草地利用方式对温性典型草原优势种植物功能性状的影响

当外界环境发生变化后植物能够改变自身功能性状及时调整适应策略, 因此植物功能性状能够有效地反映植物对草地利用变化的响应, 然而在内蒙古草原从植物功能性状角度开展草地利用方式影响的研究略少。该研究以内蒙古典型草原大针茅(Stipa grandis)、羊草(Leymus chinensis)、糙隐子草(Cleistogenes squarrosa)和冷蒿(Artemisia frigida) 4种主要优势种为研究对象, 探讨在长期自由放牧、割草、短期围封和长期无干扰的影响下优势种植物功能性状的差异, 以期从功能性状视角, 揭示植物在受到外界干扰后的适应策略, 旨为天然草地的可持续管理提供基础数据支持和科学依据。结果表明: 1)除糙隐子草外, 在长期放牧后内蒙古典型草原优势植物植株高度、根长和植物碳氮含量降低, 这些性状的变化能够使植物个体小型化, 适口性降低, 表明植物通过逃避放牧的策略适应长期自由放牧的干扰; 在割草管理方式下, 优势种的高度和比叶面积有增加的趋势, 其中冷蒿的氮含量对割草响应最敏感, 其根、茎、叶中的氮含量均在割草样地最低; 围封和长期无干扰处理下植物的碳氮含量增加, 表明在干扰强度降低后, 植物通过功能性状的改变从资源获取策略向资源储藏策略转变。2)对优势种功能性状集合分析表明, 糙隐子草具有较低的植株高度和较高的比叶面积, 冷蒿具有较高的木质素含量和氮含量, 这些性状能够使两种植物被家畜采食量减少, 并保证其具有较强的再生能力, 这可能是糙隐子草和冷蒿耐牧的原因; 大针茅具有最高的植株高度、最大的叶片干物质含量, 以及最高的茎、叶纤维素含量, 说明大针茅是非常典型的竞争物种, 在干扰较低的条件下, 大针茅采取竞争策略对其他物种产生较大的竞争压力可能是其占优势的重要原因。     

Incorporation of mineral nitrogen into the soil food web as affected by plant community composition

Although nitrogen (N) deposition is increasing globally, N availability still limits many organisms, such as microorganisms and mesofauna. However, little is known to which extent soil organisms rely on mineral‐derived N and whether plant community composition modifies its incorporation into soil food webs. More diverse plant communities more effectively compete with microorganisms for mineral N likely reducing the incorporation of mineral‐derived N into soil food webs. We set up a field experiment in experimental grasslands with different levels of plant species and functional group richness. We labeled soil with ¹⁵NH₄ ¹⁵NO₃ and analyzed the incorporation of mineral‐derived ¹⁵N into soil microorganisms and mesofauna over 3 months. Mineral‐derived N incorporation decreased over time in all investigated organisms. Plant species richness and presence of legumes reduced the uptake of mineral‐derived N into microorganisms. In parallel, the incorporation of mineral‐derived ¹⁵N into mesofauna species declined with time and decreased with increasing plant species richness in the secondary decomposer springtail Ceratophysella sp. Effects of both plant species richness and functional group richness on other mesofauna species varied with time. The presence of grasses increased the ¹⁵N incorporation into Ceratophysella sp., but decreased it in the primary decomposer oribatid mite Tectocepheus velatus sarekensis. The results highlight that mineral N is quickly channeled into soil animal food webs via microorganisms irrespective of plant diversity. The amount of mineral‐derived N incorporated into soil animals, and the plant community properties affecting this incorporation, differed markedly between soil animal taxa, reflecting species‐specific use of food resources. Our results highlight that plant diversity and community composition alter the competition for N in soil and change the transfer of N across trophic levels in soil food webs, potentially leading to changes in soil animal population dynamics and community composition. Sustaining high plant diversity may buffer detrimental effects of elevated N deposition on soil biota.     

Restoration of indigenous dominance in exotic grassland by the establishment of juvenile Microlaena stipoides plants

Two experiments were conducted to determine the establishment success of reintroducing Microlaena stipoides (pātītī, weeping rice grass) into existing high‐fertility grassland on the volcanic cones of the Auckland Isthmus. The first experiment monitored the survival and development of juveniles planted in a factorial design including two planting densities, two slope classes and two aspects across three cones. Plant survival during establishment was consistently over 90%. Maximum M. stipoides cover after 2 years (>80%) was achieved on north‐facing steep slopes (>25°) at the greater planting density (40 plants/m²). However, results were particularly idiosyncratic to specific cone/topographical combinations. The second experiment, on a flat site on one cone, monitored the survival and development of juveniles planted across four post‐planting treatments designed to suppress resident vegetation recovery. Plant survival after 6 months was relatively low (50%), and none of the treatments achieved greater than 5% cover of M. stipoides after 1 year. The chosen post‐planting treatments were unable to suppress vigorous recovery of competitive exotic grasses on a moist fertile site. Overall, juvenile planting was shown to be a potentially successful method of Microlaena establishment and could restore indigenous dominance to exotic grassland in this environment, but individual site factors and the high cost of establishment must be considered.     

The interplay between local and landscape scales on the density of pond-dwelling anurans in subtropical grasslands

We investigated the role of local and landscape environmental variables on anurans density classified as habitat specialists and generalists in grassland landscapes, known as South Brazilian grasslands (SBG). In this region, we surveyed 187 ponds distributed over 40 landscape sampling units. For each pond, 31 local environmental variables were measured. Each landscape sampling unit was embedded within a larger regional sampling unit with different landscape properties. For each landscape and regional sampling units, 16 landscape metrics were extracted from a land cover and use map. We recorded 35 species, eleven of which are specialists in the SBG. The specialists were affected by 11 local and 2 landscape environmental variables, while generalists were affected by 14 local and one landscape environmental variable. Thus, specialists and generalists presented different relationships with local and landscape variables, but in general local variables had a greater influence on the density of anurans than the landscape variables. However, the landscape indirectly influenced local variables because higher quality ponds were in landscapes with higher percentages of natural habitat. In conclusion, reproductive sites with higher local quality and located within landscapes with higher percentages of natural grasslands are essential to conserve anurans in this habitat. Effective conservation of such sites would benefit from further studies that assess effects of land use and biotic integrity of ponds, which can help to determine (a) the relative effects of local habitat quality of ponds and (b) the effectiveness of protecting ponds and their local surroundings for anuran conservation in SBG. Abstract in Portuguese is available with online material.     

The acceptability of meadow plants to the slug Deroceras reticulatum and implications for grassland restoration

Background and Aims

Despite the selective pressure slugs may exert on seedling recruitment there is a lack of information in this context within grassland restoration studies. Selective grazing is influenced by interspecific differences in acceptability. As part of a larger study of how slug–seedling interactions may influence upland hay meadow restoration, an assessment of relative acceptability is made for seedlings of meadow plants to the slug, Deroceras reticulatum.

Methods

Slug feeding damage to seedling monocultures of 23 meadow species and Brassica napus was assessed in microcosms over 14 d. The severity and rate of damage incurred by each plant species was analysed with a generalized additive mixed model. Plant species were then ranked for their relative acceptability.

Key Results

Interspecific variation in relative acceptability suggested seedlings of meadow species form a hierarchy of acceptability to D. reticulatum. The four most acceptable species were Achillea millefolium and the grasses Holcus lanatus, Poa trivialis and Festuca rubra. Trifolium pratense was acceptable to D. reticulatum and was the second highest ranking forb species. The most unacceptable species were mainly forbs associated with the target grassland, and included Geranium sylvaticum, Rumex acetosa, Leontodon hispidus and the grass Anthoxanthum odoratum. A strong positive correlation was found for mean cumulative feeding damage and cumulative seedling mortality at day 14.

Conclusions

Highly unacceptable species to D. reticulatum are unlikely to be selectively grazed by slugs during the seedling recruitment phase, and were predominantly target restoration species. Seedlings of highly acceptable species may be less likely to survive slug herbivory and contribute to seedling recruitment at restoration sites. Selective slug herbivory, influenced by acceptability, may influence community-level processes if seedling recruitment and establishment of key functional species, such as T. pratense is reduced.     

A study of the abundance and patchiness of cicada nymphs (Homoptera: Tibicinidae) in a New Zealand subalpine shrub-grassland

Abstract

Cicada emergence skins in a subalpine shrub grassland have been sampled during 1969–75 to determine the abundance and spatial distributions of nymphs feeding on plant roots. A guild of six cicada species is primarily associated with two forms of vegetation: shrubs (Dracophyllum and Cassinia) and tall tussock (Chionochloa). Skin locations were mapped relative to dominant vegetation species, litter zones, and soil and rock pavements over a range of aspects, altitudes, and vegetation types, and sampling methods were scaled at four levels: the locality, plot, quadrat, and individual plant. There were significant differences in skin counts over four years, and different measures of mean skin densities are given for the four sampling scales. The two primary vegetation types had cumulative 1969–72 mean densities of 5.2 ± 4.0 and 12.9 ± 10.0 skins/quadrat (2.3 m²) , and the 1969–72 mean productivities of the upper 25% of quadrats (adjusted for percent ground cover) were, respectively, 5.5 and 35.5 skins/m². These productivities are believed to be conservative estimates of the maturing nymph numbers per individual host plant over the span of one cicada generation. Over a 17-year span, such productivities lie within the upper range of mean densities recorded for 17-year periodical cicadas in the United States. As the dominant subalpine vegetation species are very slow-growing,it is suggested that high densities of nymphs feeding on root sap may affect plant vitality, although 1971/1987 comparisons of vitality in 52 Chionochloa tussocks could not positively demonstrate a correlation across all data. Skin dispersion analyses indicated significant levels of patchiness, in agreement with other nymphal studies and with known cicada oviposition behaviour. No single dispersion model fitted the data comprehensively, and it is suggested that a gradual shifting of the centres of cicada aggregation may occur over a cumulative period of several generations.     

TAXONOMIC STATUS OF

Association of Cattle Egrets Bubulcus ibis with large herbivores is well documented, but there are few records of their association with large birds. Here we describe the first-known records of foraging interactions between Shoebill Balaeniceps rex and Cattle Egrets. The observations were made at the Malagarasi-Muyovozi Ramsar Site in western Tanzania. Small flocks of egrets approached and foraged within 5 m of a Shoebill, which was sometimes forced to move from its hunting pool and by doing so it likely flushed more prey for egrets. Interactions occurred almost exclusively in the driest months, which suggested that prey were more difficult to locate by egrets during this period. The Shoebill inhabits inaccessible swamps and is a rare species with low density throughout its range. It is therefore possible that egret–Shoebill associations, in addition to being infrequent and highly seasonal, may have gone unnoticed.     

Altitudinal Distribution of Ammonia-Oxidizing Archaea and Bacteria in Alpine Grassland Soils Along the South-Facing Slope of Nyqentangula Mountains,Central Tibetan Plateau

Nitrogen is a major limiting nutrient for the net primary production of terrestrial ecosystems, especially on sentinel alpine ecosystem. Ammonia oxidation is the first and rate-limiting step on nitrification process and is thus crucial to nitrogen cycle. To decipher climatic influence on ammonia oxidizers, their communities were characterized by qPCR and clone sequencing by targeting amoA genes (encoding the alpha subunit of ammonia mono-oxygenase) in soils from 7 sites over an 800 m elevation transect (4400–5200 m a.s.l.), based on “space-to-time substitution” strategy, on a steppe-meadow ecosystem located on the central Tibetan Plateau (TP). Archaeal amoA abundance outnumbered bacterial amoA abundance at lower altitude (<4800 m a.s.l.), but bacterial amoA abundance was greater in surface soils at higher altitude (≥4800 m a.s.l.). Archaeal amoA abundance decreased with altitude in surface soil, while its abundance stayed relatively stable and was mostly greater than bacterial amoA abundance in subsurface soils. Conversely, bacterial amoA abundance gradually increased with altitude at all three soil depths. Statistical analysis indicated that altitude-dependent factors, in particular pH and precipitation, had a profound effect on the abundance and community of ammonia-oxidizing bacteria, but only on the community composition of ammonia-oxidizing archaea along the altitudinal gradient. These findings imply that the shifts in the relative abundance and/or community structure of ammonia-oxidizing bacteria and archaea may result from the precipitation variation along the altitudinal gradient. Thus, we speculate that altitude-related factors (mainly precipitation variation combing changed pH), would play a vital role in affecting nitrification process on this alpine grassland ecosystem located at semi-arid area on TP.     

Soil carbon sequestration and land use change associated with biofuel production: empirical evidence

Soil organic carbon (SOC) change can be a major impact of land use change (LUC) associated with biofuel feedstock production. By collecting and analyzing data from worldwide field observations of major LUCs from cropland, grassland, and forest to lands producing biofuel crops (i.e. corn, switchgrass, Miscanthus, poplar, and willow), we were able to estimate SOC response ratios and sequestration rates and evaluate the effects of soil depth and time scale on SOC change. Both the amount and rate of SOC change were highly dependent on the specific land transition. Irrespective of soil depth or time horizon, cropland conversions resulted in an overall SOC gain of 6–14% relative to initial SOC level, while conversion from grassland or forest to corn (without residue removal) or poplar caused significant carbon loss (9–35%). No significant SOC changes were observed in land converted from grasslands or forests to switchgrass, Miscanthus, or willow. The SOC response ratios were similar in both 0–30 and 0–100 cm soil depths in most cases, suggesting SOC changes in deep soil and that use of top soil only for SOC accounting in biofuel life cycle analysis (LCA) might underestimate total SOC changes. Soil carbon sequestration rates varied greatly among studies and land transition types. Generally, the rates of SOC change tended to be the greatest during the 10 years following land conversion and had declined to approach 0 within about 20 years for most LUCs. Observed trends in SOC change were generally consistent with previous reports. Soil depth and duration of study significantly influence SOC change rates and so should be considered in carbon emission accounting in biofuel LCA. High uncertainty remains for many perennial systems and forest transitions, additional field trials, and modeling efforts are needed to draw conclusions about the site‐ and system‐specific rates and direction of change.     

Does Fire Trigger Seed Germination in the Neotropical Savannas? Experimental Tests with Six Cerrado Species

The Cerrado (Brazilian savanna) is a biodiversity hotspot with a history of fire that goes back as far as 10 million years. Fire has influenced the evolution of several aspects of the vegetation, including reproduction and life cycles. This study tested how fire by‐products such as heat and smoke affect the germination of six species common to two Cerrado open physiognomies: wet grasslands and the campo sujo (grassland with scattered shrubs and dwarf trees). We subjected seeds collected in northern Brazil to heat shock and smoke treatments, both separately and combined, using different temperatures, exposure times, and smoke concentrations in aqueous solutions. High temperatures and smoke did not break seed dormancy nor stimulate germination of the Cerrado study species. However, seeds were not killed by high temperatures, indicating that they are fire‐tolerant. Our findings differed from those of other fire‐prone ecosystems (mostly of Mediterranean vegetation), where fire stimulates germination. Moreover, we provide important information regarding germination strategies of non‐woody Cerrado plants, showing the importance of considering the tolerance of seeds to high temperatures when evaluating fire‐related traits in fire‐prone ecosystems.