Moderate grazing promotes the root biomass in Kobresia meadow on the northern Qinghai–Tibet Plateau

Grazing is an important modulator of both plant productivity and biodiversity in grassland community, yet how to determine a suitable grazing intensity in alpine grassland is still controversy. Here, we explore the effects of different grazing intensities on plant biomass and species composition, both at community level and functional group level, and examines the productivity–species richness relationship under four grazing patterns: no grazing (CK), light grazing (LG), moderate grazing, (MG) and heavy grazing (HG), attempt to determine a suitable grazing intensity in alpine grassland. The results were as follows. The total aboveground biomass (AGB) reduced with increasing grazing intensity, and the response of plant functional groups was different. AGB of both sedges and legumes increased from MG to HG, while the AGB of forbs reduced sharply and the grass AGB remained steady. There was a significant positive relationship between productivity and species richness both at community level and functional group level. In contrast, the belowground biomass (BGB) showed a unimodal relationship from CK to HG, peaking in MG (8,297.72 ± 621.29 g/m²). Interestingly, the grassland community tends to allocate more root biomass to the upper soil layer under increasing grazing intensities. Our results suggesting that moderate levels of disturbance may be the optimal grassland management strategy for alpine meadow in terms of root production.     

The Establishment and Maintenance of a Species-Rich Grassland on a Reclaimed Opencast Coal Site

We conducted an experiment with the dual aims of (1) examining the feasibility of establishing a species-rich grassland using a commercially available grass and wild-flower seed mixture and (2) examining the effects of different defoliation and fertilizer managements on the productivity, species richness, diversity, and composition of a species-rich grassland established on a site reclaimed after opencast coal mining. The use of the seed mixture successfully established a sward of some 18–25 species per square meter. The species composition was enriched to some extent by recruitment of unsown species, principally from the soil seed bank. Hay-type defoliation management produced greater dry matter yield and species richness than grazing defoliation, but grazing defoliation produced greater species diversity. Fertilizer application had no significant effect on dry matter production but reduced species diversity. Ordination analysis revealed that both defoliation and fertilizer management significantly affected species composition. The response obtained by individual species was explicable largely by their comparative biology.     

模拟放牧斑块与氮素添加对半干旱草原群落植物生长的影响

放牧时,动物采食及其排泄物会影响植物的生长,但动物彩食及其排泄物的空间异质性可能会影响这咱效应.在位于我国北方典型农牧交错区的内蒙古多伦县,我们研究了模拟入牧斑块和施氧肥对植物生长的影响,实验采用模拟放牧采食斑块(观割半径分别0、10、20、40和8cm)和土壤施氮(分别为0、5、20Gn\m2)两种处理,植物地上部收获后分为绿体和立柯两部分,并分析其含氮量.结果表明,刈割降低了植物的生物量(40.5%),而施氮可增加生物量(57.8%)刈割交通规则植物生长的抑制作用在面积最水又施肥的斑块上表现更明显.土壤施氮可以促进杜物生长并且影响刈割效应.同时植物的绿-枯比阻碍施氮水平的增回而增回,因此氮会延迟植物的衰老.以上结果表明,刈割(模拟动物采食)斑块的大小会影响草原植物的生长,土壤施氮(模拟动物尿氮)可以提高草原生态系统的初级生产力,并影响刈割效应.     

Effects of grazing pattern and nitrogen availability on primary productivity

A major part of the impact of grazing on primary productivity results from the joint action of tissue removal and nutrient return to the soil via dung and urine. Grazing, however, is not uniformly distributed in space: grazed grasslands show a matrix of grazed and ungrazed patches, which in turn, may or may not be affected by faecal or urine deposition. This paper investigates the effects of grazing spatial pattern and nitrogen availability on primary productivity. We propose that grazed plants located at the edge of a grazed patch are more shaded by their taller ungrazed neighbours than plants at the center. Since the border effect is less important as patch size increases, the effects of grazing will be more positive, or less negative, when grazing pattern is coarse-grained than when it is fine-grained. We also propose that nitrogen availability will affect this response to grazing through its effects on the intensity of competition for light and on the amount of compensatory growth. We performed a field experiment in a grassland community of the Flooding Pampa, Argentina, in which we compared the productivity of undefoliated controls and defoliated patches of different size, with and without nitrogen application. Defoliation reduced primary productivity and this effect was greater in the smallest, fertilized patches. Productivity was highest at patches of intermediate and large sizes. Nitrogen addition increased productivity by two-fold. The integrated photon flux density reaching the base of the canopy was affected by defoliation and by patch size: it was lower in controls than in defoliated patches and increased with patch size. Our results showed that (a) the size of the defoliated patch modified the response of this grassland to defoliation, (b) this response was correlated with light availability, and (c) nitrogen addition, simulating urine depositions, increased primary productivity and affected the response to defoliation of the smallest patches.     

模拟放牧斑块与氮素添加对半干旱草原群落植物生长的影响(英文)

放牧时,动物采食及其排泄物会影响植物的生长,但动物采食及其排泄物的空间异质性可能会影响这种效应。在位于我国北方典型农牧交错区的内蒙古多伦县,我们研究了模拟放牧斑块和施氮肥对植物生长的影响,实验采用模拟放牧采食斑块(刈割半径分别为0、10、20、40和80 cm)和土壤施氮(分别为0、5、10、20 g N/m2)两种处理,植物地上部收获后分为绿体和立枯两部分,并分析其含氮量。结果表明,刈割降低了植物的生物量(41.5％),而施氮可增加生物量(57.8％)。刈割对植物生长的抑制作用在面积最小又施肥的斑块上表现更明显。土壤施氮可以促进植物生长并且影响刈割效应。同时植物的绿-枯比随施氮水平的增加而增加,因此氮会延迟植物的衰老。以上结果表明,刈割(模拟动物采食)斑块的大小会影响草原植物的生长,土壤施氮(模拟动物尿氮)可以提高草原生态系统的初级生产力,并且影响刈割效应。     

氮水添加对放牧背景下荒漠草原生产力的影响

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。     

Effect of competition intensity on recruitment of palatable and unpalatable grasses

In this study, we made an attempt to reveal how competition intensity from established plants impacts on palatable and unpalatable grass seedlings recruitment, in a natural mesic grassland of central Argentina. Our objective was to assess the seedling recruitment of a palatable species (Chascolytrum subaristatum) and an unpalatable species (Nassella trichotoma) in microsites differing in competition intensity from established plants. Identity (C. subaristatum and N. trichotoma) and defoliation severity were used as surrogate for competition intensity. In March 2017, we permanently marked established individuals of N. trichotoma and C. subaristatum and placed two circular plots adjacent to each individual. In one plot we added seeds of N. trichotoma and in the other seeds of C. subaristatum. After seeding, established plants were randomly assigned to one of three level of defoliation: without defoliation, low defoliation severity and high defoliation severity. From April to November 2017 (i.e. over a complete annual growing cycle), we measured seedling density, recruitment and growth. Our results supported the hypothesis that seedlings of palatable grasses are more competitive than seedlings of unpalatable grasses. Seedling of the palatable grass C. subaristatum recruited successfully regardless the intensity of competition from established plants, whereas seedlings of the unpalatable grass N. trichotoma recruited better under low competitive pressure from established plants. Our results suggest that the availability of microsites with low competitive pressure from the established vegetation, created by selective grazing of palatable grasses, promotes the recruitment of unpalatable grass seedlings. This mechanism may contribute to the species replacement process commonly observed in heavy grazed grasslands.     

Increasing nitrogen deposition enhances post-drought recovery of grassland productivity in the Mongolian steppe

Arid regions are prone to drought because annual rainfall accumulation depends on a few rainfall events. Natural plant communities are damaged by drought, but atmospheric nitrogen (N) deposition may enhance the recovery of plant productivity after drought. Here, we investigated the effect of increasing N deposition on post-drought recovery of grassland productivity in the Mongolian steppe, and we examined the influence of grazing in this recovery. We added different amounts of N to a Mongolian grassland during two sequential drought years (2006 and 2007) and the subsequent 3 years of normal rainfall (2008–2010) under grazed and nongrazed conditions. Aboveground biomass and number of shoots were surveyed annually for each species. Nitrogen addition increased grassland productivity after drought irrespective of the grazing regime. The increase in grassland productivity was associated with an increase in the size of an annual, Salsola collina, under grazed conditions, and with an increase in shoot emergence of a perennial, Artemisia adamsii, under nongrazed conditions. The addition of low N content simulating N deposition around the study area by the year 2050 did not significantly increase grassland productivity. Our results suggest that increasing N deposition can enhance grassland recovery after a drought even in arid environments, such as the Mongolian steppe. This enhancement may be accompanied by a loss of grassland quality caused by an increase in the unpalatable species A. adamsii and largely depends on future human activities and the consequent deposition of N in Mongolia.     

Patch dynamics in grazed subtropical native pastures in south‐east Queensland

Abstract Patch formation is common in grazed grasslands but the mechanisms involved in the formation and maintenance of patches are not clear. To increase our knowledge on this subject we examined possible reasons for patch formation and the influence of management on changes between patch states in three experiments in native pasture communities in the Crows Nest district, south‐east Queensland. In these communities, small‐scale patches (tall grassland (dominated by large and medium tussock grasses), short swards (dominated by short tussock grasses and sedges), and lawns (dominated by stoloniferous and/or rhizomatous grasses)) are readily apparent. We hypothesized that the formation of short sward and lawn patches in areas of tall grassland was due to combinations of grazing and soil fertility effects. This was tested in Experiment 1 by applying a factorial combination of defoliation, nutrient application and transplants of short tussock and stoloniferous species to a uniform area of tall grassland. Total species density declined during the experiment, was lower with high nutrient applications, but was not affected by defoliation. There were significant changes in abundance of species that provided support for our hypotheses. With light defoliation and low nutrients, the tall grassland remained dominated by large tussock grasses and contained considerable amounts of forbs. With heavy defoliation, the pastures were dominated by medium tussock grasses and there were significant decreases in forbs and increases in sedges (mainly with low nutrients) and stoloniferous grasses (mainly with high nutrients). Total germinable seed densities and those of most species groups were significantly lower in the heavy defoliation than the light defoliation plots. Total soil seed numbers were not affected by nutrient application but there were fewer seeds of the erect forbs and more sedge seeds in plots with high nutrients. The use of resting from grazing and fire to manage transitions between patches was tested. In Experiment 2 , changes in species density and abundance were measured for 5 years in the three patch types with and without grazing. Experiment 3 examined the effects of fire, grazing and resting on short sward patches over 4 years. In Experiment 2 , total species density was lower in lawn than short sward or tall grassland patches, and there were more species of erect forbs than other plant groups in all patch types. The lawn patches were originally dominated by Cynodon spp. This dominance continued with grazing but in ungrazed patches the abundance of Cynodon spp. declined and that of forbs increased. In the short sward patches, dominance of short tussock grasses continued with grazing but in ungrazed plots their abundance declined while that of large tussock grasses increased. The tall grassland patches remained dominated by large and medium tussock species. In Experiment 3 , fire had no effect on species abundance. On the grazed plots the short tussock grasses remained dominant but where the plots were rested from grazing the small tussock grasses declined and the large tussock grasses increased in abundance. The slow and relatively small changes in these experiments over 4 or 5 years showed how stable the composition of these pastures is, and that rapid changes between patch types are unlikely.     

Short-term regrowth responses of four steppe grassland species to grazing intensity, water and nitrogen in Inner Mongolia

Inner Mongolia steppe grasslands are widely used for livestock farming and the regrowth ability of grassland species is therefore strongly influenced not only by water and nutrient availability but also quite heavily by grazing. However, little is known on how grazing, water and nitrogen interactively affect the dominant C3 species (Leymus chinensis, Stipa grandis, Agropyron cristatum) and the C4 species (Cleistogenes squarrosa). Therefore in the 2007 and 2008 growing seasons, a field experiment was carried out to test the hypothesis that under different grazing intensities the dominant species show different short-term regrowth response to simultaneous variation in the availability of water and nitrogen. Single factor and interaction effects of the addition of water (rainfed vs. simulated wet-year) and nitrogen (0 or 25 kg N ha^?1) were analysed along a gradient of four grazing intensities (ungrazed, lightly, moderately and heavily grazed) after one month of grazing exclusion. Water and nitrogen addition affected short-term regrowth of all species in a similar way whereas species responded differently to grazing. Simulated wet-year water availability consistently resulted in higher standing biomass, relative growth rate and cellulase digestible organic matter yield. Supplementary nitrogen promoted standing biomass and crude protein concentration. The nutritive value of all species’ standing biomass showed a similar increase with more intensive grazing. However, heavy grazing led to a clear shift in the relative biomass of the species, i.e. mainly a promotion of the C4 grass, C. squarrosa. In contrast to our hypothesis, there were no differences among species in their response to water or nitrogen addition, whereas, heavy grazing induced the expected species-specific response. Our results suggest that heavy grazing rather than nitrogen or water determine short-term shifts in species composition of the investigated steppe ecosystem. Furthermore, differences in the species-specific growth response to grazing may increase the proportion of the C4 grass C. squarrosa in steppe communities, whereas higher availability of nitrogen and water may lead to higher forage biomass and nutritive value of all investigated species but in short-term cannot compensate for the grazing induced changes in species composition.     

Response of grassland biomass production to simulated climate change and clipping along an elevation gradient

Changes in rainfall and temperature regimes are altering plant productivity in grasslands worldwide, and these climate change factors are likely to interact with grassland disturbances, particularly grazing. Understanding how plant production responds to both climate change and defoliation, and how this response varies among grassland types, is important for the long-term sustainability of grasslands. For 4 years, we manipulated temperature [ambient and increased using open-top chambers (OTC)], water (ambient, reduced using rainout shelters and increased using hand watering) and defoliation (clipped, and unclipped) in three grassland types along an elevation gradient. We monitored plant cover and biomass and found that OTC reduced biomass by 15 %, but clipping and water treatments interacted with each other and their effects varied in different grassland types. For example, total biomass did not decline in the higher elevation grasslands due to clipping, and water addition mitigated the effects of clipping on subordinate grasses in the lower grasslands. The response of total biomass was driven by dominant plant species while subordinate grasses and forbs showed more variable responses. Overall, our results demonstrate that biomass in the highest elevation grassland was least effected by the treatments and the response of biomass tended to be dependent on interactions between climate change treatments and defoliation. Together, the results suggest that ecosystem function of these grasslands under altered climate patterns will be dependent on site-specific management.     

Flooding and grazing promote germination and seedling establishment in the perennial grass Paspalum dilatatum

Seed germination and seedling emergence are key processes for population recruitment. Flooding and grazing are disturbances forming gaps that may strongly influence recruitment patterns in space and time, but their combined effects and action mechanisms have rarely been addressed. In this study we analysed the effects of microhabitat conditions associated with winter flooding and spring‐summer defoliation on seed germination and seedling establishment of Paspalum dilatatum, a dominant perennial C₄ grass in native grasslands of the Flooding Pampa, Argentina. The dynamics of seedling emergence from natural seed banks and buried seeds was studied in a factorial experiment with flooding and defoliation treatments applied to soil monoliths (mesocosms) collected from natural grassland. Additional laboratory experiments were applied to investigate seed germination under different combinations of temperature, light quality and simulated flooding. Seed germination and seedling emergence of P. dilatatum were promoted by flooding and high intensity defoliation. Gaps generated by flooding were maintained by high intensity defoliation exercising a synergistic effect on survival seedlings. Flooding resulted in the breaking of seed dormancy and higher germination rates associated with alternating temperature and the activation of the phytochrome system. Our results indicate that microhabitat conditions associated with the disturbances forming gaps, such as flooding and heavy grazing, synergistically promote the recruitment process of this dominant grass species.     

Extreme weather events and plant–plant interactions: shifts between competition and facilitation among grassland species in the face of drought and heavy rainfall

Biotic interactions play an important role in ecosystem function and structure in the face of global climate change. We tested how plant–plant interactions, namely competition and facilitation among grassland species, respond to extreme drought and heavy rainfall events. We also examined how the functional composition (grasses, forbs, legumes) of grassland communities influenced the competition intensity for grass species when facing extreme events. We exposed experimental grassland communities of different functional compositions to either an extreme single drought event or to a prolonged heavy rainfall event. Relative neighbour effect, relative crowding and interaction strength were calculated for five widespread European grassland species to quantify competition. Single climatic extremes caused species specific shifts in plant–plant interactions from facilitation to competition or vice versa but the nature of the shifts varied depending on the community composition. Facilitation by neighbouring plants was observed for Arrhenatherum elatius when subjected to drought. Contrarily, the facilitative effect of neighbours on Lotus corniculatus was transformed into competition. Heavy rainfall increased the competitive effect of neighbours on Holcus lanatus and Lotus corniculatus in communities composed of three functional groups. Competitive pressure on Geranium pratense and Plantago lanceolata was not affected by extreme weather events. Neither heavy rainfall nor extreme drought altered the overall productivity of the grassland communities. The complementary responses in competition intensity experienced by grassland species under drought suggest biotic interactions as one stabilizing mechanism for overall community performance. Understanding competitive dynamics under fluctuating resources is important for assessing plant community shifts and degree of stability of ecosystem functions.     

Defoliation and below-ground herbivory in the grass Muhlenbergiaquadridentata : Effects on plant performance and on the root-feeder Phyllophaga sp. (Coleoptera, Melolonthidae)

In this study we evaluated (1) the combined effects of simulated defoliation and below-ground herbivory (BGH) on the biomass and nitrogen content of tillers and roots of the bunchgrass Muhlenbergia quadridentata and (2) the effect of defoliation on the survival of third-instar root-feeder larvae of Phyllophaga sp. The experiment was performed in a pine forest area at an altitude of 3200 m above sea level. The grass and the root-feeder species were native and dominant in the understory and in the macroarthropod root-feeder communities, respectively. Plants were established in pots in the field and were subjected to the following treatments in a factorial design: simulated defoliation (three levels) and BGH (with or without root-feeder larvae) with ten replicates per treatment. Plants were defoliated three times at 2-month intervals. The interaction between defoliation and root herbivory was significant for all components of plant biomass. In every case, light defoliation with BGH decreased live above-ground, root and total plant biomass, and the number of live tillers by more than 50% with respect to the same defoliation level without root-feeders. Plants apparently did not compensate for the carbon drain by root-feeders when a high proportion of older leaves were not removed by defoliation. Plants under heavy defoliation were not affected by the presence of root-feeders and showed a greater live/dead above-ground biomass ratio than lightly defoliated and control plants. Defoliation and BGH did not change tiller and root N concentrations but root herbivores did decrease live-tiller N content in lightly defoliated plants. Root-feeders but not defoliation decreased the root/shoot ratio by 40% and the live/dead above-ground biomass ratio by 45% through increased tiller mortality. Survivorship and final biomass of Phyllophaga sp. larvae were not affected by defoliation treatments during the 6-month study period. Received: 17 May 1996 / Accepted: 1 November 1996     

Forage quality in relation to long-term grazing history,current-year defoliation,and water resource

Forage nitrogen concentrations, nitrogen yields, and in vitro digestibilities were assessed in shortgrass steppe that had been ungrazed, lightly, or heavily grazed for 50 years. Caged plots were defoliated in amounts based upon removals observed in naturallygrazed reference plots or not defoliated. This was done in a year of average precipitation and with a supplemental water treatment to simulate a wet year. In general, current-year defoliation had positive effects, and longterm grazing and supplemental water had negative effects, on forage nitrogen concentrations and digestibilities. However, defoliation interacted with long-term grazing in determning forage nitrogen concentrations, and with grazing and with watering in determining digestibilities. Nitrogen concentration and digestibility increased with defoliation in lightly, but not in heavily, grazed treatments. The dilution effect of supplemental water an digestibilities through increased plant growth was offset by defoliation. The negative effects of long-term grazing on forage quality were small, equally or more than compensated for by defoliation in a year of average precipitation, but more pronounced in the simulated wet year. Nitrogen yields and digestible forage production were usually increased by defoliation, but this depended upon grazing and watering treatments. Increased nitrogen and digestible forage yields and concentrations in response to defoliation were greater than the biomass response in lightly grazed grassland. For both nitrogen and digestibility, yields were greater in grazed than ungrazed treatments in the year of average precipitation, but less in the simulated wet year. Optimizing quantity and year-to-year stability of nitrogen and digestible forage yield may best be achieved with light grazing rather than no or heavy grazing. Clipping was conducted in a manner closely resembling the natural pattern and intensity of defoliation by the cattle, and confirms the potential for a positive feedback of increased forage quality with defoliation observed in pot experiments. Long-term heavy grazing can diminish this response. Quantily (aboveground primary production, ANPP), quantity of quality (digestible and N yields), and quality (concentrations) do not necessarily respond similarly in interactions between current-year defoliation, long-term grazing history, and level of water resource.     

青藏高原高寒灌丛植被对长期放牧强度试验的响应特征

在青藏高原中国科学院海北高寒草甸生态系统定位研究站对金露梅高寒灌丛草场植被开展了长期不同放牧强度试验,分别在短期(4年)、中期(11年)和长期(18年)放牧阶段研究不同放牧干扰强度对草地植物物种多样性、群落结构、地上生物量和草场质量的影响.研究表明,在不同放牧阶段,随着放牧强度增加植物群落的高度和盖度都降低.在中期放牧干扰阶段,物种多样性数和均匀度指数随着放牧强度增加呈现典型的单峰曲线模式;在长期放牧干扰阶段,随着放牧强度增加,占优势地位的灌木和禾草被典型杂类草替代,其中的重度放牧干扰简化了高寒灌丛植被群落结构,减少了地上现存生物量,特别是可食优良牧草生物量.植被对放牧的响应除了与放牧强度和放牧时间阶段密切相关外,还与该地区水热条件的变化有一定的相关性.针对长期放牧干扰的反应特性可将金露梅灌丛草场中植物划分为增加型、敏感型、忍耐型和无反应型4种类型.除了丰富度指数、多样性指数和均匀度指数外,其它一些特征参数并不支持著名的中度干扰假说.本研究发现,长期重度放牧促进了青藏高原高寒草地退化,适度放牧有利于高寒灌丛草场的生物多样性保护和牧草利用;"取半留半"的放牧原则在青藏高原草场放牧管理实践中值得推荐,它将有利于防止草场退化,提高牧草利用率和维持较高的生物多样性.     

Earthworm and belowground competition effects on plant productivity in a plant diversity gradient

Diversity is one major factor driving plant productivity in temperate grasslands. Although decomposers like earthworms are known to affect plant productivity, interacting effects of plant diversity and earthworms on plant productivity have been neglected in field studies. We investigated in the field the effects of earthworms on plant productivity, their interaction with plant species and functional group richness, and their effects on belowground plant competition. In the framework of the Jena Experiment we determined plant community productivity (in 2004 and 2007) and performance of two phytometer plant species [Centaurea jacea (herb) and Lolium perenne (grass); in 2007 and 2008] in a plant species (from one to 16) and functional group richness gradient (from one to four). We sampled earthworm subplots and subplots with decreased earthworm density and reduced aboveground competition of phytometer plants by removing the shoot biomass of the resident plant community. Earthworms increased total plant community productivity (+11%), legume shoot biomass (+35%) and shoot biomass of the phytometer C. jacea (+21%). Further, phytometer performance decreased, i.e. belowground competition increased, with increasing plant species and functional group richness. Although single plant functional groups benefited from higher earthworm numbers, the effects did not vary with plant species and functional group richness. The present study indicates that earthworms indeed affect the productivity of semi-natural grasslands irrespective of the diversity of the plant community. Belowground competition increased with increasing plant species diversity. However, belowground competition was modified by earthworms as reflected by increased productivity of the phytometer C. jacea. Moreover, particularly legumes benefited from earthworm presence. Considering also previous studies, we suggest that earthworms and legumes form a loose mutualistic relationship affecting essential ecosystem functions in temperate grasslands, in particular decomposition and plant productivity. Further, earthworms likely alter competitive interactions among plants and the structure of plant communities by beneficially affecting certain plant functional groups.     

Plant responsiveness to variation in precipitation and nitrogen is consistent across the compositional diversity of a California annual grassland

Question: How does responsiveness to water and Nitrogen (N) availability vary across the compositional and functional diversity that exists in a mesic California annual grassland plant community? Location: Northern California annual grassland. Methods: A mesocosm system was used to simulate average annual precipitation totals and dry and wet year extremes observed in northern California mesic grasslands. The effects of precipitation and N availability on biomass and fecundity were measured on three different vegetation types, a mixed grass forb community, and a forb and a grass monoculture. The treatment effects on plant community composition were examined in the mixed species community. Results: While growth and seed production of the three vegetation types was inherently different, their responses to variation in precipitation and N were statistically similar. Plant density, shoot biomass, and seed production tended to increase with greater water availability in all vegetation types, with the exception of a consistent growth reduction in high precipitation (1245 mm) plots in the first year of the study. Shoot biomass responded positively to N addition, an effect that increased with greater water availability. Nitrogen addition had little effect on plant density or seed production. In the mixed grass‐forb community, biomass responsiveness to water and N treatments were consistently driven by the shoot growth of Avena barbata, the dominant grass species. Conclusions: Vegetation responses to changes in precipitation and N availability were consistent across a range of composition and structural diversity in this study. Plant growth and seed production were sensitive to both increased and decreased precipitation totals, and the magnitude of these responses to N availability varied depending on soil moisture conditions. Our results suggest the impacts of changing precipitation regimes and N deposition on annual productivity of California grasslands may be predictable under different climate scenarios across a range of plant communities.     

Simulated herbivory counteracts the effects of grass competition on the invasive fireweed (Senecio madagascariensis)

Competition, herbivory and their interaction play a significant role in determining the competitive ability and survival of individual plant species. Understanding these processes and interactions can improve the efficacy of biocontrol programs against invasive weeds. Senecio madagascariensis (fireweed) is an invasive weed of South African origin that reduces pastoral productivity and poisons livestock in several countries, notably Australia. Although competitive pastures can suppress the weed’s growth in Australia, its competitive nature is poorly understood in relation to its invasion success. This greenhouse study assessed the growth and reproductive yield of fireweed growing in competition with six native and introduced grasses present in both South Africa and Australia. Since fireweed is a target for biocontrol in Australia, we examined whether its response to grass competition changed with herbivory (simulated by 40% leaf removal). The effect of grass competition and herbivory on the weed’s biomass and floral productivity was examined during a 12‐week pot trial in South Africa. Floral numbers were unaffected by both grass competition and herbivory. Biomass was used to calculate Relative Interaction Indices (RII) to quantify the weed’s competitive or facilitative response. This index compares a specific measurable trait, such as biomass, of fireweed growing alone, to fireweed growing with grass to determine the level of competitive suppression or facilitation resulting from the interaction. Despite the lack of species‐specific effects of grass competition, the presence of grass suppressed fireweed’s foliar, root and whole plant biomass the most when herbivory was absent. With herbivory, fireweed did not suffer from any measurable competitive suppression. This lack of competitive suppression may be due to an induced allelopathic response, given the levels of pyrrolizidine alkaloids common in many Senecio species. Since this result may weaken the case for biocontrol, the weed’s competitive responses should be verified in relation to actual insect herbivory.     

Reducing Competitive Suppression of a Rare Annual Forb by Restoring Native California Perennial Grasslands

Populations of the rare annual forb Amsinckia grandiflora may be declining because of competitive suppression by exotic annual grasses, and may perform better in a matrix of native perennial bunchgrasses. We conducted a field competition experiment in which Amsinckia seedlings were transplanted into forty 0.64‐m² experimental plots of exotic annual grassland or restored perennial grassland. The perennial grassland plots were restored using mature 3 cm‐diameter plants of the native perennial bunchgrass Poa secunda planted in three densities. The exotic annual grassland plots were established in four densities through manual removal of existing plants. Both grass types reduced soil water potential with increasing biomass, but this reduction was not significantly different between grass types. Both grass types significantly reduced the production of Amsinckia inflorescences. At low and intermediate densities (dry biomass per unit area of 20–80 g/m²), the exotic annual grasses reduced Amsinckia inflorescence number to a greater extent than did Poa, although at high densities (>90 g/m²) both grass types reduced the number of Amsinckia inflorescences to the same extent. The response of Amsinckia inflorescence number to Poa biomass was linear, whereas the same response to the annual grass biomass is logarithmic, and appeared to be related to graminoid cover. This may be because of the different growth forms exhibited by the two grass types. Results of this research suggest that restored native perennial grasslands at intermediate densities have a high habitat value for the potential establishment of the native annual A. grandiflora.