Total Above-ground Plant Biomass Research Articles

Compositional variation in grassland plant communities

AbstractHuman activities are altering ecological communities around the globe. Understanding the implications of these changes requires that we consider the composition of those communities. However, composition can be summarized by many metrics which in turn are influenced by different ecological processes. For example, incidence‐based metrics strongly reflect species gains or losses, while abundance‐based metrics are minimally affected by changes in the abundance of small or uncommon species. Furthermore, metrics might be correlated with different predictors. We used a globally distributed experiment to examine variation in species composition within 60 grasslands on six continents. Each site had an identical experimental and sampling design: 24 plots × 4 years. We expressed compositional variation within each site—not across sites—using abundance‐ and incidence‐based metrics of the magnitude of dissimilarity (Bray–Curtis and Sorensen, respectively), abundance‐ and incidence‐based measures of the relative importance of replacement (balanced variation and species turnover, respectively), and species richness at two scales (per plot‐year [alpha] and per site [gamma]). Average compositional variation among all plot‐years at a site was high and similar to spatial variation among plots in the pretreatment year, but lower among years in untreated plots. For both types of metrics, most variation was due to replacement rather than nestedness. Differences among sites in overall within‐site compositional variation were related to several predictors. Environmental heterogeneity (expressed as the CV of total aboveground plant biomass in unfertilized plots of the site) was an important predictor for most metrics. Biomass production was a predictor of species turnover and of alpha diversity but not of other metrics. Continentality (measured as annual temperature range) was a strong predictor of Sorensen dissimilarity. Metrics of compositional variation are moderately correlated: knowing the magnitude of dissimilarity at a site provides little insight into whether the variation is driven by replacement processes. Overall, our understanding of compositional variation at a site is enhanced by considering multiple metrics simultaneously. Monitoring programs that explicitly incorporate these implications, both when designing sampling strategies and analyzing data, will have a stronger ability to understand the compositional variation of systems and to quantify the impacts of human activities.

Drivers of species-specific contributions to the total live aboveground plant biomass in Central European semi-natural hay grasslands

Semi-natural grasslands are known to provide numerous ecosystem services, of which one of the most important is production of biomass. However, the contribution of individual plant species to the total biomass is much less well understood. This study addressed questions concerning community structure and responses of species-specific biomass (s-AGB) to gradients in soil acidity and fertility, topographical and climatic features, and disturbance regimes in mown and abandoned grasslands in the Sudetes Mountains (Central Europe). It identified pH as the most significant environmental gradient affecting turnover in s-AGB, and mowing cessation, temperature, and precipitation also had significant effects. Further, it showed high inequality in biomass among co-occurring plant species. It also showed that biomass inequality (measured by the Gini coefficient) among interacting species decreases with increasing functional diversity (Rao’s index). This study highlights that common plant species (in terms of frequency) play a major role in contributing to the total aboveground biomass (t-AGB). However, less frequent species are also significant contributors to the t-AGB. Thus, the combined contribution of infrequent species to the t-AGB should not be neglected. Our findings support the mass ratio hypothesis stating that ecosystem functions such as biomass production depend on dominant species. On the other hand, high niche differentiation ensures the coexistence of less competitive species with the dominants by the variety and complementarity of functional traits. Infrequent and non-dominant species were the core of the diversity seen in the studied grasslands. The maintenance of species diversity in grasslands should be prioritized in nature conservation policies to ensure the sustainability of ecosystem services.

Invertebrate herbivory accelerates shift towards forbs caused by warming in a sedge‐dominated alpine meadow

AbstractInvertebrate herbivory is suggested to modulate community shifts caused by climate warming in grasslands. However, experimental evidence is lacking, especially in high elevation areas. We examined the interactive effect of experimental warming and the grassland caterpillar Gynaephora alpherakjj, a typical invertebrate herbivorous insect with sedge preference, on the structure of plant community in a sedge‐dominated alpine meadow in the Tibetan Plateau. We manipulated both herbivore and temperature in a full factorial field experiment using passive warming chambers over one plant‐growing season. Results show that simulated warming significantly increased the aboveground plant biomass (APB) of forbs and total APB, while the presence of caterpillars significantly decreased sedge APB and total APB. The increase in consumption of sedges by caterpillars under warmed conditions can largely be ascribed to the increase in feeding time of caterpillars. Importantly, the presence of caterpillars significantly decreased the APB ratio of sedges to forbs in the warmed but not in the unwarmed chambers. These results suggest that with a warming climate, invertebrate herbivory could accelerate community shifts from sedge dominance to forb dominance in only one plant‐growing season in the Tibetan alpine meadow. This shift would reduce the biomass of high‐quality forage and further threaten animal husbandry in this area. Considering the harm of G. alpherakjj to grasslands, its population should be controlled more strictly under the global warming scenario.

氮水添加对放牧背景下荒漠草原生产力的影响

PDF HTML阅读 XML下载 导出引用 引用提醒 氮水添加对放牧背景下荒漠草原生产力的影响 DOI: 10.5846/stxb202011112916 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金地区基金(31960246,31860138);内蒙古农牧业创新基金项目(2020CXJJM11);内蒙古科技计划项目(2019GG015) Influence of nitrogen and water addition on the primary productivity of Stipa breviflora in a desert steppe under different grazing intensities Author: Affiliation: Fund Project: 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:水分与氮素作为干旱和半干旱草原生产力的共同限制性因子在退化草原的生态快速修复过程中备受关注。以不同放牧强度背景下的短花针茅荒漠草原为研究对象,开展围封模拟放牧利用实验,同时添加氮素和水分。通过分析历史放牧强度与年份对生产力的影响,以及添加氮素和水分对不同功能群植物生物量的作用,探讨放牧强度对短花针茅草原生产力的内在作用机制,以及如何实现荒漠草原资源合理开发和可持续利用。研究结果显示,降雨量与放牧强度决定着短花针茅草原的植物群落结构。氮素和水分添加可分别提升11%-29%和12%-32%的群落地上生物量,且二者存在显著的交互作用。不同功能群植物的地上生物量对氮素与水分添加的响应存在差异,多年生丛生禾草对氮素和水分添加响应最敏感。氮素与水分添加可显著提高多年生丛生禾草的地上生物量,但与自然降水量相关。氮素添加对地上生物量的影响在正常降雨和稍旱年份作用显著,而水分添加在干旱年份作用显著。在正常降雨年份,以半灌木植物为优势种的轻度放牧背景以添加水分对提升生产力最宜,以多年生丛生禾草和半灌木为共优种的中度放牧背景和以多年生丛生禾草为优势种的重度放牧以同时添加水分和氮素对提升生产力最为宜;在干旱年份不同放牧强度背景下均以同时添加水分和氮素对提升生产力最为宜。我们的结果表明了养分与资源的改善有利于退化短花针茅草原的快速恢复和可持续生产。 Abstract:Nitrogen and water as a co-limitation of primary productivity in steppe has attracted much attention in the rapid restoration of the degraded grassland. In this study, we take desert steppe of Stipa breviflora under different grazing intensities as study object, and simulation grazing, nitrogen addition, and water addition were carried out at the fenced site. By analyzing the effects of historical grazing intensity, year, nitrogen addition, and water addition on total aboveground biomass and plant biomass of different functional groups, we discovered the regulation mechanism of sustainable utilization of Stipa breviflora in a desert steppe under different grazing intensities. The results showed that rainfall and grazing intensity determined the structure of plant community in the desert steppe of Stipa breviflora. Nitrogen addition and water addition enhanced total aboveground biomass by 11%-29% and 12%-32%, respectively, with significantly interaction. The response of plant growth to nitrogen and water addition was different in different functional groups, and perennial bunch grasses were most sensitive to nitrogen and water addition. Nitrogen addition and water addition significantly increased aboveground biomass affecting the perennial bunch grasses, but associated with rainfall. The effect of nitrogen addition on aboveground biomass was significant in normal rainfall and lightly dry years, while water addition was significant in dry years. In normal rainfall years, water addition to increase productivity is most appropriate in the light grazing grassland which is with semi-shrubs as dominant species, while combined nitrogen and water addition to enhance productivity is most appropriate in the moderate grazing grassland which is with semi-shrubs and perennial bunch grasses as co-dominant species, and in the heavy grazing grassland which is with perennial bunch grasses as dominant species. In dry years, combined nitrogen and water addition to increase productivity is most appropriate in grassland under different grazing intensities. Our results show that the improvement of nutrients and resources is beneficial to the rapid recovery and sustainable production of the degraded Stipa breviflora steppe. 参考文献 相似文献 引证文献

Nitrogen dynamics in grain cropping systems integrating multiple ecologically based management strategies

AbstractNitrogen (N) management is a critical agronomic challenge, as N losses are a source of pollution affecting both waterways and air quality and a potential economic loss for farmers. One approach to N conservation is through ecologically based agricultural systems that reduce tillage and incorporate cover crops. However, these systems exhibit considerable complexity resulting in potential agronomic trade‐offs. To address these concerns, four crop management systems were implemented within an organically managed corn–soy–winter grain crop rotation. These systems varied in tillage frequency and intensity, cover crop species selection, cover crop termination and establishment methods, fertilizer management, and cash crop season length. We used field measurements to investigate the impact of each system on N pools and to reveal the strengths and weaknesses of each system in addressing N provisioning services, with a focus on the supply and retention of N before and after the corn phase of the rotation. All systems had greater estimated N inputs (via manure and N‐fixation) than outputs (via crop harvest) at the end of the three‐year rotation, demonstrating the importance of prioritizing N retention in cover crops. Interactions among system components were important drivers of temporal N dynamics; cover crop species traits and timing of manure application contributed to differences in total aboveground plant biomass N among systems. For example, one cropping system which included a no‐till corn planting into a rolled cover crop mulch had soil inorganic N availability that was asynchronous with the N needs of the corn crop even though it received the same amount of N inputs as the other systems. In general, neither interseeding cover crop mixtures nor reducing tillage resulted in marked N benefits at the system level; we did not observe improved N retention from either practice in these systems, and there was no increase in N uptake by corn. What did clearly emerge from this experiment is the importance of managing for synchrony between soil inorganic N availability and cash crop N demand as influenced by the N retention capacity of cover crops and the timing of N mineralization due to tillage.

Recovery of the salt marsh periwinkle (Littoraria irrorata) 9 years after the Deepwater Horizon oil spill: Size matters

Prior studies indicated salt marsh periwinkles (Littoraria irrorata) were strongly impacted in heavily oiled marshes for at least 5 years following the Deepwater Horizon oil spill. Here, we detail longer-term effects and recovery over nine years. Our analysis found that neither density nor population size structure recovered at heavily oiled sites where snails were smaller and variability in size structure and density was increased. Total aboveground live plant biomass and stem density remained lower over time in heavily oiled marshes, and we speculate that the resulting more open canopy stimulated benthic microalgal production contributing to high spring periwinkle densities or that the lower stem density reduced the ability of subadults and small adults to escape predation. Our data indicate that periwinkle population recovery may take one to two decades after the oil spill at moderately oiled and heavily oiled sites, respectively.

Carbon Stocks in Chir Pine (Pinus roxburghii) Forests on Two Different Aspects in the Mahabharat Region of Makawanpur, Nepal

Despite the significant contribution of forests in climate change mitigation, studies to establish the potential of sub-tropical forest ecosystems at different aspects in enhancing soil health indicators are only partly known. The study was carried out to quantify vegetation and soil carbon stocks of a natural Chir Pine (Pinus roxburghii) forest at two different aspects (northern and southern) of a typical sub-tropical environment in Nepal. Stratified random sampling was used for forest inventory and soil sample collection. Aboveground forest biomass was calculated using standard allometric models. Soil was sampled up to 60 cm depth and at 20 cm intervals. Walkey and Black method was used to determine soil organic carbon. Total aboveground plant biomass carbon in southern aspect (140.20 t ha-1) was higher compared to that on the northern aspect (115.34 t ha-1). Similarly, soil carbon stock on southern aspect (46.65 t ha-1) was higher than that of northern aspect (42.14 t ha-1). This resulted to total carbon stock on southern and northern aspect of P. roxburghii forest of 186.85 t ha-1 and 157.48 t ha-1 respectively. The total carbon stock of P. roxburghii forest is significantly higher on southern aspect than on northern aspect with p value 0.001 (p<0.05). Hence, we conclude that the southern aspect of the Mahabharat range favour the growth of P. roxburghii forest compared to the northern aspect. However, the contribution of the entire Chir pine forest ecosystem to carbon sequestration and global climate warming mitigation can’t be neglected.

Estimating pasture species biomass from canopy cover

AbstractMany tools are available to measure total aboveground plant biomass in pastures, for instance the rising plate meter, but measuring the biomass contribution of individual species requires time‐consuming clipping and sorting. Pasture composition determines forage quality, carbon storage, and other ecosystem services, so an efficient nondestructive method for estimating biomass by species would be of great value. Seasonal allometric coefficients relating canopy cover to aboveground biomass were developed for individual species and for functional groups (grass, large forb, small forb) using ordinary least squares regression on data from a grazed multi‐species experimental pasture in Pennsylvania, and tested on an independent grazed experimental study. The models performed better in autumn than in spring, although performance varied greatly among species. The functional group models were generally more accurate than species‐specific models, even for estimating single species. Total grass and forb biomasses were predicted moderately well regardless of season. Unlike many allometric models, these were developed and tested on multiple years of data from grazed multispecies pastures. Although variability was high, the results have broad applicability in highly fertile mesic pastures, at least for dominant species.

Grass-Legume Mixtures for Improved Soil Health in Cultivated Agroecosystem

Planting grass-legume mixtures may be a good option to improve soil health in addition to increased forage productivity, improved forage nutritive value, and net farm profit in a hay production system. A field experiment was conducted from 2011 to 2014 at Lingle, Wyoming to evaluate soil microbial biomass under different seeding proportions of two forage grasses (meadow bromegrass, Bromus biebersteinii Roem. & Schult.; and orchardgrass, Dactylis glomerata L.) and one legume (alfalfa, Medicago sativa L.). Nine treatments included monoculture grass, monoculture legume, one grass and one legume mixture, two grasses and one legume mixture, and a control (not seeded with grass or legume). Monoculture grass received either no nitrogen (N) or N fertilizer (150 kg N ha−1 year−1 as urea) whereas monoculture legume, grass-legume mixtures, and control plots received no N fertilizer. The study was laid out as a randomized complete block design with three replications. The plots were harvested 3–4 times each year after the establishment year. Soil samples were collected and analyzed for microbial biomass using phospholipid fatty acid (PLFA) analysis at the end of May in 2013 and 2014. Soil samples were also analyzed for mineralizable carbon (C) and N in 2013 and 2014. The total above-ground plant biomass was higher in 50–50% mixture of grass and alfalfa than monoculture alfalfa and monoculture grass (with and without N fertilizer) during the entire study period. The application of N fertilizer to the grass hay production system had little effect on improving mineralizable soil C, N, and soil microbial biomass. However, grass-legume mixture without N fertilizer had great effect on improvement of mineralizable soil C and N, and total, bacterial, and actinomycetes microbial biomass in soil. The 50–50% mixture of grass and alfalfa performed consistently well and can be considered to use in Wyoming conditions for improving soil health and forage productivity.

Effects of biochar and fertilizer management on sunflower (Helianthus annuus L.) feedstock and soil properties

ABSTRACTThe need for bioenergy is increasing with increase in global energy demand, and sustainable soil and fertilizer management practices for bioenergy feedstock production are gaining importance. In this greenhouse study, we evaluated the effects of biochar and fertilizer nitrogen on soil and energy crop sunflower (Helianthus annuus L. var. Giganteus). Sunflower plants were treated with three rates of biochar, control (0 Mg ha−1), low (25 Mg ha−1) and high (50 Mg ha−1), and three rates of fertilizers, 0% (control), 50% (low) and 100% (high) of the recommended nitrogen dose. Plant height, quality (chlorophyll content), biomass yield, feedstock energy, ash content and tissue nutrients were measured along with soil moisture and pH. Results showed an 11% increase in mean plant height under low biochar compared to control biochar-treated plants. High nitrogen treatment produced 26% and 18% more stalk and total above-ground plant (whole plant) biomass, respectively, compared to the control nitrogen treatment. High biochar treatment resulted in higher soil moisture holding, but lower soil pH than the control biochar treatment. Plant quality, energy and ash contents were not affected by either biochar or nitrogen. The plant tissue analysis provides a complete tissue macro- and micronutrient information on sunflower cultivar Giganteus, which was not done previously.

Total aboveground plant biomass is more strongly affected by climate than species diversity on a grassland in Liaoning, China

The objective of this study was to analyze the relationships between total aboveground plant biomass versus altitude, latitude, longitude, mean annual temperature, mean annual precipitation, and species diversity. Simple linear regression analysis was used to study these relationships. Results showed that altitude was significantly, positively (R2= 0.038, P

Distinct impacts of different mammalian herbivore assemblages on arctic tundra CO2 exchange during the peak of the growing season

Herbivores play a key role in the carbon (C) cycle of arctic ecosystems, but these effects are currently poorly represented within models predicting land–atmosphere interactions under future climate change. Although some studies have examined the influence of various individual species of herbivores on tundra C sequestration, few studies have directly compared the effects of different herbivore assemblages. We measured peak growing season instantaneous ecosystem carbon dioxide (CO2) exchange (photosynthesis, respiration and net ecosystem exchange) on replicated plots in arctic tundra which, for 14 years, have excluded different portions of the herbivore population (grazed controls, large mammals excluded, both small and large mammals excluded). Herbivory suppressed photosynthetic CO2 uptake, but caused little change in ecosystem respiration. Despite evidence that small mammals consume a greater portion of plant biomass in these ecosystems, the effect of excluding only large herbivores was indistinguishable from that of excluding both large and small mammals. The herbivory‐induced decline in photosynthesis was not entirely attributable to a decline in leaf area but also likely reflects shifts in plant community composition and/or species physiology. One shrub species – Betula nana – accounted for only around 13% of total aboveground vascular plant biomass but played a central role in controlling ecosystem CO2 uptake and release, and was suppressed by herbivory. We conclude that herbivores can have large effects on ecosystem C cycling due to shifts in plant aboveground biomass and community composition. An improved understanding of the mechanisms underlying the distinct ecosystem impacts of different herbivore groups will help to more accurately predict the net impacts of diverse herbivore communities on arctic C fluxes.

Low Concentrations of Silver Nanoparticles in Biosolids Cause Adverse Ecosystem Responses under Realistic Field Scenario

A large fraction of engineered nanomaterials in consumer and commercial products will reach natural ecosystems. To date, research on the biological impacts of environmental nanomaterial exposures has largely focused on high-concentration exposures in mechanistic lab studies with single strains of model organisms. These results are difficult to extrapolate to ecosystems, where exposures will likely be at low-concentrations and which are inhabited by a diversity of organisms. Here we show adverse responses of plants and microorganisms in a replicated long-term terrestrial mesocosm field experiment following a single low dose of silver nanoparticles (0.14 mg Ag kg−1 soil) applied via a likely route of exposure, sewage biosolid application. While total aboveground plant biomass did not differ between treatments receiving biosolids, one plant species, Microstegium vimeneum, had 32 % less biomass in the Slurry+AgNP treatment relative to the Slurry only treatment. Microorganisms were also affected by AgNP treatment, which gave a significantly different community composition of bacteria in the Slurry+AgNPs as opposed to the Slurry treatment one day after addition as analyzed by T-RFLP analysis of 16S-rRNA genes. After eight days, N2O flux was 4.5 fold higher in the Slurry+AgNPs treatment than the Slurry treatment. After fifty days, community composition and N2O flux of the Slurry+AgNPs treatment converged with the Slurry. However, the soil microbial extracellular enzymes leucine amino peptidase and phosphatase had 52 and 27% lower activities, respectively, while microbial biomass was 35% lower than the Slurry. We also show that the magnitude of these responses was in all cases as large as or larger than the positive control, AgNO3, added at 4-fold the Ag concentration of the silver nanoparticles.

Salt marsh restoration with sediment-slurry application: Effects on benthic macroinvertebrates and associated soil–plant variables

We analyzed the effects of various levels of sediment-slurry addition on the restoration of the macroinvertebrate community and its related habitat (i.e., sediment and vegetation) 7 years after application to a subsided Louisiana salt marsh affected by sudden marsh dieback. Moderate sediment additions restored macroinvertebrate species richness, diversity, density, and total biomass to levels equivalent to those in reference marshes, although individual species and taxa had variable recovery depending on treatment-level. Total aboveground plant biomass and live Spartina alterniflora biomass, stem density and height were equivalent to those in reference marshes. In contrast, total belowground biomass had not yet reached equivalency with reference marshes. Although moderate sediment application created conditions that were ecologically equivalent with reference marshes for most macroinvertebrate and plant variables, degraded areas that received high sediment addition had impaired recovery across all metrics, even 7 years after sediment application. Thus, when sediment-slurries are applied to proper elevations, the macroinvertebrate community, as well as aboveground marsh vegetation, can recover to reference conditions. However, too much sediment impairs recovery. Consequently, greater consideration must be given to establishing suitable post-construction marsh elevations to insure successful ecosystem restoration.

Long-term microbial control of nutrient availability and plant biomass in a subarctic-alpine heath after addition of carbon, fertilizer and fungicide

Abstract A long-term field experiment lasting more than a decade was conducted on a subarctic fellfield to investigate effects of changes in nutrient availability on soil microbial C, N and P, soil nutrients, vascular plant biomass and plant-microbial interactions. Additions of NPK fertilizer, labile C (sugar) and fungicide (benomyl) were done in a fully factorial design, replicated in six blocks. The treatments were run for ten years and soil and vegetation samples were collected four years after initiating the experiment, and again after an additional 12 years, to evaluate the long-term effects. Labile C addition resulted in increased microbial biomass and nutrient immobilization after four years, and a long-term decrease in vascular plant biomass, thus suggesting the microorganisms to strongly control soil nutrient availability in periods of high microbial biomass. Fertilization increased the inorganic and total soil nutrient pools of N and P and the fine root biomass, but not the total aboveground vascular plant biomass. The vascular plant biomass increased due to benomyl addition thus indicating the plants to be strongly affected by the microbial community. Overall, the effects of benomyl resulted in more lasting changes in the soil compared to labile C and fertilizer addition. In relation to environmental changes, the indicated strong microbial control of the available nutrients in the fellfield ecosystem might limit ecosystem changes due to increased soil nutrient availability as otherwise expected in arctic soils.

Improving weed management in organic spring barley: physical weed control vs. interspecific competition

Kolb LN, Gallandt ER & Molloy T (2010). Improving weed management in organic spring barley: physical weed control vs. interspecific competition. Weed Research 50, 597–605. Summary Few options exist for effectively managing weeds in organic spring cereals. Establishing a competitive crop through increased seeding rate provides improved weed suppression, but competition with inter-row weeds is delayed, compared with an alternative, more uniform spatial pattern of sowing. We speculated that (i) enhanced interspecific competition or (ii) improved physical weed control with a strategy of wide-row sowing and inter-row hoeing would both provide better weed control than current regional organic practices. However, whether one of these strategies is superior to the other depends on an economic analysis of the relative costs of seed and equipment. We tested this hypothesis in two field experiments with spring barley (Hordeum vulgare) at three row spacings (11.4, 17.7 and 22.8 cm), and by sowing in a more uniform pattern, using condiment mustard (Sinapis alba) as a surrogate weed. Standard regional organic practice (17.7 cm rows, 200 plants m−2) had the highest weed biomass at the conclusion of both seasons. Weeds in this treatment accounted for 57% of total above-ground plant biomass in 2007 and 27% in 2008. The more uniform sowing pattern reduced weed biomass by 26% compared with the regional standard, averaged over 2 years of the experiment. Although we expected enhancing crop competition to be the most economical practice, high cost of organic seed resulted in intermediate net returns. At high weed density, the wide row strategy offered improved weed control, yields and profitability. However, with lower weed density, spring barley was sufficiently competitive that it did not benefit from cultivation.

Retention of surface nitrate additions in a temperate old field: implications for atmospheric nitrogen deposition over winter and plant nitrogen availability

In many temperate ecosystems, rates of atmospheric nitrogen deposition remain high over winter despite decreased agricultural activity over this season. The extent to which this nitrogen is accessible for plant growth over the following growing season may depend strongly on uptake by plants and soil microorganisms from late fall through early spring, when the majority of aboveground plant tissue has senesced. We added Ca(15NO3)2 (5 atom %15N) at a rate of 2 g m−2 of N (corresponding to 100 mg 15N m−2) to the surface of plots in a temperate old field during either late fall, winter, spring melt or early spring. We quantified the recovery of excess 15N in the soil microbial biomass and soil extracts following spring melt and in aboveground plant tissue at the peak of the plant growing season. Nitrate additions had no significant effect on total aboveground plant biomass, relative species abundance or percent tissue nitrogen. However, mean excess 15N in aboveground plant tissue varied significantly among treatments, with values of 8.1, 2.6, 0.3 and 7.3 mg m−2 for late fall, winter, spring melt and early spring addition plots, respectively. Corresponding values of excess 15N were 3.1, 1.4 and 0.2 mg m−2 in microbial biomass, and 0.17, 0.07 and 0.03 mg m−2 in soil extracts, for late fall, winter and spring melt addition plots, respectively. Overall, these results indicate that nitrogen retention from late fall through early spring may depend highly on plant uptake in this system, and that only a small fraction of the nitrogen that accumulates in the winter snow pack may be available to plants.

Spatial Patterns and Dynamic Responses of Arctic Food Webs Corroborate the Exploitation Ecosystems Hypothesis (EEH)

According to the exploitation ecosystems hypothesis (EEH), productive terrestrial ecosystems are characterized by community-level trophic cascades, whereas unproductive ecosystems harbor food-limited grazers, which regulate community-level plant biomass. We tested this hypothesis along arctic-alpine productivity gradients at the Joatka field base, Finnmark, Norway. In unproductive habitats, mammalian predators were absent and plant biomass was constant, whereas herbivore biomass varied, reflecting the productivity of the habitat. In productive habitats, predatory mammals were persistently present and plant biomass varied in space, but herbivore biomass did not. Plant biomass of productive tundra scrublands declined by 40% when vegetation blocks were transferred to predation-free islands. Corresponding transfer to herbivore-free islands triggered an increase in plant biomass. Fertilization of an unproductive tundra heath resulted in a fourfold increase in rodent density and a corresponding increase in winter grazing activity, whereas the total aboveground plant biomass remained unchanged. These results corroborate the predictions of the EEH, implying that the endotherm community and the vegetation of the North European tundra behaves dynamically as if each trophic level consisted of a single population, in spite of local co-occurrence of >20 plant species representing different major taxonomic groups, growth forms, and defensive strategies.

Seasonal and Diurnal Net Methane Emissions from Organic Soils of the Eastern Alps, Austria: Effects of Soil Temperature, Water Balance, and Plant Biomass

ABSTRACTAlthough the contribution of methane emission to global change is well recognized, analyses of net methane emissions derived from alpine regions are rare. Therefore, three fen sites differing in water balance and plant community, as well as one dry meadow site, were used to study the importance of soil temperature, water table, and plant biomass as controlling factors for net methane emission in the Eastern Alps, Europe, during a period of 24 months. Average methane emissions during snow-free periods in the fen ranged between 19 and 116 mg CH4 m−2 d−1. Mean wintertime emissions were much lower and accounted for 18 to 59% of annual flux. The alpine dry meadow functions as a methane sink during snow-free periods, with mean flux of −2.1 mg CH4 m−2 d−1 (2003) and −1.0 mg CH4 m−2 d−1 (2004). Seasonal methane emissions of the fen were related to soil temperature and groundwater table. During the snow-free periods the water table was the main control for seasonal methane emission. The net methane flux related to water table was much higher for the distinctly drier year 2003 than for the wetter year 2004. Methane emissions differed diurnally at sites where the water table position was high or very low. The influence of total above-ground plant biomass on methane emission was apparent only for those sites with high water table positions. Seasonal and diurnal methane uptake of the dry meadow was related to soil temperature and water-filled pore space, whereas plant biomass did not significantly influence methane fluxes. Our studies gave evidence that fens in the Eastern Alps act as a source of methane throughout the whole year and that a dry meadow site acts as a net methane sink during snow-free periods.

Modification of competition between two grass species by a hemiparasitic plant and simulated grazing

Plant parasitism and herbivory are common phenomena in natural grasslands, where they may significantly affect competition between plant species. However, only few studies have simultaneously examined these two processes. We investigated whether the root hemiparasite Odontites litoralis ssp. litoralis affects the outcome of competition between two clonal graminoids, the endangered Puccinellia phryganodes and the common species Agrostis stolonifera, and whether simulated grazing affects the interaction among these three species. This study system simulates the community of early successional stages of the Bothnian Bay salt marsh meadows, which are intensively grazed by greylag geese ( Anser anser ). We conducted a factorial greenhouse experiment to study the effects of interspecific competition (one or two host species present), hemiparasitic infection (hemiparasite present or not), and simulated grazing (host clipped or not) on Puccinellia and Agrostis. Puccinellia was clearly an inferior competitor to Agrostis , whereas the two species did not differ as hosts for the hemiparasite. Infection by the hemiparasite reduced the aboveground biomass of Puccinellia and Agrostis by 59% and 45%, respectively. Competition with Agrostis decreased the biomass of parasitised Puccinellia by 36% and that of non-parasitised Puccinellia by 56%. Parasitism thus seemed to benefit Puccinellia indirectly by decreasing the relative competitive advantage of Agrostis . Moreover, parasitism increased the relative contribution of Puccinellia to the total aboveground host plant biomass. Simulated grazing decreased the aboveground biomass of Agrostis significantly more than that of Puccinellia and thus increased the competitive ability of Puccinellia . Simulated grazing of the two host species did not affect the performance of Odontites . These results suggest that both hemiparasitic plants and herbivory may play a significant role in the maintenance of plant species diversity by promoting competitively inferior species. Parasitierung und Herbivorie sind verbreitete Phänomene in natürlichen Grasländern, wo sie die Konkurrenz zwischen Pflanzenarten maßgeblich beeinflussen können. Indessen haben nur wenige Studien die beiden Prozesse gleichzeitig betrachtet. Wir untersuchten, ob der Wurzelhemiparasit Odontites litoralis ssp. litoralis das Ergebnis der Konkurrenz zwischen zwei klonalen Gräsern, der gefährdeten Art Puccinellia phryganodes und der häufigen Art Agrostis stolonifera , beeinflusst und ob sich simulierte Beweidung auf die Interaktionen zwischen den drei Arten auswirkt. Das untersuchte System bildet die Gemeinschaft der frühen Sukzessionsstadien der Salzwiesen im nördlichen Teil des Bottnischen Meerbusens nach, die intensiv von Graugänsen ( Anser anser ) beweidet werden. In einem faktoriellen Gewächshausexperiment untersuchten wir die Effekte von interspezifischer Konkurrenz (eine oder beide Wirtsarten vorhanden), des Befalls mit dem Hemiparasiten (Hemiparasit vorhanden oder nicht), und von simulierter Beweidung (Wirtspflanzen beschnitten oder nicht) auf die beiden Gräser. Puccinellia war Agrostis als Konkurrent eindeutig unterlegen, während beide Arten sich als Wirte für den Hemiparasiten nicht unterschieden. Befall durch den Hemiparasiten reduzierte die oberirdische Biomasse von Puccinellia und Agrostis um 59 bzw. 45%. Konkurrenz mit Agrostis verminderte die Biomasse parasitierter Puccinellia um 36% und die von nicht parasitierter Puccinellia um 56%. Parasitierung schien deshalb Puccinellia indirekt zu begünstigen, indem der relative Konkurrenzvorteil von Agrostis reduziert wurde. Außerdem erhöhte die Parasitierung den relativen Beitrag von Puccinellia zur oberirdischen Gesamt-Pflanzenbiomasse. Simulierte Beweidung verringerte die oberirdische Biomasse von Agrostis signifikant stärker als die von Puccinellia und stärkte somit die Konkurrenzkraft der zweiten Art. Die simulierte Beweidung der Wirtsarten beeinflusste die Performanz von Odontites nicht. Diese Ergebnisse legen nahe, daß sowohl hemiparasitische Pflanzen als auch Beweidung eine wichtige Rolle bei der Erhaltung der Artenvielfalt der Pflanzen spielen können, indem unterlegene Konkurrenten gefördert werden.