, Molinia is well adapted to P-limiting conditions, which may explain its success in regions with increased N deposition levels. The high availability of P in many Dutch bogs compared with Irish bogs, together with prolonged high N deposition levels, may explain the strong increase in both Molinia and Betula observed in the Netherlands. As long as N and P availabilities in Dutch bogs are too high to prevent invasion of Betula and/or Molinia, management measures stimulating growth of Sphagnum mosses could probably reduce the negative effects of high N deposition levels.

Increases in atmospheric CO2 concentration and N deposition are expected to affect methane (CH4) production in soils and emission to the atmosphere, directly through increased plant litter production and indirectly through changes in substrate quality. We examined how CH4 emission responded to changes in litter quality under increased N and CO2, beyond differences in CH4 resulting from changes in litter production. We used senesced leaves from 13C‐labeled plants of Molinia caerulea grown at elevated and ambient CO2 and affected by N fertilization to carry out two experiments: a laboratory litter incubation and a pot experiment. N fertilization increased N and decreased C concentrations in litter whereas elevated CO2 decreased litter quality as reflected in litter C and N concentrations and in the composition of lignin and saturated fatty acids within the litter. In contrast to our expectations, CH4 production in the laboratory incubation decreased when using litter from N‐fertilized plants as substrate, whereas litter from elevated CO2 had no effect, compared to controls without N and at ambient CO2. Owing to high within‐treatment variability in CH4 emissions, none of the treatment effects were reflected in the pot experiment. C mineralization rates were not affected by any of the treatments. The decrease in CH4 emissions due to indirect effects of N availability through litter quality changes (described here for the first time) contrast direct effects of N fertilization on CH4 production. The complex interaction of direct effects with indirect effects of increased N on litter quality may potentially result in a net decrease in CH4 emissions from wetlands in the long term.

Multiple land‐use change drivers affect, in most cases negatively, the biodiversity in species‐rich meadows. Empirical data that can help to disentangle the effects of individual drivers and quantify the time required for a biodiversity response are seldom available. Management decisions are often based on short‐term experiments or observational data. A 15‐year field experiment, comprising a factorial combination of fertilization, mowing and removal of the dominant species Molinia caerulea, was established in an oligotrophic wet meadow in Czech Republic. Each of the eight factorial combinations was replicated three times. Percentage cover for all species was monitored annually in 1‐m² plots and species' presence recorded in each cell of a continuous square grid of 25 cells (0·1 × 0·1 m each). These data enabled various scale‐dependent estimates of species richness. The species composition of individual treatment combinations diverged over time, particularly at the start of the experiment, and by the latter stages resembled various typical grassland communities from the surrounding landscape. Fertilization had the most pronounced effect, leading to a sharp decrease in species richness, most rapidly at the smallest spatial scale. Mowing had on average a positive effect on species richness and led in most cases to spatially homogeneous species composition. The removal of Molinia had a positive effect on species richness, especially in unmown unfertilized plots. The effects of each factor were dependent on the combination of the other two factors, and also on time, with some effects continuously increasing throughout and some diminishing by the end of the experiment. The process of competitive exclusion with fertilization and cessation of mowing was, in some treatment combinations, rather slow. Synthesis and applications. Land‐use change drivers act in combination, and their effects on the structure of species‐rich wet meadows are dependent on both the temporal and spatial scales considered. Short‐term experiments might underestimate the response of vegetation and thus provide erroneous conservation recommendations. Mowing was only effective in preventing species richness decline caused by fertilization in the short term. The presence of a single dominant species can modify the effectiveness of conservation measures.

1. Current biodiversity declines in species-rich grasslands are connected with the cessation of management, eutrophication and the expansion of dominant grass species. One of the theoretical mechanisms limiting biodiversity loss is the ability of subordinate species to avoid competitive exclusion by seasonal niche separation from dominant species. Here, we explore how seasonality underpins the maintenance of diversity in temperate meadows under different management regimes and competition intensities in relation to species functional traits. 2. We studied eight different communities in a long-term meadow experiment that manipulated mowing, fertilization and dominant species (Molinia caeruiea) removal. In each community, species-specific trait and biomass data were taken five times during the year to test whether seasonal variation in species composition and functional strategies enable species to coexist. 3. Mown unfertilized meadows exhibited pronounced seasonal variations in community composition and structure, linked to differences in resource-use strategies between mid-summer dominants and the spring and autumn subordinates. Higher specific leaf area and foliar nitrogen concentration in the fast-growing dominants, and increased water use (δ¹³C) and nutrient acquisition (δ¹⁵N) efficiency in resource-retentive subordinates, best predicted their temporal niche separation. Seasonal segregation of species with contrasting strategies increased after mowing cessation, and the resulting summer dominance of Molinia. Conversely, the seasonal dynamics were markedly reduced by fertilization, promoting tall grasses over sedges and forbs throughout the entire year, thereby decreasing the overall taxonomic and functional diversity. When Molinia was removed the compositional changes during the season became less pronounced, being significant only in mown unfertilized plots. 4. Synthesis. Seasonal shifts in community composition reduced the competitive interactions and promoted the coexistence of dominant and subordinate species. Seasonality reversed the negative mid-summer diversity-productivity relationship with a positive one during the spring and autumn, and seasonality only prevented diversity loss in unfertilized conditions possibly because competition is most intense in summer. In fertilized meadows, subordinate species are not able to escape competitive exclusion by shifting their phenoiogical peaks to the spring or autumn periods because asymmetric competition is intense over the entire growing season. Studying seasonal dynamics is key to understanding the maintenance of grassland diversity under ongoing land use change.

Uncontrolled, large fires are a major threat to the biodiversity of protected heath landscapes. The severity of the fire is an important factor influencing vegetation recovery. We used airborne imaging spectroscopy data from the Airborne Prism Experiment (APEX) sensor to: (1) investigate which spectral regions and spectral indices perform best in discriminating burned from unburned areas; and (2) assess the burn severity of a recent fire in the Kalmthoutse Heide, a heathland area in Belgium. A separability index was used to estimate the effectiveness of individual bands and spectral indices to discriminate between burned and unburned land. For the burn severity analysis, a modified version of the Geometrically structured Composite Burn Index (GeoCBI) was developed for the field data collection. The field data were collected in four different vegetation types: Calluna vulgaris-dominated heath (dry heath), Erica tetralix-dominated heath (wet heath), Molinia caerulea (grass-encroached heath), and coniferous woodland. Discrimination between burned and unburned areas differed among vegetation types. For the pooled dataset, bands in the near infrared (NIR) spectral region demonstrated the highest discriminatory power, followed by short wave infrared (SWIR) bands. Visible wavelengths performed considerably poorer. The Normalized Burn Ratio (NBR) outperformed the other spectral indices and the individual spectral bands in discriminating between burned and unburned areas. For the burn severity assessment, all spectral bands and indices showed low correlations with the field data GeoCBI, when data of all pre-fire vegetation types were pooled (R2 maximum 0.41). Analysis per vegetation type, however, revealed considerably higher correlations (R2 up to 0.78). The Mid Infrared Burn Index (MIRBI) had the highest correlations for Molinia and Erica (R2 = 0.78 and 0.42, respectively). In Calluna stands, the Char Soil Index (CSI) achieved the highest correlations, with R2 = 0.65. In Pinus stands, the Normalized Difference Vegetation Index (NDVI) and the red wavelength both had correlations of R2 = 0.64. The results of this study highlight the superior performance of the NBR to discriminate between burned and unburned areas, and the disparate performance of spectral indices to assess burn severity among vegetation types. Consequently, in heathlands, one must consider a stratification per vegetation type to produce more reliable burn severity maps.

Species-rich oligotrophic meadows are affected by a wide range of management interventions that influence their functioning and capacity to deliver ecosystem services, but long-term studies on the above-and belowground adaptations to different management tools are still scarce. We focused on the interactive effects of NPK fertilization, mowing, and removal of the initially dominant species (Molinia caerulea) on plant, soil, and microbial responses in wet oligotrophic grassland in a 16-year full-factorial manipulative experiment. Changes in vegetation composition, soil pH, and nutrient availability were accompanied by altered microbial phospholipid fatty acid (PLFA) composition, whereas treatment effects on soil microbial biomass and carbon (C) mineralization were mainly related to changes in soil organic matter (SOM) content and nutrient availability. Fertilization decreased plant species richness aboveground and lowered SOM storage and microbial activity belowground. Mowing preserved high plant diversity and led to more efficient recycling of N within the grassland, whereas Molinia removal significantly affected only plant community composition. Mowing combined with fertilization maintained high species richness only in the short term. Belowground, mowing reduced N leaching from the fertilized system but did not prevent SOM depletion, soil acidification, and concomitant adverse effects on soil microbes. We conclude that annual mowing is the appropriate type of extensive management for oligotrophic species-rich meadows, but the concomitant nutrient depletion should not be compensated for by regular NPK fertilization due to its adverse effects on soil quality.

It is commonly assumed in trait‐based studies that plant functional traits are species‐specific, being more variable among species than among different environmental conditions. If the environment affects traits, it is assumed that species react in a similar direction and conserve the functional distances. The rank of species based on the trait values is then unchanged, which justifies the use of species trait averages from database values. Such assumptions of species specificity are, however, increasingly disputed by studies showing overall high intraspecific trait variability. To test the species specificity and ranking stability of functional traits, we sampled plant individuals of almost all species (66 in total) within each plot of a long‐term (19 years) land use management experiment, which comprised a factorial combination of fertilization, mowing and removal of the dominant species Molinia caerulea in an oligotrophic wet meadow in the Czech Republic. Plant individuals were measured for eight commonly used traits: height, leaf dry matter content (LDMC), specific leaf area (SLA), leaf δ13C content, leaf carbon content, leaf δ15N content and leaf nitrogen content. Height, LDMC and SLA were also extracted from the LEDA trait database for comparison. Species identity consistently explained the largest portion of trait variability (40%‐68%). Land use managements had a considerably lower effect (0.4%–9% of explained trait variability for individual traits). The species trait averages computed for each land use management regime separately were mutually correlated, showing the stable trait‐based species ranking. Ranking stability of species trait averages was observed despite land use management changing absolute trait values and despite the tremendous intraspecific trait variability (causing substantial overlap of trait values for different species). For all treatments, our measured species averages for LDMC and SLA were also stably ranked with species averages from the LEDA database. Synthesis. Our results showed that species conserve the functional distances in different environmental conditions from where they were measured. Species trait averages can describe general trends in functional composition, although averaging reduces the ecologically interesting information of the intraspecific trait variability. A plain language summary is available for this article. Plain Language Summary

Aims Belowground carbon transfer from plant to plant has been extensively described, but such transfer for nitrogen has been less thoroughly investigated when the donor is a non-N 2 -fixing species. This study, applied to forest regeneration, aimed to determine whether tree seedlings facilitated neighbouring grass growth through nitrogen transfer at an early stage of development, thus facilitating nitrogen acquisition by understory species. Methods Quercus petraea seedlings were planted in pots either sole-grown or mixed-grown with Molinia caerulea tufts or another oak seedling. 15 N-urea pulse-chase labelling (cotton wick method) was performed in oak shoots and the fate of 15 N in each soil and plant compartment was tracked for one year. N transfer pathways were investigated using two degrees of physical separation between root systems. Results Molinia dry weight was higher when mixed-grown with oak seedlings than when sole-grown. Increase in grass dry weight correlated with N transfer from donor oak to receiver Molinia . Interestingly, the presence of Molinia increased N rhizodeposition of oak. N allocation in donor oak towards root in winter and shoot in spring was enhanced. Conclusions Oak seedlings facilitated Molinia growth through rapid N transfer, underlining the ability of non-N 2 -fixing species to supply N to neighbours. 15 N allocation within donor oak and its rhizodeposition depended on neighbour identity.

Key messageMolinia caerulea (L.) Moench has been observed to significantly reduce budburst in Quercus petraea (Matt.) Liebl. seedlings through water-based M. caerulea root exudates. This suggests direct allelopathic effects between the two species. In terms of nutrient uptake, oak roots primarily take up nitrogen in the forms of ammonium and glycine. Interestingly, the application of root exudates from M. caerulea doubled the nitrate uptake in oak roots. Moreover, gene sets involved in nitrogen metabolism within oak roots exhibited strong deregulation when treated with M. caerulea root exudate, indicating that the interaction between these two plant species alters the nitrogen metabolism in the oak roots.ContextOak regeneration encounters numerous impediments, including interactions with Molinia caerulea (L.) Moench, an understory grass species widespread in temperate forests. Besides competition for resources, our previous work suggested that the interaction between oak and M. caerulea may involve allelopathic processes.AimThis study tested the hypothesis that M. caerulea affects the budburst dynamics, early growth of oak seedlings, and root N uptake transport and assimilation systems.MethodsPotted oak seedlings (Quercus petraea (Matt.) Liebl.) were watered with M. caerulea root exudates or water for 6 weeks during April and May 2021. The capacity of oak seedlings to take up nitrogen in its main molecular forms was characterized by an influx analysis of isotopically labeled (15N) nitrate, ammonium, and glycine on excised roots using the “teabag” technique. Concomitantly, targeted transcriptomics were carried out to monitor changes in gene expression related to nitrogen metabolism.ResultsThe treatment resulted in reduced budburst rates (− 50%), together with an early reduction in height increment. Kinetics of N influx rates revealed that ammonium, and to a lesser degree glycine, were the predominant forms in which N was taken up by roots. Application of M. caerulea root exudate barely increased the ammonium influx rate and had no effect on the glycine influx rate. By contrast, the nitrate influx rate, despite its low values, doubled after application of the exudate. Targeted transcriptomic analysis revealed a regulatory shift in gene sets associated with key mechanisms underlying N uptake, assimilation, and long-distance transport in oak roots.ConclusionInteractions between a perennial grass, M. caerulea, and a woody species, Q. petraea, do not only rely upon direct competition for resources. Allelopathic processes must be taken into consideration when designing regeneration operations.

Background and aimsHeathlands are a broad vegetation type characterized by the dominance of evergreen shrub species that thrive on nutrient-poor soils, thus sensitive to small changes in soil conditions. Here we aimed to identify soil gradients related to climate, management and main species in wet heathlands.MethodsSoil nutrient levels, organic matter and acidity were studied at two soil depths on ninety plots from 18 sites in Erica mackayana wet heaths of NW Iberian Peninsula, in relation to presence and cover of structural plant species (shrubs and graminoids) at two scales: plot and site (landscape) scale.ResultsWe identified one main soil gradient explained by soil organic matter (SOM), the effective cation exchange complex (eCEC), available phosphorus (P), and Aluminium-Calcium ratio (Al:Ca). Cattle density had a positive correlation with the main gradient in the surface layer, all other climate and management factors were unrelated to soil conditions. Molinia caerulea had a positive relation with SOM, eCEC, basic cations and low Al:Ca ratio. Erica cinerea showed the opposite reaction at both scales. Ulex gallii showed a negative correlation with C:N ratio at the plot scale and deeper layer.ConclusionSOM accumulation, low nutrient levels and Al toxicity explain the uniqueness of E. mackayana heathland vegetation and soils. Main indicator species react to soil conditions at plot and landscape scales. Cattle density correlated positively with soil nutrient levels, but density of wild ponies is unrelated to soil conditions. Large herbivores, especially ponies, are needed for conservation management of these heathlands.