Increased community compositional dissimilarity alleviates species loss following nutrient enrichment at large spatial scales

Aims Anthropogenic activities have drastically increased nutrient availability, resulting in declines in species richness in many plant communities. However, most previous studies have explored only species-loss patterns and mechanisms over small sampling areas, so their results might overestimate species loss at larger spatial scales. The aim of this research was to explore species diversity change patterns and species-loss rates at multiple scales in alpine meadow communities following nutrient enrichment. Specifically, we asked two closely related questions: (i) do changes in species diversity and species-loss patterns differ among spatial scales? and (ii) how does community compositional dissimilarity and species turnover change among spatial scale?     

功能多样性比物种多样性更好解释氮添加对高寒草地生物量的影响

为进一步了解氮添加条件下群落功能多样性如何驱动生物量变化, 该研究在位于天山山脉的巴音布鲁克高寒草地开展氮添加实验, 通过连续两年调查群落物种组成并测量常见物种的功能性状, 分析物种多样性、功能多样性及群落水平功能性状的响应模式及其在驱动生物量变化中的相对贡献。结果表明, 短期氮添加同时增加群落地上和地下生物量, 且地上生物量的增加比例高于地下生物量, 氮添加导致功能多样性降低但是物种多样性未发生显著变化; 氮添加增加群落水平上的植株高度和叶片碳含量, 但导致比叶面积、种子质量及叶片磷含量下降; 物种多样性对生物量变化解释非常有限, 而功能多样性与群落水平功能性状可以很好地解释生物量变化, 以上研究结果支持质量比假说。综上, 该研究表明功能多样性与群落水平功能性状比物种多样性对短期氮添加的响应更加迅速, 且两者在解释高寒草地群落生物量对氮添加的响应中起到关键作用。     

Aboveground resilience to species loss but belowground resistance to nitrogen addition in a montane plant community

Aims Decades of empirical work have demonstrated how dominant plant species and nitrogen fertilization can influence the structure and function of plant communities. More recent studies have examined the interplay between these factors, but few such studies use an explicit trait-based framework. In this study, we use an explicit trait-based approach to identify potential mechanisms for community-level responses and to test ecological niche theory.Methods We experimentally manipulated plant communities (control, ?dominant species, ?random biomass) and nitrogen (N) inputs (control, +organic N, +inorganic N) in a fully factorial design. We predicted that traits related to plants' ability to take up different forms of soil N would differ between dominant and subordinate species, resulting in interactive effects of dominant species loss and N fertilization on plant community structure and function. The study took place in a montane meadow in the Rocky Mountains, Colorado, USA.Important findings After four years, the plant community in removal plots converged toward a species composition whose leaf and root functional traits resembled those of the previously removed dominant species. Ecosystem productivity generally increased with N addition: soil carbon efflux was ~50% greater when either form of N was added, while inorganic N addition increased aboveground biomass production by ~60% relative to controls. The increase in production was mediated by increased average height, leaf mass:area ratio and leaf dry matter content in plant communities to which we added inorganic N. Contrary to our predictions, there were no interactive effects of N fertilization and dominant species loss on plant community structure or ecosystem function. The plant community composition in this study exhibited resistance to soil N addition and, given the functional convergence we observed, was resilient to species loss. Together, our results indicate that the ability of species to compensate functionally for species loss confers resilience and maintains diversity in montane meadow communities.     

Atmospheric nitrogen deposition explains patterns of plant species loss

Atmospheric nitrogen (N) deposition across Europe increased substantially from the 1950s to the 1990s. Targeted surveys suggest a negative correlation between N deposition and species richness within quadrats in sensitive habitats. However, it remains unclear whether plant species losses at national recording scales are correlated with nitrogen deposition. We relate plant species losses before 1987 in Great Britain to reduced and oxidized N deposition, land use change and climate change. The mean Ellenberg fertility (N) indices of plant species lost in each 100 km² cell before 1987 was compared with those of species that were recorded between 1987 and 1999. In 45% of squares, indices of species lost were significantly lower than those for species present after 1986. For 17%, primarily upland, squares, the opposite effect was found. A generalized least squares regression model, with difference in the mean Ellenberg N index between samples as the dependent variable, showed that higher deposition of reduced N was significantly associated with selective loss of species with a lower index. Arable land use and change in arable land use also demonstrated this positive relationship. Rough grazing, change in rough grazing, change in pasture and change in annual precipitation showed negative effects. Difference in Ellenberg R index was highly correlated with difference in Ellenberg N and was negatively correlated with oxidized N deposition, suggesting that the lack of a significant effect of oxidized N deposition on Ellenberg N was because it had effects through both acidification and eutrophication, while the effect of reduced N deposition was primarily through eutrophication. Our results suggest that N deposition, along with land use and precipitation changes, has been a significant driver of local plant extinctions. With N deposition increasing in many parts of the world, local extinctions of plant species may be experienced in other regions.     

高寒草甸植物物种丧失对草原毛虫的影响

植物多样性是调控食物网结构和生态系统功能最重要的生物因素, 植物多样性丧失深刻影响食草动物, 但由于小型食草动物种群数量波动明显、统计随机性较大等困难, 我们对植物多样性丧失如何影响小型食草动物依然知之甚少。基于在青藏高原高寒草甸设置的长期植物物种剔除试验, 本研究于2016-2020年7-8月连续调查了植物物种剔除各处理中草原毛虫(Gynaephora alpherakiif)的数量, 分析了植物物种及功能群丧失对草原毛虫的影响。结果表明, 虽然时空差异及统计随机性是影响草原毛虫数量变化的主要因素, 但植物物种剔除介导的群落差异对草原毛虫数量的影响依然显著: (1)在各观测时段, 优势种线叶嵩草(Kobresia capillifolia)的丧失导致群落中草原毛虫数量显著减少; 禾草类物种丧失也会减少草原毛虫数量, 但其影响仅在8月显著; (2)杂类草物种丧失通过增加群落中禾草物种多度, 可增加草原毛虫数量; 豆科物种丧失使莎草增多, 也会增加草原毛虫数量; (3)各植物功能群部分物种剔除并未显著影响草原毛虫数量。本研究证实了高寒草甸中草原毛虫数量会因优势植物嵩草和禾草的多度减少或禾草物种丧失而显著减少, 但群落总生物量、个体数和物种丰富度、豆科多度以及各功能群植物同比减少, 都对草原毛虫数量没有明显影响。这些结果说明在随机作用主导下, 植物群落中的特定功能群相对多度(而非物种多样性)变化深刻影响草原毛虫适合度, 进而影响生态系功能及服务; 未来生物多样性研究及草地虫害生物防控中应更多考虑统计随机性及植物功能多样性对小型食草动物的影响。     

天坑森林植物群落叶功能性状、物种多样性和功能多样性特征

以天坑内部-边缘-外部森林植物群落为研究对象,通过调查植物的群落结构、叶功能性状,探究天坑内外森林植物群落叶功能性状、物种多样性和功能多样性变化特征及其内在关联,为深入了解负地形森林生态系统的功能和恢复退化喀斯特地区的植被提供一定参考。研究结果如下：（1）比叶面积（SLA: 198.75 cm2/g)）、叶面积（LA: 42.70 cm2）、叶磷含量（LPC: 1.70 g/kg）和叶钾含量（LKC: 10.27 g/kg）在天坑内部最高,叶组织密度（LTD: 0.32 g/cm3）和叶干物质含量（LDMC: 0.41 g/g）在天坑外部最高,天坑内外森林均易受到磷限制,表明随天坑内部-边缘-外部生境变化,植物对环境的适应机制和生存策略发生了部分调整,物种的防御策略增强,生长投入策略减弱。（2）Shannon-Wiener指数（2.82）、Simpson指数（0.92）和Pielou’s均匀度指数（0.87）均以天坑外部最高,功能丰富度（1.05）、功能离散度（1.88）和Rao’s二次熵（4.52）以天坑内部最高,表明随天坑内部-边缘-外部生境的变化,植物功能性状的差异减少,物种分布及其功能性状分布总体上更为均匀、物种数量增多。（3）物种多样性指数之间、功能多样性指数之间存在较强的相关性,表明物种多样性指数之间、功能多样性指数之间存在不同的制约关系。（4）叶功能性状与物种多样性、功能多样性的相关性强,物种多样性和功能多样性之间相关性较弱,表明叶性状对生态学过程的变化较为敏感,叶功能性状与物种多样性之间存在较强的耦合关系。     

Cumulative nitrogen input drives species loss in terrestrial ecosystems

Aim Elevated inputs of biologically reactive nitrogen (N) are considered to be one of the most substantial threats to biodiversity in terrestrial ecosystems. Several attempts have been made to scrutinize the factors driving species loss following excess N input, but generalizations across sites or vegetation types cannot yet be made. Here we focus on the relative importance of the vegetation type, the local environment (climate, soil pH, wet deposition load) and the experimentally applied (cumulative) N dose on the response of the vegetation to N addition. Location Mainly North America and Europe. Methods We conducted a large‐scale meta‐analysis of in situ N addition experiments in different vegetation types, focusing on the response of biomass and species richness. Results Whereas the biomass of grasslands and salt marshes significantly increased with N fertilization, forest understorey vegetation, heathlands, freshwater wetlands and bogs did not show any significant response. Graminoids significantly increased in biomass following N addition, whereas bryophytes significantly lost biomass; shrubs, forbs and lichens did not significantly respond. The yearly N fertilization dose significantly influenced the biomass response of grassland and salt marshes, while for the other vegetation types none of the collected predictor variables were of significant influence. Species richness significantly decreased with N addition in grasslands and heathlands [Correction added on 23 March 2011, after first online publication: ‘across all vegetation types’ changed to ‘in grasslands and heathlands’]. The relative change in species richness following N addition was significantly driven by the cumulative N dose. Main conclusions The decline in species richness with cumulative N input follows a negative exponential pathway. Species loss occurs faster at low levels of cumulative N input or at the beginning of the addition, followed by an increasingly slower species loss at higher cumulative N inputs. These findings lead us to stress the importance of including the cumulative effect of N additions in calculations of critical load values.     

Mechanisms of plant species impacts on ecosystem nitrogen cycling

Plant species are hypothesized to impact ecosystem nitrogen cycling in two distinctly different ways. First, differences in nitrogen use efficiency can lead to positive feedbacks on the rate of nitrogen cycling. Alternatively, plant species can also control the inputs and losses of nitrogen from ecosystems. Our current understanding of litter decomposition shows that most nitrogen present within litter is not released during decomposition but incorporated into soil organic matter. This nitrogen retention is caused by an increase in the relative nitrogen content in decomposing litter and a much lower carbon‐to‐nitrogen ratio of soil organic matter. The long time lag between plant litter formation and the actual release of nitrogen from the litter results in a bottleneck, which prevents feedbacks of plant quality differences on nitrogen cycling. Instead, rates of gross nitrogen mineralization, which are often an order of magnitude higher than net mineralization, indicate that nitrogen cycling within ecosystems is dominated by a microbial nitrogen loop. Nitrogen is released from the soil organic matter and incorporated into microbial biomass. Upon their death, the nitrogen is again incorporated into the soil organic matter. However, this microbial nitrogen loop is driven by plant‐supplied carbon and provides a strong negative feedback through nitrogen cycling on plant productivity. Evidence supporting this hypothesis is strong for temperate grassland ecosystems. For other terrestrial ecosystems, such as forests, tropical and boreal regions, the data are much more limited. Thus, current evidence does not support the view that differences in the efficiency of plant nitrogen use lead to positive feedbacks. In contrast, soil microbes are the dominant factor structuring ecosystem nitrogen cycling. Soil microbes derive nitrogen from the decomposition of soil organic matter, but this microbial activity is driven by recent plant carbon inputs. Changes in plant carbon inputs, resulting from plant species shifts, lead to a negative feedback through microbial nitrogen immobilization. In contrast, there is abundant evidence that plant species impact nitrogen inputs and losses, such as: atmospheric deposition, fire‐induced losses, nitrogen leaching, and nitrogen fixation, which is driven by carbon supply from plants to nitrogen fixers. Additionally, plants can influence the activity and composition of soil microbial communities, which has the potential to lead to differences in nitrification, denitrification and trace nitrogen gas losses. Plant species also impact herbivore behaviour and thereby have the potential to lead to animal‐facilitated movement of nitrogen between ecosystems. Thus, current evidence supports the view that plant species can have large impacts on ecosystem nitrogen cycling. However, species impacts are not caused by differences in plant quantity and quality, but by plant species impacts on nitrogen inputs and losses.     

Plant community responses to nitrogen addition and increased precipitation: the importance of water availability and species traits

Global nitrogen (N) enrichment and changing precipitation regimes are likely to alter plant community structure and composition, with consequent influences on biodiversity and ecosystem functioning. Responses of plant community structure and composition to N addition and increased precipitation were examined in a temperate steppe in northern China. Increased precipitation and N addition stimulated and suppressed community species richness, respectively, across 6 years (2005–2010) of the manipulative experiment. N addition and increased precipitation significantly altered plant community structure and composition at functional groups levels. The significant relationship between species richness and soil moisture (SM) suggests that plant community structure is mediated by water under changing environmental conditions. In addition, plant height played an important role in affecting the responses of plant communities to N addition, and the effects of increased precipitation on plant community were dependent on species rooting depth. Our results highlight the importance and complexity of both abiotic (SM) and biotic factors (species traits) in structuring plant community under changing environmental scenarios. These findings indicate that knowledge of species traits can contribute to mechanistic understanding and projection of vegetation dynamics in response to future environmental change.     

Functional diversity (FD), species richness and community composition

Functional diversity is an important component of biodiversity, yet in comparison to taxonomic diversity, methods of quantifying functional diversity are less well developed. Here, we propose a means for quantifying functional diversity that may be particularly useful for determining how functional diversity is related to ecosystem functioning. This measure of functional diversity “FD” is defined as the total branch length of a functional dendrogram. Various characteristics of FD make it preferable to other measures of functional diversity, such as the number of functional groups in a community. Simulating species' trait values illustrates how the relative importance of richness and composition for FD depends on the effective dimensionality of the trait space in which species separate. Fewer dimensions increase the importance of community composition and functional redundancy. More dimensions increase the importance of species richness and decreases functional redundancy. Clumping of species in trait space increases the relative importance of community composition. Five natural communities show remarkably similar relationships between FD and species richness.     

Environmental determinants of leaf litter ant community composition along an elevational gradient

Ant communities are extremely diverse and provide a wide variety of ecological functions in tropical forests. Here, we investigated the abiotic factors driving ant composition turnover across an elevational gradient at Mont Itoupé, French Guiana. Mont Itoupé is an isolated mountain whose top is covered by cloud forests, a biogeographical rarity that is likely to be threatened according to climate change scenarios in the region. We examined the influence of six soil, climatic, and LiDAR‐derived vegetation structural variables on leaf litter ant assembly (267 species) across nine 0.12‐ha plots disposed at three elevations (ca. 400, 600, and 800m asl). We tested (a) whether species cooccurring within a same plot or a same elevation were more similar in terms of taxonomic, functional, and phylogenetic composition, than species from different plots/elevations, and (b) which environmental variables significantly explained compositional turnover among plots. We found that the distribution of species and traits of ant communities along the elevational gradient was significantly explained by a turnover of environmental conditions, particularly in soil phosphorus and sand content, canopy height, and mean annual relative humidity of soil. Our results shed light on the role exerted by environmental filtering in shaping ant community assembly in tropical forests. Identifying the environmental determinants of ant species distribution along tropical elevational gradients could help predicting the future impacts of global warming on biodiversity organization in vulnerable environments such as cloud forests.     

Mycorrhizal fungi reduce the negative effects of nitrogen enrichment on plant community structure in dune grassland

Nitrogen (N) inputs to ecosystems have increased worldwide, often leading to large changes in plant community structure and reducing plant diversity. Yet, the interaction of increased N availability with other factors that determine plant community composition, are still poorly understood. Here, we test whether the impact of N addition on plant communities depends on the presence of arbuscular mycorrhizal fungi (AMF). AMF are widespread plant symbionts that facilitate growth of many plant species. We hypothesize that AM fungi reduce the negative impact of N addition on plant communities by supporting growth of species that are sensitive to N enrichment.We established experimental grassland microcosms consisting of 18 plant species. These microcosms were subjected to high and low N supply and were inoculated with AMF or remained nonmycorrhizal. Both N addition and AMF had a big impact on plant community composition, but with opposite effects. N addition induced a 2.8‐fold increase in grass biomass and reduced legume biomass. Grasses dominated the microcosms at high N supply, especially when AMF were absent. In contrast, AMF enhanced biomass of all legumes species (on average 6.8‐fold) and reduced the relative abundance of grasses. The proportion of legume biomass out of total shoot biomass at high N supply was 19% with AMF and only 3% without AMF. Our results show that responses of plant communities to N enrichment depend on AMF and that AMF can reduce the negative impact of increased N availability on plant community structure by reducing grass dominance.     

Impacts of nitrogen deposition on terrestrial plant diversity: a meta-analysis in China

Aims With the global atmospheric nitrogen (N) deposition increasing, the effect of N deposition on terrestrial plant diversity has been widely studied. Some studies have reviewed the effects of N deposition on plant species diversity; however, all studies addressed the effects of N deposition on plant community focused on species richness in specific ecosystem. There is a need for a systematic meta-analysis covering multiple dimensions of plant diversity in multiple climate zones and ecosystems types. Our goal was to quantify changes in species richness, evenness and uncertainty in plant communities in response to N addition across different environmental and experimental contexts.     

Neutral responses of plant community Ca concentration to nitrogen enrichment in a semiarid grassland

氮沉降对半干旱草原植物群落钙浓度的影响 钙(Ca)是植物生长所必需的营养物质,牧草中钙的含量对反刍动物的饲粮和健康具有重要意义。目前,关于氮沉降增加是否改变牧草的Ca浓度仍不是十分清楚。我们于2008–2015年在中国北方半干旱草原开展了氮沉降速率增加实验,测定了不同氮处理下每个样方内所有植物的Ca浓度,并且 根据各样方内每个物种的Ca浓度及其相对生物量,计算出功能群水平和群落水平的植物Ca浓度。研究结果表明,尽管物种水平和功能群水平的Ca浓度对氮沉降表现为负响应,但群落水平Ca浓度在整个氮添 加速率梯度上保持稳定。由于杂类草Ca浓度显著高于禾草,杂类草相对生物量的增加抵消了物种水平和功能群水平Ca浓度对氮沉降的负响应。另外,群落Ca库对氮添加速率表现为正饱和响应,且阈值为10 g N m−² yr−¹。本研究表明了氮沉降背景下植物相对生物量的变化对调控牧草Ca浓度和储量具有重要作用。     

Functional traits but not environmental gradients explain seed weight in Mongolian plant species

• Seed weight varies by several orders of magnitude among vascular plant species. However, the importance of potential drivers such as environmental conditions and plant functional traits have rarely been assessed for a larger taxonomic sample.
• We collected seeds of 148 species from 237 sites spread across Mongolia and compared their weight among the major zonal vegetation types, taxonomic groups and a set of functional traits (growth form, dispersal mode, fruit type, storage organs and palatability).
• Seed weight strongly varied among all functional traits and taxonomic groups, but no differences among vegetation zones were detected.
• These results suggest a low impact of environmental conditions on the evolution of seed weight, contrasting the strong phylogenetic signal.

     

内蒙古和青藏高原草原植物叶片与根系氮利用效率空间格局及影响因素

氮利用效率是植物的关键功能性状, 同时紧密关联生态系统功能, 但是目前对氮利用效率的区域格局及影响因素仍然不清楚。该研究分析了内蒙古和青藏高原草原82个调查地点、139种植物叶片和根系的氮利用效率及其与环境因素、植物功能群之间的关系, 实验结果显示: 1)草甸草原植物叶片的氮利用效率为53 g·g ^-1, 显著大于高寒草甸(46 g·g ^-1)、荒漠草原(41 g·g ^-1)和典型草原(39 g·g ^-1)。高寒草甸根系氮利用效率为108 g·g ^-1, 显著高于其他生态系统。2)叶片氮利用效率比根系对温度更加敏感, 但随着干旱指数的增加, 两者均表现出显著的降低趋势。3)杂类草叶片和根系氮利用效率低于莎草科和禾本科植物, 豆科植物叶片和根系氮利用效率分别比非豆科植物低48%和60%。4)植物氮利用效率与土壤氮含量之间没有显著关系。总体上, 内蒙古和青藏高原草原植物叶片和根系氮利用效率的空间格局存在差异, 主要影响因素为植物功能群和干旱指数。本研究系统揭示内蒙古和青藏高原草原植物氮利用效率的空间格局及关键驱动因子, 有助于在全球变化背景下了解我国草地生产力维持机制, 同时为草原生态系统管理提供科学依据。