Grazer exclusion alters plant spatial organization at multiple scales,increasing diversity

Grazing is one of the most important factors influencing community structure and productivity in natural grasslands. Understanding why and how grazing pressure changes species diversity is essential for the preservation and restoration of biodiversity in grasslands. We use heavily grazed subalpine meadows in the Qinghai‐Tibetan Plateau to test the hypothesis that grazer exclusion alters plant diversity by changing inter‐ and intraspecific species distributions. Using recently developed spatial analyses combined with detailed ramet mapping of entire plant communities (91 species), we show striking differences between grazed and fenced areas that emerged at scales of just one meter. Species richness was similar at very small scales (0.0625 m²), but at larger scales diversity in grazed areas fell below 75% of corresponding fenced areas. These differences were explained by differences in spatial distributions; intra‐ and interspecific associations changed from aggregated at small scales to overdispersed in the fenced plots, but were consistently aggregated in the grazed ones. We conclude that grazing enhanced inter‐ and intraspecific aggregations and maintained high diversity at small scales, but caused decreased turnover in species at larger scales, resulting in lower species richness. Our study provides strong support to the theoretical prediction that inter‐ and intraspecific aggregation produces local spatial patterns that scale‐up to affect species diversity in a community. It also demonstrates that the impacts of grazing can manifest through this mechanism, lowering diversity by reducing spatial turnover in species. Finally, it highlights the ecological and physiological plant processes that are likely responding to grazing and thereby altering aggregation patterns, providing new insights for monitoring, and mediating the impacts of grazing.     

Committee for the study of the Scottish Flora

Background: Damage to alpine plant communities is likely to occur when hikers and pack animals trample vegetation. Currently, there is limited research that quantifies and compares impacts from these activities.

Aims: A manipulative experimental protocol was used to assess damage to alpine meadows by pack animals and hikers in the Aconcagua Provincial Park, Andes, Argentina.

Methods: Vegetation height, overall cover, cover of dominant species and species richness were measured immediately after, and 2 weeks after different numbers of passes (0, 25, 100 and 300) by hikers or pack animals in an experiment, using a randomised block design.

Results: Pack animals had two to three times the impact of hiking on the meadows, with greater reductions in plant height, the cover of one of the dominant sedges and declines in overall vegetation cover after 300 passes. Impacts of pack animals were also apparent at lower levels of use than for hikers. These differences occurred despite the meadow community having relatively high resistance to trampling due to the traits of one of the dominant sedges (Carex gayana).

Conclusions: Pack animals caused more damage than hikers to the alpine meadow, but the scale of the difference in short-term impacts depends on the characteristics of the plant community, the amount of use and the vegetation parameters measured. Use of the meadows by hikers and pack animals should be minimised as these meadows are scarce, and have high conservation values.     

Intensified plant N and C pool with more available nitrogen under experimental warming in an alpine meadow ecosystem

Nitrogen (N) availability is projected to increase in a warming climate. But whether the more available N is immobilized by microbes (thus stimulates soil carbon (C) decomposition), or is absorbed by plants (thus intensifies C uptake) remains unknown in the alpine meadow ecosystem. Infrared heaters were used to simulate climate warming with a paired experimental design. Soil ammonification, nitrification, and net mineralization were obtained by in situ incubation in a permafrost region of the Qinghai‐Tibet Plateau (QTP). Available N significantly increased due to the stimulation of net nitrification and mineralization in 0–30 cm soil layer. Microbes immobilized N in the end of growing season in both warming and control plots. The magnitude of immobilized N was lower in the warming plots. The root N concentration significantly reduced, but root N pool intensified due to the significant increase in root biomass in the warming treatment. Our results suggest that a warming‐induced increase in biomass is the major N sink and will continue to stimulate plant growth until plant N saturation, which could sustain the positive warming effect on ecosystem productivity.     

火烧及微地貌对高原湿草甸优势种华扁穗草地上部灰分含量及热值的影响

为探讨火烧干扰以及微地貌差异对高原湿草甸植被的影响,以高原湿草甸优势种华扁穗草( Blysmus sinocompressus Tang et F. T. Wang)为研究对象,分别在经历火烧干扰和未烧的湿地中选择平地和地垄作为样地,研究了不同生长期以及不同年份华扁穗草地上部的灰分含量和热值(包括干质量热值和去灰分热值)变化。结果表明：在火烧平地、火烧地垄、未烧平地和未烧地垄4类样地中,不同生长期以及不同年份间华扁穗草地上部的灰分含量和热值均有一定差异。在火烧平地、火烧地垄和未烧平地3类样地中,与生长初期相比,生长旺盛期华扁穗草地上部的灰分含量均降低,其干质量热值和去灰分热值均升高;而在未烧地垄样地中,与生长初期相比,生长旺盛期华扁穗草地上部的灰分含量显著升高,其干质量热值和去灰分热值则均略降低。从样地类型看,在生长旺盛期,平地样地中华扁穗草地上部的灰分含量均低于地垄样地,其干质量热值和去灰分热值均高于地垄样地,其中,其灰分含量在火烧平地样地中最低、在未烧地垄样地中最高;其干质量热值和去灰分热值在火烧地垄样地中均最低、在未烧平地样地中均最高,但总体无显著差异。随时间推移,火烧样地特别是火烧平地样地中华扁穗草地上部的灰分含量增加,而其干质量热值和去灰分热值则有所降低但总体上与未烧样地间无显著差异。总体上看,在火烧样地中,华扁穗草地上部的干质量热值和去灰分热值低于未烧样地;而在平地样地中,其干质量热值和去灰分热值高于地垄样地。研究结果显示：火烧干扰和微地貌差异对华扁穗草地上部的灰分含量和热值均有一定的影响,据此,建议将植物地上部的灰分含量和热值作为高原湿草甸植被优势种响应环境扰动的参考指标。     

Effects of experimental warming on plant reproductive phenology in Xizang alpine meadow北大核心CSCD

Aims: Climate warming strongly influences reproductive phenology of plants in alpine and arctic ecosystems. Here we focus on phenological shifts caused by warming in a typical alpine meadow on the Qinghai-Xizang Plateau. Our objective was to explore phenological responses of alpine plant species to experimental warming. Methods: Passive warming was achieved using open-top chambers (OTCs). The treatments included control (C), and four levels of warming (T1, T2, T3, T4). We selected Kobresia pygmaea, Potentilla saundersiana, Potentilla cuneata, Stipa purpurea, Festuca coelestis and Youngia simulatrix as the focal species. Plant phenology was scored every 3-5 days in the growing season. The reproductive phenology phases of each species were estimated through fitting the phenological scores to the Richards function. Important findings: Under soil water stress caused by warming, most plants in the alpine meadow advanced or delayed their reproductive events. As a result, warming significantly delayed phenological development of K. pygmaea. Warming significantly advanced reproductive phenology of P. saundersiana, S. purpurea and F. coelestis, but not of P. cuneata and Y. simulatrix. In addition, warming significantly shortened the average flowering duration of alpine plant species. The potentially warmer and drier growing seasons under climate change may shift the reproductive phenology of the alpine systems in similar pattern.     

高寒草甸高原早熟禾个体性状对放牧与围封的响应

植物对放牧的响应是植物在响应过程中为生存和繁殖所形成的适应策略,研究植物表型反应对揭示草原生态系统的放牧响应机制具有重要意义。以青藏高原高寒草甸主要植物高原早熟禾(Poa alpigena)为研究对象,通过放牧和围封试验,研究其个体性状对放牧与围封的响应,旨在为草原生态系统放牧机制提供理论依据。结果表明:1)在长期放牧干扰下,除叶片数和根重外,其他性状株高、分枝数、叶长、叶宽、叶面积、茎粗、茎长、根长、根粗、穗长、总叶质量、单叶质量、茎重、穗重、全株重均出现显著变小的特征(P0.05);而短期围封对于高原早熟禾叶长、叶宽、总叶质量、单叶质量、茎重、穗重、根重等功能性状的恢复效果并不显著(P0.05),表明放牧退化草原植物性状具有保守性;2)通过构建高原早熟禾性状可塑性变化谱,发现穗重、茎重、全株重、分蘖数、茎长、株高等可塑性幅度较大,为放牧响应的敏感指标,叶片数、叶宽、茎粗、根粗的可塑性变化幅度较小,为惰性性状。     

Indirect effect of nitrogen enrichment modified invertebrate herbivory through altering plant community composition in an alpine meadow

Aims Nitrogen enrichment may affect ‘community invertebrate herbivory’ (hereafter ‘herbivory’) directly by changing plant species’ specific herbivory, or indirectly by altering the composition of natural plant communities. Here, we investigated how community composition altered the community herbivory in natural ecosystems and compared the relative importance of direct and indirect effects of nitrogen addition on community herbivory.     

Precipitation and seasonality affect grazing impacts on herbage nutritive values in alpine meadows on the Qinghai-Tibet Plateau

Aims Grasslands used for animal husbandry are chosen depending on the nutritive values of dominant herbage species. However, the influence of grazing in combination with precipitation and growing season on the nutritive values of dominant species has not been explicated.     

Low‐tech riparian and wet meadow restoration increases vegetation productivity and resilience across semiarid rangelands

Restoration of riparian and wet meadow ecosystems in semiarid rangelands of the western United States is a high priority given their ecological and hydrological importance in the region. However, traditional restoration approaches are often intensive and costly, limiting the extent over which they can be applied. Practitioners are increasingly trying new restoration techniques that are more cost‐effective, less intensive, and can more practically scale up to the scope of degradation. Unfortunately, practitioners typically lack resources to undertake outcome‐based evaluations necessary to judge the efficacy of these techniques. In this study, we use freely available, satellite remote sensing to explore changes in vegetation productivity (normalized difference vegetation index) of three distinct, low‐tech, riparian and wet meadow restoration projects. Case studies are presented that range in geographic location (Colorado, Oregon, and Nevada), restoration practice (Zeedyk structures, beaver dam analogs, and grazing management), and time since implementation. Restoration practices resulted in increased vegetation productivity of up to 25% and increased annual persistence of productive vegetation. Improvements in productivity with time since restoration suggest that elevated resilience may further enhance wildlife habitat and increase forage production. Long‐term, documented outcomes of conservation are rare; we hope our findings empower practitioners to further monitor and explore the use of low‐tech methods for restoration of ecohydrologic processes at meaningful spatial scales.     

Wood chip soil amendments in restored wetlands affect plant growth by reducing compaction and increasing dissolved phenolics

Adding chipped wood to soil ameliorates compaction, allowing faster plant growth that is critical to successful wetland restorations. Following the filling and planting of an erosion gully in Halstead Meadow, Sequoia National Park, the tallest leaf height and maximum clone width of transplanted Scirpus microcarpus seedlings were negatively correlated with soil compaction. Plant height decreased by 9.8 cm and width decreased by 11.9 cm per MPa of soil compaction (range of 0.74–4.50 MPa). We experimentally amended mineral soil in a test trench and found that every 0.10 cm³/cm³ addition of wood chips (range of 0.00–0.75 cm³/cm³) reduced compaction by 0.174 MPa. Had the Halstead Meadow gully fill contained an equivalent volume of wood chips to the reference area soil organic matter content (0.64 cm³/cm³), we predict compaction would have been reduced by 1.11 MPa, increasing individual transplant width spread by 36%, approximately doubling the vegetated area after two growing seasons. In a greenhouse phytometer experiment, conifer bark leachate (phenolics 211 mg/L) significantly reduced plant growth and, in the presence of added nutrients, increased the production of the enzyme polyphenol oxidase (PPO). However, phenolics concentration in bark‐free conifer wood leachate (12 mg/L), similar to field‐sampled concentrations, did not affect plant growth or PPO production. Pure conifer bark is not recommended as a soil amendment, but the addition of low‐bark‐content wood chips to gully fill may be a feasible and effective means of reducing soil compaction, accelerating plant establishment, and lowering wetland restoration project costs.