Reciprocal causality between marine macroalgal diversity and productivity in an upwelling area

The relationship between biodiversity and ecosystem functioning has become a prominent topic in the ecological literature. However, the contemporary approach that species diversity controls primary productivity contrasts with the historical perspective that species diversity responds to productivity. Moreover, previous experimental results have not been consistent with the patterns observed in nature. To resolve these questions, the multivariate productivity–diversity (MPD) hypothesis proposes a bidirectional relationship between diversity and productivity. It predicts that the resource supply, expressed in terms of resource availability and imbalance, establishes the number of species that can locally coexist. Simultaneously, the resource supply also indirectly affects biomass production, determining the form and cause of the effects of species richness on resource use and biomass. To test the MPD hypothesis, we conducted three field experiments with a subtidal marine macroalgal community using a seasonal upwelling process as a driver of distinct levels of nutrient supply. Seasonally, macroalgal species richness and biomass were assessed and experimental manipulations conducted to investigate the relative importance of species richness and identity effects on biomass production and the mechanisms underlying these. Changes in macroalgal biomass and species richness were observed in response to the nutrient supply. Stronger effects of species identity were detected for all periods investigated, although species richness effects also occurred to some extent. The magnitudes of the net biodiversity and of the complementarity effects were a unimodal function of nutrient supply, whereas a concave‐up curve was observed for selection effects. The nutrient supply directly affected the number of species that dominated the local community and, consequently, determined the efficiency with which resources were exploited and converted to biomass. Our results provide evidence consistent with the MPD hypothesis and aids in explaining the discrepancies between experimental results and natural patterns through the merging of two contrasting perspectives in ecology.     

耕作和养分管理方式对冬小麦-夏玉米轮作农田春季杂草群落的影响

保护性耕作和杂草多样性是现代生态农业关注的热点问题.尽管保护性耕作下农田杂草多样性有所提高,但耕作方式与养分管理方式对农田杂草群落多样性的交互影响尚未探明.本研究以山东省济南市一块连续3年施行不同耕作方式(免耕、深松、旋耕、深耕)和不同养分管理措施(农户常规:年施氮肥480 kg·hm^-2,高产高效:年施氮肥360 kg·hm^-2,优化管理:年施氮肥300 kg·hm^-2)的冬小麦-夏玉米轮作农田为研究样地,调查并比较了不同管理模式下农田春季杂草群落特征.结果表明: 该冬小麦-夏玉米农田春季杂草群落共发现杂草15种,其中优势物种为马唐和稗;与深耕和旋耕相比,免耕和深松下杂草密度较高.在群落多样性方面,深耕处理下的物种丰富度和均匀度最低,而群落优势度高于其余3种耕作方式;在养分管理中,随着施肥量的增加,物种丰富度和均匀度均升高;深耕和旋耕处理下,随着施肥量的增加,群落优势度升高;而在免耕深松处理下,群落优势度会随着施肥量的增加而减少.在杂草群落生物量方面,免耕和深松的生物量显著高于深耕和旋耕,农户常规处理均高于其余两种养分管理方式,杂草生物量最高的组合是免耕+农户常规.免耕和施肥将提高农田春季杂草群落丰富度、均匀度和生产力.     

Using root traits to understand temporal changes in biodiversity effects in grassland mixtures

Biodiversity–ecosystem functioning (BEF) studies typically show that species richness enhances community biomass, but the underlying mechanisms remain debated. Here, we combine metrics from BEF research that distinguish the contribution of dominant species (selection effects, SE) from those due to positive interactions such as resource partitioning (complementarity effects, CE) with a functional trait approach in an attempt to reveal the functional characteristics of species that drive community biomass in species mixtures. In a biodiversity experiment with 16 plant species in monocultures, 4‐species and 16‐species mixtures, we used aboveground biomass to determine the relative contributions of CE and SE to biomass production in mixtures in the second, dry year of the experiment. We also measured root traits (specific root length, root length density, root tissue density and the deep root fraction) of each species in monocultures and linked the calculated community weighted mean (CWM) trait values and trait diversity of mixtures to CE and SE. In the second year of the experiment, community biomass, CE and SE increased compared to the first year. The contribution of SE to this positive effect was greater than that of CE. The increased contribution of SE was associated with root traits: SE increased most in communities with high abundance of species with deep, thick and dense roots. In contrast, changes in CE were not related to trait diversity or CWM trait values. Together, these results suggest that increased positive effects of species richness on community biomass in a dry year were mainly driven by increased dominance of deep‐rooting species, supporting the insurance hypothesis of biodiversity. Positive CE indicates that other positive interactions did occur, but we could not find evidence that belowground resource partitioning or facilitation via root trait diversity was important for community productivity in our biodiversity experiment.     

Biodiversity effects on productivity and stability of marine macroalgal communities: the role of environmental context

The influence of biodiversity on ecosystem functioning has been the focus of much recent research, but the role of environmental context and the mechanisms by which it may influence diversity effects on production and stability remain poorly understood. We assembled marine macroalgal communities in two mesocosm experiments that varied nutrient supply, and at four field sites that differed naturally in environmental conditions. Concordant with theory, nutrient addition promoted positive species richness effects on algal growth in the first mesocosm experiment; however, it tended to weaken the positive diversity relationship found under ambient conditions in a second experiment the next year. In the field experiments, species richness increased algal biomass production at two of four sites. Together, these experiments indicate that diversity effects on algal biomass production are strongly influenced by environmental conditions that vary over space and time. In decomposing the net biodiversity effect into its component mechanisms, seven of the eight experimental settings showed positive complementarity effects (suggesting facilitation or complementary resource use) countered by negative selection effects (i.e. enhanced growth in mixture of otherwise slow growing species) to varying degrees. Under no conditions, including nutrient enrichment, did we find evidence of positive selection effects commonly thought to drive positive diversity effects. Species richness enhanced stability of algal community biomass across a range of environmental settings in our field experiments. Hence, while species richness can increase production, enhanced stability is also an important functional outcome of maintaining diverse marine macroalgal communities.     

干热河谷植物化学计量特征与生物量之间的关系

了解植物化学计量学特征对生物量变化的响应机制对预测全球变化下植物生产力以及生态系统功能具有重要意义。为了了解干热河谷地区植物化学计量学塑性变化与植物生物量变化的关系, 该研究以当地的典型燥红土为基质, 观察水分、养分以及二者的交互作用对6种植物的生长的促进作用, 并分析这种作用与植物化学计量学特征变化的关系。研究结果显示: 水分、养分、物种及其二元交互作用对植物生长具有显著的作用。养分添加处理增加了32.55%的生物量, 高频次水分处理增加了31.35%的生物量, 水分与养分复合处理下生物量增加了110.60%。植物化学计量学特征的变化与植物生物量对处理的响应具有显著相关性。其中, 植物总体K:Ca、K:Mg、K:Mn、K:Zn、Mg:Mn的变化与植物生物量的变化呈正相关关系, 表明水分和养分处理对植物生长的促进作用影响了植物养分的平衡, 主要的变化趋势是高含量元素与低含量元素的计量比随着生物量的增加而不断增加。此外, 相对于植物生物量变化, 处理类型和物种因素对多数化学计量学特征变化无显著影响, 表明水分和养分处理对化学计量学的影响具有相同的驱动机制, 即通过生物量变化最终影响化学计量学变化。植物生物量对水分和养分的响应可对植物化学计量学特征以及生态系统功能产生深远的影响。     

Relationships between plant stoichiometry and biomass in an arid-hot valley,Southwest China

Aims The micro-elemental stoichiometry as well as nitrogen (N) and phosphorus (P) plays an important role in ecosystem process. However, the drivers of the variations in these stoichiometric ratios in plants are less explored in compared with N and P. Plant productivity and plant stoichiometry can response simultaneously to environmental changes, such as water and nutrient supply levels. However, the relationships between the changes in plant stoichiometry and biomass were unclear yet although both of them play important roles in ecosystem functioning. Our object was to investigate the changes in plant stoichiometry (including multiple macro- and micro-elements) and in biomass under different nutrient and water supply. Methods We collected seeds from six grass species in an arid-hot valley and performed a nutrient-water addition experiment in 2012 with a complete factorial design (nutrient × water). The concentrations of N, P, K, Ca, Mg, Zn and Mn in different organs and plant biomass were measured. The effects of species, water and nutrient on element concentration and plant biomass were analyzed by three-way ANOVA. Linear regressions were used to test the relationships between changes in plant stoichiometry and changes in biomass after nutrient and water addition. Important findings Nutrient addition increased plant biomass by 32.55% compared with control. High-level water supply increased plant biomass by 31.35% and the combination of nutrient and high-level water addition increased plant biomass by 110.60%. Nutrient, water, species identity and their two-way interactions significantly affected plant biomass. Changes in total plant K:Ca, K:Mg, K:Mn, K:Zn and Mg:Mn were significantly and positively related to changes in plant biomass. The ratio between the concentrations of macro-elements and micro-elements tended to increase with biomass. Species identity and treatment had no effects in most of these relationships, suggesting that the changes in stoichiometry were mostly driven by the variations in biomass. The relationships between changes in stoichiometry and in biomass also occurred in leaves, stems and roots. The covariation between plant stoichiometry and biomass can have profound effects on ecosystem functioning under the global environmental changes.     

Impact of management on biodiversity-biomass relations in Estonian flooded meadows

A negative species richness–productivity relationship is often described in grasslands at smaller scales. We aimed to study the effect of management on this relationship. In particular, we addressed the relative importance of biomass cutting, hay removal and nutrient impoverishment on species richness and growth form structure. We conducted fieldwork in flooded meadows in Alam-Pedja Nature Reserve, central Estonia. We sampled vegetation in managed and abandoned stands of two types of alluvial meadows, sedge and tall forb meadow. Aboveground biomass and litter were harvested, weighed and analysed for major plant nutrients by near infrared reflectance spectroscopy. Three groups of general additive models were developed and compared, addressing the impact of (i) productivity, (ii) nutrients and (iii) management regime on species richness. Management—mowing and hay removal—reduced the amount of litter but not aboveground biomass. Management led to a decrease in nitrogen in the biomass and enhanced species richness, particularly in the tall forb meadow. The strongest determinant of species richness was the amount of litter, exhibiting a hump-shaped relationship with richness. The effect of nitrogen supply was significant, but explained less variation. Management increased the proportion of sedges in the sedge meadow and of small herbs in the tall forb meadow. We conclude that litter removal is the most important management means to support biodiversity. On highly productive sites, reducing nutrients via hay removal is of secondary importance within a timeframe of 10 years.     

Impacts of species interactions on grass community productivity under contrasting management regimes

Productivity–diversity relationships are routinely described mainly in terms of species richness. However, these relationships can be affected by the functional strategy and physiological plasticity characterizing each species as they respond to environment and management changes. This study, therefore, aimed to analyze species interactions in grass communities presenting the same number of species (n = 6) but different growth strategies, and the impact on community productivity across several forms of field management (two different fertilizer application rates, i.e. 120 and 360 kg N ha⁻¹ year⁻¹, and two cutting frequencies, i.e. 3 and 6 cuts per year). For this purpose, we applied the tripartite partitioning method introduced for the analysis of biodiversity effects (BE). Grass species were cultivated on small plots (4.2 m²) in both mixtures and monocultures. Different management regimes altered both net BE and its component effects: dominance and potential for complementarity. A higher cutting frequency significantly reduced net BE, via a reduction in dominance effect. We found that increased N supply could either increase or decrease complementary effect according to grass mixture composition, i.e. species strategy. Regardless of management intensity, net BE was in general significantly positive especially when including individual species-specific plasticity effects. We conclude that a combination of different grasses has a positive effect on community biomass. Furthermore, both the functional strategy and the functional plasticity of component species play an important role in the intensity of BE. Therefore, biological mechanisms leading to enhanced biomass in six-grass communities are as effective for productivity as management conditions.     

Resource stoichiometry and consumers control the biodiversity-productivity relationship in pelagic metacommunities

Recent theory suggests that both biodiversity and productivity are constrained by resource supply rates and ratios and that resource stoichiometry is the key to understanding the relationship between biodiversity and productivity. We experimentally tested this theory using pelagic metacommunities. We amended existing predictions by explicitly considering evenness as an aspect of biodiversity and including control of algal biomass by consumption in addition to competition. The metacommunities received a different phosphorus (P) supply and the three patches within each metacommunity differed in their nitrogen (N) supply, which created different N∶P ratios (2, 16, and 128). All patches were inoculated with a phytoplankton assemblage consisting of five species, and half of the metacommunities received two ciliate species as consumers. At the level of the entire metacommunity, algal biomass increased with increasing P supply, whereas species richness and evenness decreased with increasing P supply. Without consumers, resource use efficiency (RUE; realized biomass per unit of P) increased with increasing richness and evenness. Consumer presence reduced overall biomass and richness and precluded a correlation between RUE and biodiversity. At the patch level, local evenness correlated with higher RUE at both imbalanced N∶P ratios (2 and 128) but not at a balanced N∶P ratio. In conclusion, overall P supply constrained realized biomass and altered diversity, whereas resource stoichiometry shaped the relationship between biodiversity and RUE.     

Multiple nutrients and herbivores interact to govern diversity,productivity, composition,and infection in a successional grassland

In spite of increasing awareness that interactions between herbivory and the supply rates of multiple nutrients control biodiversity, ecosystem functions and ecosystem services in ecological communities, few experimental studies have concurrently examined the independent and joint effects of multiple nutrients and mammalian consumers on these responses in natural systems. Here we quantify the independent and interactive effects of multiple concurrent changes to resources and consumers in an invaded annual grassland community in California. In a two‐year study using thirty‐seven 400‐m² plots, we examine interactions among four nutrient treatments (N, P, K and micronutrients) and a keystone herbivore (pocket gopher Thomomys bottae) on four plant community outcomes: 1) plant diversity, 2) functional group composition, 3) net biomass production, an important ecosystem function, and 4) infection risk by a group of viral pathogens shared by crop and non‐crop grasses (barley and cereal yellow dwarf viruses), an important regulating ecosystem service. We found that grassland biodiversity and infection risk were controlled by nutrient identity and supply ratio whereas nutrients interacted strongly with consumers to control grassland composition and net primary productivity. The most important insights arising from this multi‐factor experiment are that net biomass production increased with phosphorus or nitrogen supply; however, when gophers were present, nitrogen caused no net effect on biomass production. In addition, infection risk was driven by phosphorus, nitrogen and micronutrient supply. Infection in a sentinel host increased strongly with the addition of micronutrients or phosphorus; however, infection declined with increasing N/P supply ratio, indicating stoichiometric control of infection risk. Finally, in spite of manipulating multiple factors, plant species richness declined with nitrogen, alone. The importance of higher‐order interactions demonstrates that a multi‐factor approach is critical for effective predictions in a world in which anthropogenic activities are simultaneously changing herbivore abundance and the relative supply of many nutrients.     

Effects of ramet clipping and nutrient availability on growth and biomass allocation of yellow nutsedge

A glasshouse experiment was conducted to examine how the interactions of nutrient availability and partial ramet clipping affect growth, reproduction and biomass allocation of Cyperus esculentus, an invasive sedge. The plants sprouting from tubers were grown at low and high nutrient levels, and were subject either to no clipping, one, two or three clippings, with each clipping cutting half of the existing ramets at soil level. Our results show that nutrient availability and clipping frequency tended to independently affect most of growth, reproduction and biomass allocation parameters of Cyperus esculentus examined in the present study. Increased supply of nutrients led to an increase in plant productivity and its associated traits. All of the traits, except for the number of ramets, displayed a decreasing pattern with increasing clipping frequency, indicating that Cyperus esculentus had undercompensatory responses to ramet clipping. It is likely that the patterns of plants response to clipping are species specific, and depend on morphological characters of species. Its susceptibility to ramet clipping can offer opportunities for controlling this invasive species through mechanical methods such as mowing. Clipping had little effects on biomass allocation; however, root weight fraction increased with increasing clipping frequency. While nutrient availability and clipping frequency had no influence on leaf carbon concentration at harvest, both of them increased leaf nitrogen concentration, and hence reduced leaf C/N ratio.     

干热河谷草本植物生物量分配及其对环境因子的响应

以干热河谷6种草本植物为对象,研究了水分、养分、刈割对生物量在根、茎、叶的分配及异速生长关系的影响.结果表明:刈割处理叶生物量质量分数从25.1%显著增加到31.2%,茎生物量质量分数从43.7%显著降低到34.2%;养分添加处理根生物量质量分数从34.0%显著降低到30.8%;水分处理对生物量分配没有显著影响.物种对根、茎、叶生物量分配有显著影响,适应贫瘠土壤的物种将更多生物量分配给叶和根,对茎生物量的分配相对较低.物种与环境因子存在显著的互作效应,表明环境因子对不同物种的生物量分配影响不同.适应贫瘠土壤的物种叶-茎标度指数和异速生长常数大于其他物种,而茎-根标度指数和异速生长常数小于其他物种.养分显著增加了叶-茎和叶-根的异速生长常数,刈割显著降低了茎-根的标度指数,水分处理则没有显著效应.环境因素对器官间异速生长关系的影响存在种间差异.生物量分配的种间差异及其对环境因素的响应特征可能对植物适应环境变化产生重要影响.     

Sawdust Addition Reduces the Productivity of Nitrogen‐Enriched Mountain Grasslands

Anthropogenic nutrient enrichment of mountain grasslands has boosted grasses and fast‐growing unpalatable plants at the expense of slow‐growing species, resulting in a significant loss in biodiversity. A potential tool to reduce nutrient availability and aboveground productivity without destroying the perennial vegetation is carbon (C) addition. However, little is known about its suitability under severe climatic conditions. Here, we report the results of a 3‐year field study assessing the effects of sawdust addition on soil nutrients, aboveground productivity, and vegetational composition of 10 grazed and ungrazed mountain grasslands. Of particular interest was the effect of C addition on grasses and on the tall unpalatable weed Veratrum album. After 3 years, soil pH, ammonium, and plant‐available phosphorus were not altered by sawdust application, and nitrate concentrations were marginally higher in treatment plots. However, the biomass of grasses and forbs (without V. album) was 20–25% lower in sawdust‐amended plots, whereas the biomass of V. album was marginally higher. Sawdust addition reduced the cover of grasses but did not affect evenness, vegetation diversity, or plant species richness, although species richness generally increased with decreasing biomass at our sites. Our results suggest that sawdust addition is a potent tool to reduce within a relatively short time the aboveground productivity and grass cover in both grazed and ungrazed mountain grasslands as long as they are not dominated by tall unpalatable weeds. The technique has the advantage that it preserves the topsoil and the perennial soil seed bank.     

Effect of functional group richness and species richness in manipulated productivity–diversity studies: a glasshouse pot experiment

Species and functional group (grasses, legumes, creeping nonlegume forbs, rosette nonlegume forbs) richness of species assemblages composed of 16 species from four functional plant groups were manipulated to evaluate the productivity-diversity relationships in a greenhouse pot experiment. Pots were filled with sand, and supplied at two levels of nutrients. The plants were grown in monocultures, two, four, eight and 16 species mixtures. Individual two, four, and eight species mixtures differed in the richness of functional groups. Although the two characteristics of biodiversity, i.e. species and functional group richness, were necessarily correlated, it was shown that it is possible to separate their effect statistically, and also test for their common effect without pronounced loss of test power. There was a pronounced increase of average aboveground biomass and a mild increase in belowground biomass with biodiversity. The effect of functional group richness was more pronounced than the effect of the number of species. By using the method of Loreau and Hector (Nature 411 (2001) 72), selection and complementarity effects were statistically separated, and the overyielding index was calculated as a ratio of the productivity of a mixture to the productivity of its most productive component (to demonstrate transgressive overyielding). Positive values of complementarity and transgressive overyielding were both found, particularly in some rich communities and under high nutrient levels. Complementarity significantly increased only with functional group richness and mainly under high nutrients in the belowground biomass. Some species, when grown in monocultures, had decreased productivity under higher nutrients, and thus were more productive in mixtures than in monocultures. It seems that those species suffered from too high nutrient levels when grown in monocultures, but not in the presence of other species, which were able to use the nutrients in high concentrations and effectively decrease the nutrient levels. As a consequence, mixtures of high diversity were always more productive under high nutrients. The difference in species proportions between high and low nutrients, characterized by chord distance, increased with species richness. The relative change in productivity decreased with the number of functional groups. This suggests that species richness might lead to stabilization of aggregate characteristics (like total productivity) under changing environmental conditions by changing the proportions of individual species.     

Functional traits determine tree growth and ecosystem productivity of a tropical montane forest: Insights from a long‐term nutrient manipulation experiment

Trait‐response effects are critical to forecast community structure and biomass production in highly diverse tropical forests. Ecological theory and few observation studies indicate that trees with acquisitive functional traits would respond more strongly to higher resource availability than those with conservative traits. We assessed how long‐term tree growth in experimental nutrient addition plots (N, P, and N + P) varied as a function of morphological traits, tree size, and species identity. We also evaluated how trait‐based responses affected stand scale biomass production considering the community structure. We found that tree growth depended on interactions between functional traits and the type or combination of nutrients added. Common species with acquisitive functional traits responded more strongly to nutrient addition, mainly to N + P. Phosphorous enhanced the growth rates of species with acquisitive and conservative traits, had mostly positive effects on common species and neutral or negative effects in rare species. Moreover, trees receiving N + P grew faster irrespective of their initial size relative to trees in control or to trees in other treatment plots. Finally, species responses were highly idiosyncratic suggesting that community processes including competition and niche dimensionality may be altered under increased resource availability. We found no statistically significant effects of nutrient additions on aboveground biomass productivity because acquisitive species had a limited potential to increase their biomass, possibly due to their generally lower wood density. In contrast, P addition increased the growth rates of species characterized by more conservative resource strategies (with higher wood density) that were poorly represented in the plant community. We provide the first long‐term experimental evidence that trait‐based responses, community structure, and community processes modulate the effects of increased nutrient availability on biomass productivity in a tropical forest.     

Nutrient concentrations and fibre contents of plant community biomass reflect species richness patterns along a broad range of land-use intensities among agricultural grasslands

Understanding changes in biodiversity in agricultural landscapes in relation to land-use type and intensity is a major issue in current ecological research. In this context nutrient enrichment has been identified as a key mechanism inducing species loss in Central European grassland ecosystems. At the same time, insights into the linkage between agricultural land use and plant nutrient status are largely missing. So far, studies on the relationship between chemical composition of plant community biomass and biodiversity have mainly been restricted to wetlands and all these studies neglected the effects of land use. Therefore, we analyzed aboveground biomass of 145 grassland plots covering a gradient of land-use intensities in three regions across Germany. In particular, we explored relationships between vascular plant species richness and nutrient concentrations as well as fibre contents (neutral and acid detergent fibre and lignin) in the aboveground community biomass.We found the concentrations of several nutrients in the biomass to be closely linked to plant species richness and land use. Whereas phosphorus concentrations increased with land-use intensity and decreased with plant species richness, nitrogen and potassium concentrations showed less clear patterns. Fibre fractions were negatively related to nutrient concentrations in biomass, but hardly to land-use measures and species richness. Only high lignin contents were positively associated with species richness of grasslands. The N:P ratio was strongly positively related to species richness and even more so to the number of endangered plant species, indicating a higher persistence of endangered species under P (co-)limited conditions. Therefore, we stress the importance of low P supply for species-rich grasslands and suggest the N:P ratio in community biomass to be a useful proxy of the conservation value of agriculturally used grasslands.     

浙江定海次生林内物种丰富度与生物量和生产力关系的环境依赖性

迄今生物多样性与生态系统功能关系的研究主要在物种组成随机配置的人工生态系统中进行, 在自然生态系统中研究较少, 且未考虑环境因子如何影响生态系统功能及其与生物多样性的关系。本研究选取亚热带广泛分布的次生林为研究对象, 利用模型拟合的方法, 探讨亚热带次生林中物种丰富度与生物量和生产力之间的关系, 以及环境因子(海拔、坡度、坡向、土层厚度)和次生林恢复时间(林龄)对生物量、生产力、物种丰富度与生物量和生产力间关系的影响。结果表明, 当不考虑环境因子时, 物种丰富度与生物量之间存在显著的线性正相关关系, 而与生产力之间存在显著的二次关系(先增加后减少的驼峰型)。当考虑环境因子时, 个体密度和土层厚度对生物量具有显著影响, 而环境因子对生产力并无显著效应。在坡度较陡、坡向朝南及土层较厚的环境条件下, 物种丰富度与生物量具有显著的线性正相关关系; 而在坡度较缓、坡向朝北及土层较薄的环境条件下, 物种丰富度不影响生物量。在较高海拔环境条件下, 生产力随物种丰富度先增加后减少(驼峰形状), 而在其他环境条件下, 生产力均不响应物种丰富度。以上结果说明自然森林生态系统中物种丰富度与生物量和生产力的关系存在差异, 且其相互间的关系依赖于环境因子。     

Combined effects between temporal heterogeneity of water supply,nutrient level,and population density on biomass of four broadly distributed herbaceous species

Temporal heterogeneity of water supply affects grassland community productivity and it can interact with nutrient level and intraspecific competition. To understand community responses, the responses of individual species to water heterogeneity must be evaluated while considering the interactions of this heterogeneity with nutrient levels and population density. We compared responses of four herbaceous species grown in monocultures to various combinations of water heterogeneity, nutrient level, and population density: two grasses (Cynodon dactylon and Lolium perenne), a forb (Artemisia princeps), and a legume (Trifolium repens). Treatment effects on shoot and root biomass were analyzed. In all four species, shoot biomass was larger under homogeneous than under heterogeneous water supply. Shoot responses of L. perenne tended to be greater at high nutrient levels. Although root biomass was also larger under homogeneous water supply, effects of water heterogeneity on root biomass were not significant in the grasses. Trifolium repens showed marked root responses, particularly at high population density. Although greater shoot and root growth under homogeneous water supply appears to be a general trend among herbaceous species, our results suggested differences among species could be found in the degree of response to water heterogeneity and its interactions with nutrient level and intraspecific competition.     

Short-term effects of nitrogen enrichment,litter removal and cutting on a Mediterranean grassland

Semi-natural grassland communities are of great interest in conservation because of their high species richness. These communities are being threatened by both land abandonment and nitrogen eutrophication, and their continued existence will depend upon correct management. However, there is a distinct lack of studies of the ecological mechanisms that regulate species diversity and productivity in Mediterranean grasslands. We have conducted a 3-year field experiment in a species-poor grassland in central Italy to investigate the effects of nitrogen fertilization coupled with removal of plant litter and artificial cutting on species diversity and community productivity. Vegetation cutting reduced living biomass but increased species diversity. In fact, cutting had positive effects on the cover of almost all of the annual and biennial species, while it had a negative effect on the dominant perennial grasses Brachypodium rupestre and Dactylis glomerata. Litter removal had similar effects to cutting, although it was far less effective in increasing species diversity. In contrast, nitrogen enrichment strongly increased the living biomass while maintaining very low species diversity. Our results have indicated that semi-natural Mediterranean grasslands need specific management regimes for maintenance and restoration of species diversity. In the management of these grasslands, attention should be paid to the potential threat from nitrogen enrichment, especially when coupled with land abandonment.     

Short-term effects of nitrogen enrichment, litter removal and cutting on a Mediterranean grassland

Semi-natural grassland communities are of great interest in conservation because of their high species richness. These communities are being threatened by both land abandonment and nitrogen eutrophication, and their continued existence will depend upon correct management. However, there is a distinct lack of studies of the ecological mechanisms that regulate species diversity and productivity in Mediterranean grasslands. We have conducted a 3-year field experiment in a species-poor grassland in central Italy to investigate the effects of nitrogen fertilization coupled with removal of plant litter and artificial cutting on species diversity and community productivity. Vegetation cutting reduced living biomass but increased species diversity. In fact, cutting had positive effects on the cover of almost all of the annual and biennial species, while it had a negative effect on the dominant perennial grasses Brachypodium rupestre and Dactylis glomerata. Litter removal had similar effects to cutting, although it was far less effective in increasing species diversity. In contrast, nitrogen enrichment strongly increased the living biomass while maintaining very low species diversity. Our results have indicated that semi-natural Mediterranean grasslands need specific management regimes for maintenance and restoration of species diversity. In the management of these grasslands, attention should be paid to the potential threat from nitrogen enrichment, especially when coupled with land abandonment.