Use of near-infrared reflectance spectroscopy to predict species composition in tree fine-root mixtures

Assessment of belowground interactions in mixed forests has been largely constrained by the ability to distinguish fine roots of different species. Here, we explored near infrared reflectance spectroscopy (NIRS) to predict the proportion of woody fine roots in mixed samples and analyzed whether the prediction quality of NIRS models is related to the complexity of the fine-root mixture. For model calibration and validation purposes, 11 series of artificial mixed species samples containing known amounts of fine roots of up to four temperate tree species and non-woody plants were prepared. Three types of models with different calibration/validation approaches were developed and tested against external independent data for additional validation. With these models the proportion of each species in root mixtures was predicted accurately with low standard error of prediction (RMSECV/RMSEP <6.5%) and high coefficient of determination (r²?>?0.93) for all fine-root mixtures. In addition, NIRS models also provided satisfactory estimates for samples with low (<15%) or no content of particular components. The predictive power of the NIRS models did not decrease substantially with increasing complexity of the root samples. The approach presented here is a promising alternative to hand sorting of fine roots, which may be influenced substantially by operator variation, and it will facilitate investigating belowground interactions between woody species.     

The potential use of n-alkanes, long-chain alcohols and long-chain fatty acids as diet composition markers: indoor validation with sheep and herbage species from the rangeland of Inner Mongolia of China

To investigate the potential use of n-alkanes (alkanes), long-chain alcohols (alcohols) and long-chain fatty acids (acids) for estimating the diet composition of sheep, in a feeding trial. A total of 18 sheep were assigned randomly to three different diets (diet A, diet B and diet C) containing up to eight herbage species (Leymus chinensis, Leymus dasystachys, Elymus sibiricum, Chenopodium album, Puccinellia chinampoensis, Medicago sativa, Saussurea sinuata and Bromus inermis). Faecal recoveries of alkanes, alcohols and acids were determined, and diet compositions were estimated using different combinations of alkanes, alcohols and acids. The faecal concentrations of individual alkanes, alcohols and acids were corrected using the mean recovery of the dietary treatment that the respective animal belonged to (diet recovery), or the mean recovery across all dietary treatments (general recovery). In general, diets did not affect the faecal recovery values for alkanes, alcohols and acids, and no difference in accuracy was found between diet composition estimates based on dietary recovery and general recovery. The accuracy of diet composition estimates declined as the number of dietary components increased from four to eight herbage species (P < 0.001). Better (P < 0.05) estimates of diet composition were obtained with the combinations of two or three marker types instead of alkanes alone. Moreover, results showed that excluding minor diet components from the calculations decreased (P < 0.05) the accuracy of diet composition estimates, whereas including extra non-grazed herbage species did not reduce (P > 0.05) the quality of diet composition estimates. These results confirmed the usefulness of alkanes, alcohols and acids as markers for determining complex diet composition of sheep. However, a negative impact on the accuracy of diet composition estimates, caused by missing minor diet components from the calculation of diet composition, could happen when plant wax markers are used to estimate the diet composition of free-ranging animals.     

In vitro studies of the metabolism of [14C]- n-alkanes using ruminal fluid of sheep as substrate

Whether the rumen microbes are able to synthesize and/or degrade long-chain alkanes in anaerobic conditions remains a question to be answered before these hydrocarbons can be confidently used as duodenal flow or rumen transit markers. In this context, an experiment in vitro was carried out to establish whether within a rumen liquor fermentation system, n-alkanes can be derived from de-waxed structures of the plant or from non-alkane wax components (long-chain fatty alcohols, long-chain fatty acids and esters), or may be metabolized by bacteria to other components or to shorter-chain hydrocarbons. Ryegrass was labelled with 14C in growth chambers under controlled conditions in order to use it as a substrate. The labelled material obtained was separated in three fractions: labelled alkanes, labelled de-waxed plant and labelled wax components without the alkanes. These fractions were used for three different incubations in vitro, which objectives were as follows: 1. To check whether rumen bacteria can synthesize alkanes from carbon structures other than waxes (e.g. sugars). 2. To verify whether rumen bacteria can metabolize the n-alkanes to other compounds. 3. To check whether rumen bacteria can synthesize n-alkanes from other carbon compounds from waxes. The results showed that there was neither bacterial synthesis nor metabolism of the n-alkanes in in vitro conditions.     

Evaluation of long-chain alcohols as diet composition markers in goats grazing heathland areas

An experiment was conducted to assess the potential of long-chain alcohols (LCOH), in alternative or combined with alkanes and long-chain fatty acids (LCFA), as faecal markers to estimate the diet composition of goats grazing heathland vegetation with associated improved pastures. A total of seven diets were offered across the grazing season. The diets were composed of mixtures of herb species (Lolium perenne and Trifolium repens) and woody species (Erica spp., Calluna vulgaris and Ulex gallii) in an attempt to simulate diet selection of goats on these complex vegetation areas. The diet composition was estimated using LCOH markers alone or combined with alkanes, LCFA and alkanes+LCFA, by least square optimization procedures. The data showed large differences between plant species in their LCOH profile. Generally, plant species showed higher LCOH concentrations than those of alkanes and lower than LCFA markers. Faecal recovery of LCOH was incomplete and increased in a linear manner (P < 0.001) with the carbon-chain length, and was influenced by diet composition and its digestibility. The diet composition estimates based on LCOH alone were more accurate (P < 0.05) than those using alkanes or LCFA alone. Results showed that the combination of LCOH with alkanes, LCFA and alkanes+LCFA resulted in more accurate (P < 0.05) estimates of diet composition, indicating that LCOH provided different discriminatory information to that of alkanes and LCFA, helping in the discrimination of the plant species used in this experiment. Results indicate that correction of faecal LCOH concentrations to incomplete faecal recovery is necessary to obtain more accurate estimates of diet composition. Nevertheless, it seemed that the use of a less accurate recovery correction on LCOH markers had a lower impact on the accuracy of estimates than that of alkanes and LCFA.     

Assessment of Root System Dynamics of Species Grown in Mixtures under Field Conditions using Herbicide Injection and 15N Natural Abundance Methods: A Case Study with Pea, Barley and Mustard

Two methods were developed and used to study the root system dynamics of two species grown together or separately under field conditions. The first method, based on herbicide injection at different soil depths, was used to determine the rooting depth penetration rate of each species in pea–barley and pea–mustard mixtures. The roots absorbed the herbicide when they reached the treated zone leading to visible symptoms on the leaves which could be readily monitored. The second method used differences in ¹⁵N natural abundance and N concentration between legume and non-legume species to quantify the contribution of each species to root biomass of a pea–barley mixture. Each contribution was calculated using ¹⁵N abundance and N concentration of root mixtures and of subsamples of roots of individual species within mixtures. Both methods can indeed be used to distinguish roots of species in mixtures and thus to study belowground competition between associated species. The use of these methods demonstrated species differences in root system dynamics between species but also significant effects of interactions between species in mixtures. The rooting depth penetration rate was mainly species specific whereas root biomass was dependant on plant growth, allocation of dry matter between shoot and root components and growth factors such as N fertilization. Root biomass of each species may vary therefore with the level of competition between species.     

Interspecific interactions and biomass allocation among grassland plant species

Although a handful of studies have shown how interspecific interactions may influence plant shoot to root ratios, the issue of how these interactions influence biomass partitioning among coexisting plant species remains largely unexplored. In this study, we determined whether a given plant species could induce other plant species to allocate relative biomass to each of four zones (aboveground, and three soil depth layers) in a different manner to what they would otherwise, and whether this may influence the nature of competitive or facilitative interactions amongst coexisting plant species. We used a glasshouse study in which mixtures and monocultures of ten grassland plant species were grown in cylindrical pots to determine the effects of plant species mixtures versus monocultures on the production of shoots and of roots of other species for each of three soil depths. Across all experiments, stimulation of production in mixtures was far less common than suppression of production. Different plant species shifted their allocation to shoots or roots at different depths, suggesting that interspecific interactions can either: (1) increase the ratio of deep to shallow roots, perhaps because competition reduces root growth in the uppermost part of the soil profile; or (2) decrease this ratio by reducing plant vigour to such an extent that the plant cannot produce roots that can reach deep enough to exploit resources at lower depths. Further, these results suggest that there are instances in which competition may have the potential to enforce resource partitioning between coexisting plant species by inducing different species to root at different depths to each other.     

Solvation thermodynamics of xenon in n-alkanes, n-alcohols and water

Experimental thermodynamic data for the solvation of xenon in n-alkanes, water and n-alcohols, from different sources, are analysed by means of a general theory of solvation. The standard solvation Gibbs energy change is given by the sum of the work spent to create a cavity suitable to host xenon and the energy gained to turn on xenon-solvent attractive interactions. The latter contribution is larger in magnitude than the former in both n-alkanes and n-alcohols; the reverse holds in water. This finding is due to the fact that liquid water is characterized by the largest work of cavity creation, because of the smallness of its molecules. Since the two contributions to the solvation Gibbs energy change are both larger in magnitude in n-alcohols with respect to n-alkanes of the same number of heavy atoms, the xenon solubility is not so different between the two classes of solvents. The H-bonds play an indirect role, being important to determine the density of the liquid. It is also shown that there is no clear correlation between the cohesive energy density of the liquid and the solubility of xenon in the liquid itself. The present study confirms that the effective molecular size of solvent molecules is the principal factor in controlling the solvation Gibbs energy changes.     

Do species evenness and plant density influence the magnitude of selection and complementarity effects in annual plant species mixtures?

Abstract Plant species richness influences primary productivity via mechanisms that (1) favour species with particular traits (selection effect) and (2) promote niche differentiation between species (complementarity). Influences of species evenness, plant density and other properties of plant communities on productivity are poorly defined, but may depend on whether selection or complementarity prevails in species mixtures. We predicted that selection effects are insensitive to species evenness but increase with plant density, and that the converse is true for complementarity. To test predictions, we grew three species of annuals in monocultures and in three‐species mixtures in which evenness of established plants was varied at each of three plant densities in a cultivated field in Texas, USA. Above‐ground biomass was smaller in mixtures than expected from monocultures because of negative ‘complementarity’ and a negative selection effect. Neither selection nor complementarity varied with species evenness, but selection effects increased at the greatest plant density as predicted.     

Moving beyond the ubiquitous: the diversity and biosynthesis of specialty compounds in plant cuticular waxes

Cuticular waxes coat aerial plant surfaces to protect tissues against biotic and abiotic stress. The waxes are complex mixtures of fatty-acid-derived lipids formed on modular biosynthetic pathways, with varying chain lengths and oxygen functional groups. The waxes of most plant species contain C₂₆–C₃₂ alcohols, aldehydes, alkanes, and fatty acids together with their alkyl esters, and comparisons between diverse wax mixtures have revealed matching chain length distributions between some of these compound classes. Based on such patterns, the biosynthetic pathways leading to the ubiquitous wax constituents were hypothesized early on, and most of these pathway hypotheses have since been confirmed by biochemical and molecular genetic studies in model species. However, the most abundant wax compounds on many species, including many important crop species, contain secondary functional groups and thus their biosynthesis differs at least in part from the ubiquitous wax compounds with which they co-occur. Here, we survey the chemical structures of these species-specific specialty wax compounds based on a comprehensive CAS SciFinder search and then review relevant reports on wax compositions to help develop and refine hypotheses for their biosynthesis. Across the plant kingdom, specialty wax compounds with one, two, and three secondary functional groups have been identified, with most studies focusing on Angiosperms. Where multiple specialty wax compounds were reported, they frequently occurred as homologous series and/or mixtures of isomers. Among these, it is now possible to recognize series of homologs with predominantly odd- or even-numbered chain lengths, and mixtures of isomers with functional groups on adjacent or on alternating carbon atoms. Using these characteristic molecular geometries of the co-occurring specialty compounds, they can be categorized and, based on the common structural patterns, mechanisms of biosynthesis may be predicted. It seems highly likely that mixtures of isomers with secondary functions on adjacent carbons arise from oxidation catalyzed by P450 enzymes, while mixtures of isomers with alternating group positions are formed by malonate condensation reactions mediated by polyketide synthase or ketoacyl-CoA synthase enzymes, or else by the head-to-head condensation of long-chain acyls. Though it is possible that some enzymes leading to ubiquitous compounds also participate in specialty wax compound biosynthesis, comparisons between co-occurring ubiquitous and specialty wax compounds strongly suggest that, at least in some species, dedicated specialty wax compound machinery exists. This seems particularly true for the diverse species in which specialty wax compounds, most notably nonacosan-10-ol, hentriacontan-16-one (palmitone), and very-long-chain β-diketones, accumulate to high concentrations.     

Selective transport and accumulation of alkanes by Rhodococcus erythropolis S+14He

Selective transport and accumulation of n-alkanes by Rhodococcus erythropolis S+14He was studied by growing cells on n-hexadecane, n-octadecane or the branched alkane pristane, and on mixtures of hydrocarbons. Ultrastructural analysis by transmission electron microscopy (TEM) revealed hydrocarbon inclusion bodies present in cells grown on the three alkanes, but not in cells grown on soluble media or exposed to nonmetabolized 2,2,4,4,6,8,8-heptamethylnonane (HMN). n-Hexadecane had the highest rates of accumulation within the cells and higher overall consumption rates relative to the other alkanes. These rates decreased when the molar concentration of n-hexadecane was decreased in hydrocarbon mixtures, but at the same time the accumulation of n-hexadecane in intracellular inclusions became increasingly selective. Sodium azide significantly inhibited the accumulation of n-hexadecane, consistent with an active transport mechanism for accumulation. These results indicate that R. erythropolis S+14He is able to selectively discriminate and preferentially transport n-hexadecane from mixtures of structurally similar alkanes into intracellular inclusions by an energy-driven transport system. This selective membrane transport of hydrocarbon isomers has potential application for separations, bioprocessing, and the development of novel biosensors.     

Facilitation and interference among three predators affect their consumption of a stream-dwelling mayfly

1. We experimentally tested if a multiplicative risk model accurately predicted the consumption of a common mayfly at risk of predation from three predator species in New Zealand streams. Deviations between model predictions and experimental observations were interpreted as indicators of ecologically important interactions between predators. 2. The predators included a drift‐feeding fish [brown trout (T), Salmo trutta], a benthivorous fish [galaxiid (G), koaro, Galaxias brevipennis] and a benthic predatory stonefly (S; Stenoperla sp.) with Deleatidium sp. mayflies as prey. Eight treatments with all predator species combinations and a predator‐free control were used. Experiments were performed in aquaria with cobbles as predator refuges for mayflies and we measured the proportion of prey consumed after 6 h for both day and night trials. 3. Trout consumed a higher proportion of prey than other predators. For the two predator treatments we found less than expected prey consumption in the galaxiid + trout treatment (G + T) for both day and night trials, whereas a higher than expected proportion of prey was consumed during night time in the stonefly + trout (S + T) treatment. 4. The results indicate interference (G + T) and facilitation (S + T) between predators depending on predator identity and time of day. Thus, to make accurate predictions of interspecific interactions, it is necessary to consider the ecology of individual species and how differences influence the direction and magnitude of interactions.     

Substrate interactions in dehalogenation of 1,2-dichloroethane, 1,2-dichloropropane,and 1,1,2-trichloroethane mixtures by Dehalogenimonas spp.

When chlorinated alkanes are present as soil or groundwater pollutants, they often occur in mixtures. This study evaluated substrate interactions during the anaerobic reductive dehalogenation of chlorinated alkanes by the type strains of two Dehalogenimonas species, D. lykanthroporepellens and D. alkenigignens. Four contaminant mixtures comprised of combinations of the chlorinated solvents 1,2-dichloroethane (1,2-DCA), 1,2-dichloropropane (1,2-DCP), and 1,1,2-trichloroethane (1,1,2-TCA) were assessed for each species. Chlorinated solvent depletion and daughter product formation determined as a function of time following inoculation into anaerobic media revealed preferential dechlorination of 1,1,2-TCA over both 1,2-DCA and 1,2-DCP for both species. 1,2-DCA in particular was not dechlorinated until 1,1,2-TCA reached low concentrations. In contrast, both species concurrently dechlorinated 1,2-DCA and 1,2-DCP over a comparably large concentration range. This is the first report of substrate interactions during chlorinated alkane dehalogenation by pure cultures, and the results provide insights into the chlorinated alkane transformation processes that may be expected for contaminant mixtures in environments where Dehalogenimonas spp. are present.     

Cuticular lipids of insects. VI. Cuticular lipids of the grasshoppers Melanoplus sanguinipes and Melanoplus packardii

The cuticular lipids of the grasshoppers Melanoplus sanguinipes and Melanoplus packardii contain 60 and 68% alkanes and 28 and 18% secondary alcohol wax esters, respectively, with lesser amounts of normal and sterol wax esters, triglycerides, alcohols, sterols, and free fatty acids. All the hydrocarbons are saturated, and four types of alkanes are present: n-alkanes, 3-methylalkanes, internally branched monomethylalkanes, and internally branched dimethylalkanes. The principal n-alkanes in both insects are C(29) and C(27), with a range from C(21) to C(33). Trace amounts of 3-methylalkanes of 28, 30, and 32 total carbons are present. The principal internally branched monomethylalkanes are C(32) and C(34), whereas the main dimethylalkane contains 35 carbons. The n-alkanes do not correspond in chain length to the secondary alcohols. The primary alcohols range from C(22) to C(32) in both insects, with C(24) and C(26) predominating. The fatty acids in the triglyceride and free fatty acid fractions range from C(12) to C(24) in M. sanguinipes and from C(12) to C(18) in M. packardii.     

Estimating belowground plant abundance with DNA metabarcoding

Most work on plant community ecology has been performed above ground, neglecting the processes that occur in the soil. DNA metabarcoding, in which multiple species are computationally identified in bulk samples, can help to overcome the logistical limitations involved in sampling plant communities belowground. However, a major limitation of this methodology is the quantification of species’ abundances based on the percentage of sequences assigned to each taxon. Using root tissues of five dominant species in a semi‐arid Mediterranean shrubland (Bupleurum fruticescens, Helianthemum cinereum, Linum suffruticosum, Stipa pennata and Thymus vulgaris), we built pairwise mixtures of relative abundance (20%, 50% and 80% biomass), and implemented two methods (linear model fits and correction indices) to improve estimates of root biomass. We validated both methods with multispecies mixtures that simulate field‐collected samples. For all species, we found a positive and highly significant relationship between the percentage of sequences and biomass in the mixtures (R² = .44–.66), but the equations for each species (slope and intercept) differed among them, and two species were consistently over‐ and under‐estimated. The correction indices greatly improved the estimates of biomass percentage for all five species in the multispecies mixtures, and reduced the overall error from 17% to 6%. Our results show that, through the use of post‐sequencing quantification methods on mock communities, DNA metabarcoding can be effectively used to determine not only species’ presence but also their relative abundance in field samples of root mixtures. Importantly, knowledge of these aspects will allow us to study key, yet poorly understood, belowground processes.     

Biotic interactions boost spatial models of species richness

Biotic interactions are known to affect the composition of species assemblages via several mechanisms, such as competition and facilitation. However, most spatial models of species richness do not explicitly consider inter‐specific interactions. Here, we test whether incorporating biotic interactions into high‐resolution models alters predictions of species richness as hypothesised. We included key biotic variables (cover of three dominant arctic‐alpine plant species) into two methodologically divergent species richness modelling frameworks – stacked species distribution models (SSDM) and macroecological models (MEM) – for three ecologically and evolutionary distinct taxonomic groups (vascular plants, bryophytes and lichens). Predictions from models including biotic interactions were compared to the predictions of models based on climatic and abiotic data only. Including plant–plant interactions consistently and significantly lowered bias in species richness predictions and increased predictive power for independent evaluation data when compared to the conventional climatic and abiotic data based models. Improvements in predictions were constant irrespective of the modelling framework or taxonomic group used. The global biodiversity crisis necessitates accurate predictions of how changes in biotic and abiotic conditions will potentially affect species richness patterns. Here, we demonstrate that models of the spatial distribution of species richness can be improved by incorporating biotic interactions, and thus that these key predictor factors must be accounted for in biodiversity forecasts.     

Nitrogen and Phosphorus Release from Mixed Litter Layers is Lower than Predicted from Single Species Decay

Ecosystem-level nutrient dynamics during decomposition are often estimated from litter monocultures. If species effects are additive, we can statistically predict nutrient dynamics in multi-species systems from monoculture work, and potential consequences of species loss. However, if species effects are dependent on interactions with other litter species (that is, non-additive), predictions based on monoculture data will likely be inaccurate. We conducted a 3-year, full-factorial, mixed-litter decomposition study of four dominant tree species in a temperate forest and measured nitrogen and phosphorus dynamics to explore whether nutrient dynamics in mixtures were additive or non-additive. Following common approaches, we used litterfall data to predict nutrient dynamics at the ecosystem-level. In mixtures, we observed non-additive effects of litter mixing on nutrient dynamics: the presence of nutrient-rich species in mixture facilitated nutrient release, whereas nutrient-poor species facilitated nutrient retention. Fewer nutrients were released from mixtures containing high-quality litter, and more immobilized from mixtures containing low-quality litter, than predicted from monocultures, creating a difference in overall nutrient release between predicted and actual dynamics in litter mixtures. Nutrient release at the ecosystem-level was greatly overestimated when based on monocultures because the effect of species interactions on nutrient immobilization was not accounted for. Our data illustrate that the identity of species in mixtures is key to their role in non-additive interactions, with repercussions for mineral nutrient availability and storage. These results suggest that predictions of ecosystem-level nutrient dynamics using litter monoculture data likely do not accurately represent actual dynamics because the effects of litter species interactions are not incorporated. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Competition between grassland plants of different initial sizes

Summary Four species of grassland plant, Plantago lanceolata, Holcus lanatus, Lolium perenne and Rumex acetosa, were grown as monocultures and mixtures in pots of nutrient poor soil in a glasshouse for 8 months. There were four plants per pot and these were arranged in two competition modes: either root and shoot interactions were permitted, or only roots allowed to interact by using above-ground partitions. Time of introduction of seedlings was varied to give a range of plant size ratios at the start of the experiment. The factorial design catered for all combinations of species, competition modes and planting times, replicated in four blocks. The shoots were clipped at a fixed height at each of five harvests. Rumex grew badly and was mostly omitted from analysis of the data.By (i) following the change in the relationship of clip dry weights against planting time with successive harvests, (ii) plotting the change in the logarithm of the ratio of cumulative clip dry weights with time and (iii) the use of de Wit logarithmic ratio plots it was demonstrated that each monoculture and mixture combination's ratios of plant weights converged towards stable equilibrium values. Three hypotheses are put forward to explain why in monocultures a smaller plant was at a competitive advantage relative to a larger neighbour and was not suppressed in its growth by the latter. In mixtures this plant size effect was superimposed to different extents on the relative aggressiveness of the species considered. It was concluded that in a nutrient poor soil, when competition for light was low, root interactions can promote the co-existence of neighbouring plant species.     

Predicting competition coefficients for plant mixtures: reciprocity, transitivity and correlations with life-history traits

There are few empirical or theoretical predictions of how per capita or per individual competition coefficients for pairs of plant species should relate to each other. In contrast, there are a considerable number of general hypotheses that predict competitive ability as a function of a range of ecological traits, together with a suite of increasingly sophisticated models for competitive interactions between plant species. We re-analyse a data set on competition between all pairwise combinations of seven species and show that competition coefficients relate strongly to differences between the maximum sizes, root allocation, emergence time and seed size of species. Regressions suggest that the best predictor of competition coefficients is the difference in the maximum size of species and that correlations of the other traits with the competition coefficients occur through effects on the maximum size. We also explore the patterns of association between coefficients across the competition matrix. We find significant evidence for coefficient reciprocity (inverse relationships between the interspecific coefficients for species pairs) and transitivity (numerically predictable hierarchies of competition between species) across competition matrices. These results therefore suggest simple null models for plant community structure when there is competition for resources.     

Overyielding in grassland communities: testing the sampling effect hypothesis with replicated biodiversity experiments

We derive and test some assumptions and predictions of the Sampling Effect Hypothesis (SEH) by examining the relationship between the traits of species in monoculture and their relative abundance in mixture, and by comparing polyculture performance with single-species plots. Although we found a positive relationship between production in monoculture and dominance in mixtures as predicted by the SEH, the relationship had low explanatory power. Counter to predictions, the species with the highest monoculture biomass were not able to strongly dominate all mixtures; instead the dominance of these species decreased with increasing species richness. On average, polycultures did not achieve greater biomass than (transgressively overyield) the species in each mixture, or at each site, that was most productive in monoculture. However, mixture yields did transgressively overyield both the monoculture biomass of the dominant species in the mixtures, and the weighted average of all monocultures (non-transgressive overyielding), both of which were positively related to increasing species richness. The varying responses of different overyielding tests resulted because resource partitioning and positive interactions were often counter-balanced by selection for species with lower biomass than the highest-yielding monocultures. Judging whether or not mixtures overyield therefore depends in part upon which species is the basis for comparison. We present a new general framework for overyielding analysis where every monoculture provides a potential comparison and from which the most relevant tests can be selected.     

内蒙古天然草原不同放牧压力下亚洲小车蝗的食性及营养生态位（英文）

植物表皮蜡质中的饱和链烷作为内源指示剂广泛用于评价放牧家畜的食性和食量, 但用于天然草原蝗虫食性的评价研究较少。为了探讨天然草原蝗虫的食性及其生态位变化, 本研究以内蒙古天然草原为研究对象, 于2003年7-8月沿降水梯度选择3种典型植物群落（小针茅Stipa klemenzii、 羊草Leymus chinensis和大针茅Stipa grandis群落）, 在每个植物群落不同放牧压力下小区随机做20个植被样方, 样方内植物齐地面刈割, 测定其地上生物量和物种多样性, 取主要植物种测定其链烷模式, 同时采集放牧小区优势蝗虫种亚洲小车蝗Oedaleus asiaticus的粪便, 测定其链烷模式, 运用链烷技术评价蝗虫的食性及其营养生态位。结果表明： 不同植物群落中优势牧草种类及其比例不同, 其链烷模式存在种间差异, 链烷技术可以评价亚洲小车蝗的食性。亚洲小车蝗的食性在不同植物群落及不同放牧压力下存在显著的差异, 在羊草和大针茅群落中, 亚洲小车蝗是禾草采食者, 主要采食羊草和糙隐子草Cleistogenes squarrosa, 且与绵羊的营养生态位重叠指数较低, 分别为0.0619和0.0172; 在小针茅群落中亚洲小车蝗是杂类草采食者, 主要采食无芒隐子草Cleistogenes songorica、 猪毛菜Salsola collina和小针茅, 且与绵羊的营养生态位重叠指数较高, 达到0.1815。因此, 放牧不仅改变了群落的植物种类组成, 而且直接影响了亚洲小车蝗的食物组成, 二者对食物资源利用存在一定程度的竞争。