Relative importance of spatial processes and environmental factors in shaping alpine meadow communities

Aims Spatial processes and environmental control are the two distinct, yet not mutually exclusive forces of community structuring, but the relative importance of these factors is controversial due to the species-specific dispersal ability, sensitivity towards environmental variables, organism's abundance and the effect of spatial scale. In the present paper, we explored spatial versus environmental control in shaping community composition (i.e. β-diversity) and species turnover (i.e. change of β-diversity) at an alpine meadow along a slope aspect gradient on the Qinghai–Tibetan Plateau at different spatial scales of sampling (quadrats and plots), by taking account of seed dispersal mode and abundance.Methods We examined the relative importance of spatial processes and environmental factors using all species and four additional subsets of selected species. Moreover, we attempted to explore the effect of scale (quadrat refers to scale of ~0.3 m and plot of ~8 m) on their counter balance. The data were analyzed both by variation partitioning and multiple regressions on distance matrices. The spatial structure was modelled using Moran's eigenvector maps (MEM).Important findings Both spatial processes and environmental factors were important determinants of the community composition and species turnover. The community composition in the alpine meadow was controlled by spatially structured environment (17.6%), space independent of environment (18.0%) and a negligible effect of environment independent of space (4.4%) at the scale of quadrats. These three components contributed 21.8, 9.9 and 13.9%, respectively, at the scale of plots. The balance between the forces at different spatial scales drove community structures along the slope aspect gradient. The importance of environmental factors on β-diversity at alpine meadow increased with scale while that of spatial processes decreased or kept steady, depending on dispersal mode and abundance of species comprising the subset. But the 'pure' effect of spatial processes on species turnover increased with scale while that of environmental factors decreased. This discrepancy highlights that β-diversity and species turnover were determined jointly by spatial processes and environmental factors. We also found that the relative roles of these processes vary with spatial scale. These results underline the importance of considering species-specific dispersal ability and abundance of species comprising the communities and the appropriate spatial scale in understanding the mechanisms of community assembly.     

Effects of grazing and fertilization on the relationship between species abundance and functional traits in an alpine meadow community on the Tibetan Plateau

Trait‐based approaches can provide a useful tool for linking plant attributes to community structure and ecosystem function. Seed mass and plant height play important roles in the dynamics of plant communities, but few empirical community level studies have tested this, especially in stressful environments. The aim of the present study was to determine if there is a relationship between functional traits (seed mass and plant height) and changes in species relative abundance (SRA) in response to grazing and fertilization. We measured SRA and plant functional traits for 40 common species in a Tibetan Plateau alpine meadow. In the fertilized meadow, seed mass and plant height was significantly positively correlated with the relative abundance of the species. In the grazed meadow, these variables were significantly negatively correlated. Our results demonstrates that plant functional traits can be used to predict the change of SRA in plant community. Grazing promotes the dominance of small‐seeded and short‐stature species, and fertilization facilitates the occurrence of large‐seeded and tall‐stature species.     

非生长季降水对青藏高原高寒草甸优势种生物量稳定性的影响

受全球气候变化的影响,青藏高原在过去的几十年间整体上呈现暖湿化的趋势,相比于年际之间温度和降水的变化外,生长季和非生长季气候变化模式的差异可能会对生态系统产生更重要的影响,但相关的研究尚不充分。以青藏高原东部的高寒草甸为研究对象,基于2001年至2017年17年的野外观测数据,包括优势植物紫花针茅的高度、多度以及生物量、次优势物种洽草的生物量,结合生长季和非生长季平均温度和降水量的变化,通过线性回归以及结构方程模型,探究生长季/非生长季不对称气候变化对于青藏高原高寒草甸优势物种生物量稳定性的影响。研究结果表明：1)青藏高原东部年均温和年降水在过去的17年间显著增加,呈现暖湿化的趋势,但是非生长的降水却变化不明显;2)紫花针茅的高度、多度以及生物量在过去17年没有显著的趋势,但是洽草的生物量稳定性显著减少;3)非生长降水结合紫花针茅的高度、多度以及洽草的生物量稳定性促进了紫花针茅的生物量稳定性。研究结果可以为青藏高原高寒草甸在未来气候变化的背景下合理保护与利用提供科学依据。     

The role of biomass allocation strategy in diversity loss due to fertilization

Proposed mechanisms for explaining biodiversity loss due to fertilization include interspecific competition and assemblage-level thinning. The interspecific competition hypothesis (ICH) assumes a link between population changes and species competitive ability, which is related to functional traits such as biomass allocation patterns. Based on a 2-year fertilization experiment in an alpine meadow on the Tibetan Plateau, we attempted to explore the relationships between individual and population responses. Individual response was measured by changes in plant biomass and biomass allocation, and population response was estimated by changes in species abundance. The results suggested that following fertilization (1) changes in individual biomass differ among species and functional groups, (2) reproductive allocation tends to decrease for all species whereas leaf allocation generally increases for grasses but decreases for forbs, (3) a strong positive correlation exists between species relative abundance change and individual biomass response, and (4) species relative abundance change has a positive correlation with leaf allocation response, a negative correlation with stem allocation response, and no significant correlation with reproductive allocation response. We conclude that the individual biomass responses and biomass allocation strategy can partly explain diversity loss due to fertilization, a result consistent with the ICH.     

高寒草甸植物群落中相似性、物种多样性与地上生物量的年际变异性

相似性似说通过物种构成的相似性来解释物种丧失是如何影响生物量的变异性的,但还没有得到检验。本研究通过设置在青藏高原东部地区的高寒草甸植物群落中的74个永久样方．采集3年(1999～2001)植物生长高峰期的群落数据,试图检验物种构成的相似性是如何解释物种多样性对地上生物量年际变异性的影响。结果表明：随着物种丰富度增加,生物量变异性降低;而随着均匀度的增加,生物量的变异性尽管在均匀度中等程度时似乎保持在同一水平,但总体上呈下降趋势;物种构成上的相似性解释了地上生物量变异性的大部分,而且随着物种构成上的相似性的增加,生物量的变异性降低;物种丰富度和均匀度均与物种构成上的相似性没有显著相关关系。这些结果表明：尽管生物多样性的丧失可能不必导致物种丰富群落中物种构成上的相似性,但相似性与地上生物量的变异性的因果联系可能是稳健的．由于本研究是在自然群落中进行的,对物种构成的相似性没有进行直接控制,因此,要深入理解相似性是如何影响生物多样性对生态系统功能变异性的效应的机制,可能还需要直接对物种构成的相似性进行控制的实验研究。     

Arbuscular mycorrhiza regulate inter-specific competition between a poisonous plant, <Emphasis Type="Italic">Ligularia virgaurea</Emphasis>, and a co-existing grazing grass, <Emphasis Type="Italic">Elymus nutans</Emphasis>, in Tibetan Plateau Alpine meadow ecosystem

In order to investigate the function of arbuscular mycorrhizae in Tibetan Plateau Alpine meadow ecosystems, experiments were carried out to evaluate the influence of arbuscular mycorrhizal (AM) fungi on inter-specific competition between the poisonous plant, Ligularia virgaurea, and the grazing grass, Elymus nutans, at different relative densities. Our results showed that the biomass of L. virgaurea significantly declined in AM treatments while the biomass of E. nutans increased in our inter-specific competition system. Relative yields and root/shoot ratios of these two species indicated that AM fungi had a significant influence on the growth of L. virgaurea and E. nutans. This implies that arbuscular mycorrhiza do not benefit both plant species equally but rather contribute to the growth of E. nutans more than L. virgaurea. We thus provide evidence that AM fungi could regulate plant interactions and influence the plant community structure in the Tibetan Plateau Alpine meadow ecosystem.     

The effects of fertilization on the trait-abundance relationships in a Tibetan alpine meadow community

Aims Comparisons of the trait–abundance relationships from various habitat types are critical for community ecology, which can offer us insights about the mechanisms underlying the local community assembly, such as the relative role of neutral vs. niche processes in shaping community structure. Here, we explored the responses of trait–abundance relationships to nitrogen (N), phosphorus (P) and potassium (K) fertilization in an alpine meadow.Methods Five fertilization treatments (an unfertilized control and additions of N, P, K and NPK respectively) were implemented using randomized block design in an alpine Tibetan meadow. Species relative abundance (SRA), plant above-ground biomass and species richness were measured in each plot. For 24 common species, we measured species functional traits: saturated height, specific leaf area (SLA) and leaf dry matter content (LDMC) in each treatment but seed size only in the unfertilized control. Standard major axis (SMA) regression and phylogenetically independent contrasts (PICs) analysis were used to analyse species trait–abundance relationships in response to different fertilization treatments.Important findings Positive correlations between SRA and saturated height were raised following N, P and NPK fertilizations, which indicated an increase in light competition in these plots. In P fertilized plots, SRA was also positively correlated with LDMC because tall grasses with a nutrients conservation strategy often have a relative competitive advantage in capturing limited light and soil nutrients. In K fertilized plots, neither the trait–abundance relationships nor above-ground biomass or species richness significantly differed from that in the control, which suggests that K was not a limiting resource in our study site. These significant correlations between species traits and relative abundance in fertilized treatment suggest that trait-based selection plays an important role in determining species abundance within local communities in alpine meadows.     

Linking species performance to community structure as affected by UV-B radiation: an attenuation experiment

Aims UV-B radiation is known to affect plant physiology and growth rate in ways that can influence community species composition and structure. Nevertheless, comparatively little is known about how UV-B radiation induced changes in the performance of individual species cascades to affect overall community properties. Because foliage leaves are primarily responsible for photosynthesis and carbon gain and are the major organ that senses and responds to UV-B radiation, we hypothesized that, under reduced UV-B radiation, species with larger leaf areas per plant would manifest higher growth rates and hence tend to improve their community status compared to species with smaller leaf areas per plant in herbaceous plant communities.Methods We tested this hypothesis by examining plant traits (leaf area per plant and plant height), plant growth rate (aboveground biomass per plant and plant biomass per area) and community status (species within-community relative biomass) for 19 common species in a two-year field experiment in an alpine meadow on Tibetan Plateau.Important findings Aboveground biomass per plant, as well as per area, progressively increased in a 39% reduced (relative to ambient) UV-B treatment during the experimental period. At the second year, 11 out of 19 species significantly or marginally significantly increased their plant height, leaf area per plant and aboveground biomass per plant. No species was negatively affected by reducing UV-B. As hypothesized, the increase in aboveground biomass per plant increased with increasing leaf area per plant, as indicated by cross-species regression analysis. Moreover, the change in species within-community status increased with increasing leaf area per plant. Our study demonstrates that UV-B radiation has differential effects on plant growth rate across species and hence significantly affects species composition and plant community structure. We suggest that UV-B radiation is an ecological factor structuring plant communities particularly in alpine and polar areas.     

Seasonal Dynamics of the Plant Community and Soil Seed Bank along a Successional Gradient in a Subalpine Meadow on the Tibetan Plateau

Background

Knowledge about how change the importance of soil seed bank and relationship between seed mass and abundance during vegetation succession is crucial for understanding vegetation dynamics. Many studies have been conducted, but their ecological mechanisms of community assembly are not fully understood.

Methodology

We examined the seasonal dynamics of the vegetation and soil seed bank as well as seed size distribution along a successional gradient. We also explored the potential role of the soil seed bank in plant community regeneration, the relationship between seed mass and species abundance, and the relative importance of deterministic and stochastic processes along a successional gradient.

Principal Findings

Species richness of seed bank increased (shallow layer and the total) and seed density decreased (each layer and the total) significantly with succession. Species richness and seed density differed significantly between different seasons and among soil depths. Seed mass showed a significant negative relationship with relative abundance in the earliest successional stage, but the relationships were not significant in later stages. Seed mass showed no relationship with relative abundance in the whole successional series in seed bank. Results were similar for both July 2005 and April 2006.

Conclusions

The seed mass and abundance relationship was determined by a complex interaction between small and larger seeded species and environmental factors. Both stochastic processes and deterministic processes were important determinants of the structure of the earliest stage. The importance of seed bank decreased with succession. The restoration of abandoned farmed and grazed meadows to the species-rich subalpine meadow in Tibetan Plateau can be successfully achieved from the soil seed bank. However, at least 20 years are required to fully restore an abandoned agricultural meadow to a natural mature subalpine meadow.     

Relationships between aboveground biomass and plant cover at two spatial scales and their determinants in northern Tibetan grasslands

The relationships between cover and AGB for the dominant and widely distributed alpine grasslands on the northern Tibetan Plateau is still not fully examined. The objectives of this study are to answer the following question: (1) How does aboveground biomass (AGB) of alpine grassland relate to plant cover at different spatial scales? (2) What are the major biotic and abiotic factors influencing on AGB–cover relationship? A community survey (species, cover, height, and abundance) was conducted within 1 m × 1 m plots in 70 sites along a precipitation gradient of 50–600 m. Ordinary linear regression was employed to examine AGB–cover relationships of both community and species levels at regional scale of entire grassland and landscape scale of alpine meadow, alpine steppe, and desert steppe. Hierarchical partitioning was employed to estimate independent contributions of biotic and abiotic factors to AGB and cover at both scales. Partial correlation analyses were used to discriminate the effects of biotic and abiotic factors on AGB–cover relationships at two spatial scales. AGB and community cover both exponentially increased along the precipitation gradient. At community level, AGB was positively and linearly correlated with cover for all grasslands except for alpine meadow. AGB was also linearly correlated with cover of species level at both regional and landscape scales. Contributions of biotic and abiotic factors to the relationship between AGB and cover significantly depended on spatial scales. Cover of cushions, forbs, legumes and sedges, species richness, MAP, and soil bulk density were important factors that influenced the AGB–cover relationship at either regional or landscape scale. This study indicated generally positive and linear relationships between AGB and cover are at both regional and landscape scales. Spatial scale may affect ranges of cover and modify the contribution of cover to AGB. AGB–cover relationships were influenced mainly by species composition of different functional groups. Therefore, in deriving AGB patterns at different spatial scales, community composition should be considered to obtain acceptable accuracy.     

Spatial patterns and associations of four species in an old-growth temperate forest

The spatial pattern of a tree species is an important characteristic of plant communities, providing critical information to explain species coexistence. The spatial distribution and association of four different successional species were analyzed among different life-history stages in an old-temperate forest. Significant aggregation patterns were found, and the degree of aggregation decreased with the scales and life-history stages. Significant interspecific spatial associations were detected. In comparing the relationships among the different life-history stages, positive associations were found at small scales in all of the juvenile species pairs. In the adult stage, negative associations were detected in coniferous vs. deciduous species pairs, while the deciduous species pairs, which have identical resource requirements, showed a positive association in this study. The coniferous species pairs showed a positive association at small scales. We infer that seed dispersal, competitive ability, or the requirement for specific topographic and light environments may contribute to the coexistence of these species.     

Linking grazing response of species abundance to functional traits in the Tibetan alpine meadow

Whether grazing response of species abundance can be predicted by plant functional trait remains a challenge untested in a specific ecosystem. We measured species abundance and relevant functional traits for 30 common component species in a moderately grazed and a control community over 3 years (2005, 2006 and 2007) in a Kobresia capillifolia dominated alpine meadow on the Tibetan Plateau. Our objective was to examine species response to grazing and to test the relationship between the response of species abundance and functional traits in grazed and control habitats. Our results showed: i) in terms of species relative abundance and saturated height (the maximum height of an adult individual), most component species significantly decrease in response to moderate grazing and the effect differed among species and between functional groups. ii) The response of species abundance was significantly negatively correlated with saturated height, but not correlated with specific leaf area or seed size. We concluded that the response direction of species abundance to grazing can be predicted by functional traits, whereas it is a weak predictor of the extent of grazing response in species abundance.     

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.     

Geographic variation in primary sex allocation per flower within and among 12 species of Pedicularis (Orobanchaceae): Proportional male investment increases with elevation

? Premise of the study: The study of geographic variation in ecologically important traits within and among taxa is a first step toward understanding the environmental factors that contribute to population differentiation and species divergence. This study examines variation in mean sex allocation per flower (androecium mass/gynoecium mass) among 49 wild populations representing 12 Pedicularis species across an elevation gradient on the eastern Tibetan Plateau. ? Methods: We used population means to evaluate sources of variation in per-flower sex allocation within and across species. In particular, we evaluate the relative influence of intrinsic (i.e., plant size, estimated as aboveground stem biomass) vs. extrinsic factors affecting mean sex allocation among populations. ? Key results: Mean sex allocation per flower (the relative investment in male floral organs) is negatively correlated with mean plant size; populations of large plants produce relatively female-biased flowers. This relationship between mean plant size and mean sex allocation is not statistically significant, however, when the effect of elevation is controlled statistically. Among populations within and across species, mean sex allocation increases with elevation. This relationship persists even when the effect of mean plant size is controlled statistically. Factors associated with increasing elevation appear to favor genotypes and/or taxa with male-biased flowers. ? Conclusion: Extrinsic environmental conditions may be more important than intrinsic resource status in determining patterns of geographic variation in mean sex allocation among populations or species of Pedicularis. We cannot conclude whether the effect of elevation on mean sex allocation is the result of environmentally induced plasticity, genetically based adaptation, or species sorting, but it is only partly mediated by mean plant size.     

Niche and Neutral Processes Together Determine Diversity Loss in Response to Fertilization in an Alpine Meadow Community

Fertilization via nutrient deposition and agricultural inputs is one of the most important factors driving decreases in plant diversity. However, we still do not fully understand which processes (niche process or neutral process) are more important in leading to decreases in plant diversity caused by fertilization. A hypothesis-based approach was used to test the relative importance of niche versus neutral processes along a fertilization gradient in an alpine meadow community on the eastern Tibetan plateau, China. Niche overlap values were calculated for species biomass, and the null model was used to generate the values of niche overlap expected at random. A linear regression modeling was used to evaluate the relationship between functional traits (specific leaf area, leaf dry matter content, and leaf total nitrogen concentration) and species relative abundance. Our results demonstrated that observed niche overlap for species biomass was significantly higher than expected at lower fertilization gradients. Moreover, we also found a significantly negative correlation between species relative abundance and specific leaf area and leaf dry matter content, but a significantly positive correlation between relative abundance and leaf nitrogen concentration at lower fertilization gradients. However, these relationships were not significant at higher fertilization gradients. We concluded that community assembly is dynamic progression along the environmental gradients, and niche and neutral processes may together determine species diversity loss in response to fertilization.     

科尔沁沙地流动沙丘3种常见植物的空间分布格局与异速生长

研究流动沙丘上物种小尺度分布格局和植物异速分配特点有助于理解植物在流动沙丘生境上的适应策略.调查了科尔沁沙地28个1 m×1 m样方内沙蓬、烛台虫实和狗尾草3个物种的位置、多度、高度以及生物量信息.采用最近邻体法与O-ring O(r)函数方法进行空间格局分析.结果表明： 在1 m×1 m样方内,群落水平上植物个体间以聚集分布为主,而在种群水平上物种大体以随机分布为主;个体间距离在0～50 cm范围内无论种内还是种间,它们以互助作用为主,在特定尺度上物种出现聚集分布并随1 m×1 m样方内植物多度增加而变化.在0～40 cm内,沙蓬和狗尾草聚集分布增加;在10～20 cm内,3个物种的种内和种间聚集均增加;在10～30 cm内,狗尾草与沙蓬、烛台虫实出现种间正相关的概率增加;在40～50 cm内,沙蓬与烛台虫实以及沙蓬与狗尾草出现正相关的概率增加.物种构件结果表明：3个物种地上部分生长速度显著大于地下部分,狗尾草根冠比最大且营养器官的生物量比例居中,烛台虫实分配给生殖器官和茎的生物量比重较大,沙蓬分配给营养器官的比重较大.结合小尺度上的格局分析与构件分配特点,可以推断3种植物是典型的r对策者.     

藏香猪翻拱型放牧对不同类型高寒草甸植物特征的影响及土壤环境驱动力

生态系统脆弱的高寒草甸是生态学研究的热点。藏香猪翻拱土壤并取食地下草根层,是藏族自治区一种特有的放牧类型,关于该放牧类型对藏区高寒草甸的影响尚缺乏研究。以滇西北高原国际重要湿地——纳帕海流域的典型高寒草甸为对象,研究了不同高寒草甸类型(陆生草甸与沼泽化草甸)中的植物群落结构特征对藏香猪翻拱型放牧的响应。结果表明,翻拱型放牧显著降低了植被盖度(63.5%)、地上生物量(84.6%)与地下生物量(97.4%),并促进植物生物量向地上部分的再分配。翻拱型放牧下随着植被盖度与生物量降低,植物竞争强度减小,植株高度与空间利用率提高。然而,不同草甸类型中的植物群落结构对翻拱型放牧表现出差异响应。陆生草甸上的群落组成基本未变,而沼泽化草甸上的优势植物华扁穗草(Blysmus sinocompyessus)向1年生水生植物水蓼(Polygonum hydropiper)演替。土壤含水率是导致植物响应差异的关键驱动力。综上,藏香猪翻拱型放牧对植物生物量及盖度等特征均产生不利影响,导致高寒草甸生物量积累锐减,而植物群落结构的变化是放牧干扰与土壤水分、空间竞争协同作用的结果,反映出植物在不同环境中对放牧干扰的差异化响应。研究区域环境特征与放牧干扰的耦合关系,可以为藏香猪放牧作用下的高寒草地植物多样性保护及恢复提供理论依据。     

Multiple scales and the relationship between density and spatial aggregation in littoral zone communities

Understanding the constraints on community composition at multiple spatial scales is an immense challenge to community and ecosystem ecologists. As community composition is basically the composite result of species’ spatial patterning, studying this spatial patterning across scales may yield clues as to which scales of environmental heterogeneity influence communities. The now widely documented positive interspecific relationship between ‘regional’ range and mean ‘local’ abundance has become a generalisation describing the spatial patterning of species at coarse scales. We address some of the shortcomings of this generalisation, as well as examine the cross‐scale spatial patterning (aggregation and density levels) of littoral‐benthic invertebrates in very large lakes. Specifically, we (a) determine whether the positive range‐abundance relationship can be reinterpreted in terms of the actual spatial structure of species distributions, (b) examine the relationship between aggregation and density across different spatial scales, and (c) determine whether the spatial patterning of species (e.g. low density/aggregated distribution) is constant across scales, that is, whether our interpretation of a species spatial pattern is dependent on the scale at which we choose to observe the system. Spatial aggregation of littoral invertebrates was generally a negative function of mean density across all spatial scales and seasons (autumn and spring). This relationship may underlie positive range‐abundance relationships. Species that were uncommon and highly aggregated at coarse spatial scales can be abundant and approach random distributions at finer spatial scales. Also, the change in spatial aggregation of closely related taxa across spatial scales was idiosyncratic. The idiosyncratic cross‐scale spatial patterning of species implies that multiple scales of environmental heterogeneity may influence the assembly of littoral communities. Due to the multi‐scale, species‐specific spatial patterning of invertebrates, littoral zone communities form a complex spatial mosaic, and a ‘spatially explicit’ approach will be required by limnologists in order to link littoral‐benthic community patterns with ecosystem processes in large oligotrophic lakes.     

Aggregated spatial distributions of species in a subtropical karst forest,southwestern China

Aims Spatial distribution patterns of species reflect not only the ecological processes but also the habitat features that are related to species distribution. In karst topography, species distribution patterns provide more specific information about their environments. The objectives of this study are as follows: (i) to analyse and explain the spatial distribution patterns of conspecific trees in an old-growth subtropical karst forest; (ii) to investigate pattern changes at different spatial scales; (iii) to test the spatial pattern similarity (or dissimilarity) between trees at different abundances, diameter at breast height classes, canopy layers and different functional groups (shade tolerance and seed dispersal mode); (iv) to examine whether habitat heterogeneity has an important effect on the species spatial distribution.Methods The spatial distributions of woody species with ≥20 individuals in a 1-ha subtropical karst forest plot at Maolan in southwestern China were quantified using the relative neighbourhood density Ω based on the average density of conspecific species in a circular neighbourhood around each species.Important findings Aggregated distribution is the dominant pattern in the karst forest, but the ratio of aggregated species in total species number decreases with an increase in spatial scale. Less abundant species are more aggregated than most abundant species. Aggregation is weaker in larger diameter classes, which is consistent with the prediction of self-thinning. Seed dispersal mode influences spatial patterns, with species dispersed by animals being less aggregated than those dispersed by wind and gravity. Other species functional traits (e.g. shade tolerance) also influence the species spatial distributions. Moreover, differences among species habitat associations, e.g. with rocky outcrops, play a significant role in species spatial distributions. These results indicate that habitat heterogeneity, seed dispersal limitation and self-thinning primarily contribute to the species spatial distributions in this subtropical karst forest.     

Estimation of aboveground biomass using in situ hyperspectral measurements in five major grassland ecosystems on the Tibetan Plateau

Aims There are numerous grassland ecosystem types on the Tibetan Plateau. These include the alpine meadow and steppe and degraded alpine meadow and steppe. This study aimed at developing a method to estimate aboveground biomass (AGB) for these grasslands from hyperspectral data and to explore the feasibility of applying air/satellite-borne remote sensing techniques to AGB estimation at larger scales.Methods We carried out a field survey to collect hyperspectral reflectance and AGB for five major grassland ecosystems on the Tibetan Plateau and calculated seven narrow-band vegetation indices and the vegetation index based on universal pattern decomposition (VIUPD) from the spectra to estimate AGB. First, we investigated correlations between AGB and each of these vegetation indices to identify the best estimator of AGB for each ecosystem type. Next, we estimated AGB for the five pooled ecosystem types by developing models containing dummy variables. At last, we compared the predictions of simple regression models and the models containing dummy variables to seek an ecosystem type-independent model to improve prediction of AGB for these various grassland ecosystems from hyperspectral measurements.Important findings When we considered each ecosystem type separately, all eight vegetation indices provided good estimates of AGB, with the best predictor of AGB varying among different ecosystems. When AGB of all the five ecosystems was estimated together using a simple linear model, VIUPD showed the lowest prediction error among the eight vegetation indices. The regression models containing dummy variables predicted AGB with higher accuracy than the simple models, which could be attributed to the dummy variables accounting for the effects of ecosystem type on the relationship between AGB and vegetation index (VI). These results suggest that VIUPD is the best predictor of AGB among simple regression models. Moreover, both VIUPD and the soil-adjusted VI could provide accurate estimates of AGB with dummy variables integrated in regression models. Therefore, ground-based hyperspectral measurements are useful for estimating AGB, which indicates the potential of applying satellite/airborne remote sensing techniques to AGB estimation of these grasslands on the Tibetan Plateau.