Atmospheric CO2 and O3 alter competition for soil nitrogen in developing forests

Plant growth responses to rising atmospheric CO₂ and O₃ vary among genotypes and between species, which could plausibly influence the strength of competitive interactions for soil N. Ascribable to the size‐symmetric nature of belowground competition, we reasoned that differential growth responses to CO₂ and O₃ should shift as juvenile individuals mature, thereby altering competitive hierarchies and forest composition. In a 12‐year‐long forest FACE experiment, we used tracer ¹⁵N and whole‐plant N content to assess belowground competitive interactions among five Populus tremuloides genotypes, between a single P. tremuloides genotype and Betula papryrifera, as well as between the same single P. tremuloides genotype and Acer saccharum. Under elevated CO₂, the amount of soil N and ¹⁵N obtained by the P. tremuloides genotype common to each community was contingent on the nature of belowground competition. When this genotype competed with its congeners, it obtained equivalent amounts of soil N and tracer ¹⁵N under ambient and elevated CO₂; however, its acquisition of soil N under elevated CO₂ increased by a significant margin when grown in competition with B. papyrifera (+30%) and A. saccharum (+60%). In contrast, elevated O₃ had no effect on soil N and ¹⁵N acquisition by the P. tremuloides genotype common in each community, regardless of competitive interactions. Under elevated CO₂, the rank order of N acquisition among P. tremuloides genotypes shifted over time, indicating that growth responses to CO₂ change during ontogeny; this was not the case under elevated O₃. In the aspen‐birch community, the competitive advantage elevated CO₂ initially conveyed on birch diminished over time, whereas maple was a poor competitor for soil N in all regards. The extent to which elevated CO₂ and O₃ will shape the genetic structure and composition of future forests is, in part, contingent on the time‐dependent effects of belowground competition on plant growth response.     

Vegetation in contrasting soil water sites of upland herbaceous grasslands and N:P ratios as indicators of nutrient limitation

Effects of differences in long-term water supply were examined on soil characteristics, primary production and species composition in a wet and a dry site of an upland herbaceous grassland. Also the responses of species to N and P enrichments were examined. N and P concentrations of non-legume species were positively related, indicating that biomass N:P ratios seem to be mainly determined by N:P supply ratios. Forbs had generally higher concentrations than graminoids. Intermittent water inundation of soil in the wet site resulted in greater soil N and P availability. The greater productivity of this site promoted the growth of forbs. A fertilizer experiment showed that biomass was limited by N only in the wet site, but by both nutrients in the dry one. The species with the higher N and P concentrations were favored more after N and P enrichment, respectively; however, species enhancement was not related to N:P ratios of species. This indicates that N and P concentrations of species, rather than N:P ratios of species, are better predictors of species responses to N and P enrichment. N:P ratios of whole communities were 8.73 for the wet and 11.36 for the dry site. These values in comparison with the responses of plant communities to N and P fertilization show that thresholds of N:P ratios indicative of N or P limitation are much lower than those found for European wetlands.     

Resource allocation to growth or luxury consumption drives mycorrhizal responses

Highly variable phenotypic responses in mycorrhizal plants challenge our functional understanding of plant‐fungal mutualisms. Using non‐invasive high‐throughput phenotyping, we observed that arbuscular mycorrhizal (AM) fungi relieved phosphorus (P) limitation and enhanced growth of Brachypodium distachyon under P‐limited conditions, while photosynthetic limitation under low nitrogen (N) was exacerbated by the fungus. However, these responses were strongly dependent on host genotype: only the faster growing genotype (Bd3‐1) utilised P transferred from the fungus to achieve improved growth under P‐limited conditions. Under low N, the slower growing genotype (Bd21) had a carbon and N surplus that was linked to a less negative growth response compared with the faster growing genotype. These responses were linked to the regulation of N : P stoichiometry, couples resource allocation to growth or luxury consumption in diverse plant lineages. Our results attest strongly to a mechanism in plants by which plant genotype‐specific resource economics drive phenotypic outcomes during AM symbioses.     

Patterns of root activity and responses of species to nutrients in vegetation of fertile alluvial soil

Temporal pattern of growth, time and depth of root activity, and responses to N, P and K enrichment were measured for the three most abundant species in species-poor vegetation on fertile alluvial soil to examine whether differences in these characteristics could account for their co-existence. The responses of these co-existing species to nutrients were tested in a factorial experiment of N, P and K additions. In the control plots, repeated harvests and injections of Sr at different depths in the soil were used to test differences among species in temporal and spatial pattern of root activity. Root activities were assessed from the Sr concentrations in the aboveground biomass. Differences in temporal pattern of growth and root activity, but not differences in spatial root activity between species, could account for the co-existence of species, since Conium, the most abundant species, was the earliest grown and the deepest-rooted species and it died back when the other two species started to maximize their growth and root activity. In comparison with the second most abundant species Lactuca, Conium had higher N but lower P tissue concentrations. Addition of N favoured Conium and almost eliminated Lactuca, while P and/or K additions increased the abundance of Lactuca and restricted that of Conium. These results provide indications that the differential responses of species to nutrients could be explained by species co-existence also in fertile soils. The changes in vegetation composition after nutrient enrichment could merely be predicted by the species' tissue concentrations of nutrients. The addition of a particular nutrient tended to favor the species with the highest tissue concentration of this nutrient.     

Nitrogen fertilization reduces Sphagnum production in bog communities

The effects of increased nitrogen influx on Sphagnum growth and on interspecific competition between Sphagnum species were studied in a 3-yr experiment in mires situated in two areas with different rates of airborne N deposition. Sphagnum growth was recorded after various supplementary N influxes (0, 1, 3, 5 and 10 g m ⁻² yr⁻¹) in hummocks and lawn communities. Sphagnum biomass production decreased with increasing N influx in both areas. After the first season at the low-deposition site, Sphagnum showed an increased growth in length with the intermediate N treatment, but in the second and third seasons the control treatment had the highest growth in length. Capitulum dry mass increased with increasing N influx. Sphagnum N concentration and N/P quotient were higher at the high- than at the low-deposition site. The low quotient at the low-deposition site, together with the initial growth increase with intermediate N supplements, indicates that growth was N-limited at this site, but our lowest N supplement was sufficient to reduce growth. The N treatments had no effect on interspecific competition between the Sphagnum species. This indicates that the species have similar responses to N. The species studied all occur naturally on ombrotrophic, N-poor sites and show low tolerances to increased N influx. Reduced Sphagnum production may affect the carbon balance, changing the mires from C sinks to sources.     

Nitrogen enrichment modifies plant community structure via changes to plant–soil feedback

We tested the hypothesis that N enrichment modifies plant-soil feedback relationships, resulting in changes to plant community composition. This was done in a two-phase glasshouse experiment. In the first phase, we grew eight annual plant species in monoculture at two levels of N addition. Plants were harvested at senescence and the effect of each species on a range of soil properties was measured. In the second phase, the eight plant species were grown in multi-species mixtures in the eight soils conditioned by the species in the first phase, at both levels of N addition. At senescence, species performance was measured as aboveground biomass. We found that in the first phase, plant species identity strongly influenced several soil properties, including microbial and protist biomass, soil moisture content and the availability of several soil nutrients. Species effects on the soil were mostly independent of N addition and several were strongly correlated with plant biomass. In the second phase, both the performance of individual species and overall community structure were influenced by the interacting effects of the species identity of the previous soil occupant and the rate of N addition. This indicates that N enrichment modified plant-soil feedback. The performance of two species correlated with differences in soil N availability that were generated by the species formerly occupying the soil. However, negative feedback (poorer performance on the soil of conspecifics relative to that of heterospecifics) was only observed for one species. In conclusion, we provide evidence that N enrichment modifies plant-soil feedback relationships and that these modifications may affect plant community composition. Field testing and further investigations into which mechanisms dominate feedback are required before we fully understand how and when feedback processes determine plant community responses to N enrichment.     

羊草草原群落6种植物叶片功能特性对氮素添加的响应

 该文比较了羊草草原群落中包括建群种和优势种在内的6种植物,羊草（Leymus chinensis）、西伯利亚羽茅（Achnatherum sibiricum）、大 针茅（Stipa grandis）、 冰草（Agropyron cristatum）、糙隐子草（Cleistogenes squarrosa）和黄囊苔草（Carex korshinskyi）的比叶 面积(Specific leaf area, SLA)、叶片含氮量和叶绿素含量等叶片功能特性( Leaf functional trait)对氮素添加的响应,旨在探讨草原生态 系统中,不同物种对氮素可利用性改变的响应和适应对策。结果表明：随着氮素添加量的增加,物种对光资源的竞争增强,不同物种在光资源 的竞争策略和竞争力间存在着显著的差异。羊草通过提高SLA、单位质量叶片的叶绿素含量和含氮量,使单位面积叶片含氮量和叶绿素含量均呈 线性提高,进而增强了其对光的竞争力。西伯利亚羽茅主要通过提高SLA增加光合总面积,来增强自身的光竞争力。冰草在SLA和单位质量叶片 的叶绿素和氮含量均有一定的可塑性,但对光的竞争力明显弱于羊草和西伯利亚羽茅。大针茅和黄囊苔草对SLA的调节能力较低,加之大针茅 SLA较低,而黄囊苔草处于群落的下层,这两种植物的光竞争力较弱。糙隐子草具有较高的SLA,对单位质量叶片叶绿素和氮含量的调节能力相 对较强,其光竞争力高于黄囊苔草。同时,糙隐子草叶片叶绿素a与叶绿素b的比值沿氮素添加梯度显著降低,进一步证实氮素添加改变了群落 的光环境。     

Effects of Plant Growth Characteristics on Biogeochemistry and Community Composition in a Changing Climate

Vegetation growth characteristics influence ecosystem biogeochemistry and must be incorporated in models used to project biogeochemical responses to climate variations. We used a multiple-element limitation model (MEL) to examine how variations in nutrient use efficiency (NUE) and net primary production to biomass ratio (nPBR) affect changes in ecosystem C stocks after an increase in temperature and atmospheric CO₂. nPBR influences the initial rates of response, but the magnitude and direction of long-term responses are determined by NUE. MEL was used to simulate responses to climate change in communities composed of two species differing in nPBR and/or NUE. When only nPBR differed between the species, the high-nPBR species outgrew the low-nPBR species early in the simulations, but the shift in dominance was transitory because of secondary N limitations. High-NUE species were less affected by secondary N limitations and were therefore favored under elevated CO₂. Increased temperature stimulated N release from soil organic matter (SOM) and therefore favored low-NUE species. The combined release from C and N limitation under the combination of increased temperature and elevated CO₂ favored high-NUE species. High C:N litter from high-NUE species limited the N-supply rate from SOM, which favors the dominance of the high-NUE species in the short term. However, in the long term increased litter production resulted in SOM accumulation, which reestablished a N supply rate favorable to the reestablishment and dominance of the low-NUE species. Conditions then reverted to a state favorable to the high-NUE species. Received 8 October 1998; accepted 9 April 1999.     

Comparative Behavioural Studies of Larval and Adult Stages of the Phytoseiids (Acari: Mesostigmata) Typhlodromus Athiasae and Neoseiulus Californicus

We compared the behaviours of the indigenous Typhlodromus athiasae Porath and Swirski and the exotic Neoseiulus californicus (McGregor) (= Amblyseius chilenensis Dosse) relative to their persistence in apple orchards in Israel. We studied (1) larval feeding, walking, intraspecific interactions (cannibalism, touch-avoidance responses and/or touching with palps and tarsi) and tendency to aggregate (when resting), (2) predation and cannibalism on phytoseiid eggs by young females and (3) the effects of starvation for 10 days on young females relative to ambulation speed, longevity, fecundity, progeny survival and sex ratio. Larvae of T. athiasae were almost inert, did not feed and hardly walked or interacted whereas larvae of N. californicus fed, walked and interacted, mainly by touching with palps and tarsi. No cannibalism in the larval stage was observed for either species. The presence of prey increased the larval walking and intraspecific interactions of N. californicus but not of T. athiasae. Egg predation by adult females of both species was substantially higher than cannibalism, implying that both are capable of distinguishing their eggs from those of other species. Soaking eggs for 30 min in deionized water increased cannibalism in both species. During the 10 days of starvation, the ambulation speed of adult female N. californicus ranged from 1.8 to 10.1 times that of T. athiasae. The 50% lethal time value (LT₅₀) of T. athiasae (6.0 days) was significantly lower than that of N. californicus (10.4 days). None of the starved T. athiasae recuperated following the reintroduction of prey, whereas 75% of N. californicus did and oviposited after 2 days. These traits should enable N. californicus to persist when prey is scarce; however the selective predation of N. californicus eggs by T. athiasae could prevent establishment of N. californicus. The degree of specialization of these two predators is discussed.     

Interspecific variations in responses of <Emphasis Type="Italic">Festuca rubra</Emphasis> and <Emphasis Type="Italic">Trifolium pratense</Emphasis> to a severe clipping under environmental changes

Understanding and predicting possible responses of grassland species to global change is of important meaning for adapting grassland management to a changed and changing environment. A laboratory clipping experiment was conducted to examine the interspecific responses in an ecological context of competition and environmental changes. Festuca rubra and Trifolium pratense, either in monoculture or two- and three-species mixtures, were grown in three environmental combinations (ambient and increased temperature, repetitive N supply, and simulated acid rain), respectively. After a growth time of three months, plants were clipped at the height of 1.0 cm above soil surface. Plant height and aboveground biomass prior clipping, and survival rate and regrowth (height and biomass) after clipping were analyzed. F. rubra and T. pratense responded differently in compensatory growth and competition intensity to environmental change and co-existing species. The differences in their physiological and ecological traits may account for species-dependent responses. The present study emphasizes that predicting the plant assemblage response in the face of global change requires in understanding the integrating effects of abiotic and biotic factors.     

Testing mechanisms of N-enrichment-induced species loss in a semiarid Inner Mongolia grassland: critical thresholds and implications for long-term ecosystem responses

The increase in nutrient availability as a consequence of elevated nitrogen (N) deposition is an important component of global environmental change. This is likely to substantially affect the functioning and provisioning of ecosystem services by drylands, where water and N are often limited. We tested mechanisms of chronic N-enrichment-induced plant species loss in a 10-year field experiment with six levels of N addition rate. Our findings on a semi-arid grassland in Inner Mongolia demonstrated that: (i) species richness (SR) declined by 16 per cent even at low levels of additional N (1.75 g N m^–2 yr⁻¹), and 50–70% species were excluded from plots which received high N input (10.5–28 g N m⁻² yr⁻¹); (ii) the responses of SR and above-ground biomass (AGB) to N were greater in wet years than dry years; (iii) N addition increased the inter-annual variations in AGB, reduced the drought resistance of production and hence diminished ecosystem stability; (iv) the critical threshold for chronic N-enrichment-induced reduction in SR differed between common and rare species, and increased over the time of the experiment owing to the loss of the more sensitive species. These results clearly indicate that both abundance and functional trait-based mechanisms operate simultaneously on N-induced species loss. The low initial abundance and low above-ground competitive ability may be attributable to the loss of rare species. However, shift from below-ground competition to above-ground competition and recruitment limitation are likely to be the key mechanisms for the loss of abundant species, with soil acidification being less important. Our results have important implications for understanding the impacts of N deposition and global climatic change (e.g. change in precipitation regimes) on biodiversity and ecosystem services of the Inner Mongolian grassland and beyond.     

Teasing apart plant community responses to N enrichment: the roles of resource limitation,competition and soil microbes

Although ecologists have documented the effects of nitrogen enrichment on productivity, diversity and species composition, we know little about the relative importance of the mechanisms driving these effects. We propose that distinct aspects of environmental change associated with N enrichment (resource limitation, asymmetric competition, and interactions with soil microbes) drive different aspects of plant response. We test this in greenhouse mesocosms, experimentally manipulating each factor across three ecosystems: tallgrass prairie, alpine tundra and desert grassland. We found that resource limitation controlled productivity responses to N enrichment in all systems. Asymmetric competition was responsible for diversity declines in two systems. Plant community composition was impacted by both asymmetric competition and altered soil microbes, with some contributions from resource limitation. Results suggest there may be generality in the mechanisms of plant community change with N enrichment. Understanding these links can help us better predict N response across a wide range of ecosystems.     

Soil nutrient heterogeneity interacts with elevated CO2 and nutrient availability to determine species and assemblage responses in a model grassland community

Interactive effects of atmospheric CO(2) concentration ([CO(2)]), soil nutrient availability and soil nutrient spatial distribution on the structure and function of plant assemblages remain largely unexplored. Here we conducted a microcosm experiment to evaluate these interactions using a grassland assemblage formed by Lolium perenne, Plantago lanceolata, Trifolium repens, Anthoxanthum odoratum and Holcus lanatus. Assemblages exhibited precise root foraging patterns, had higher total and below-ground biomass, and captured more nitrogen when nutrients were supplied heterogeneously. Root foraging responses were modified by nutrient availability, and the patterns of N capture by interactions between nutrient distribution, availability and [CO(2)]. Greater above-ground biomass was observed under elevated CO(2) only under homogeneous conditions of nutrient supply and at the highest availability level. CO(2) interacted with nutrient distribution and availability to determine foliar percentage N and below : above-ground biomass ratios, respectively. Interactions between nutrient distribution and CO(2) determined the relative contribution to above-ground biomass of four of the species. The responses of dominant and subordinate species to [CO(2)] were dependent on the availability and distribution of nutrients. Our results suggest that soil nutrient distribution has the potential to influence the response of plant species and assemblages to changes in [CO(2)] and nutrient availability.     

Different growth responses of C3 and C4 grasses to seasonal water and nitrogen regimes and competition in a pot experiment

Understanding temporal niche separation between C₃ and C₄ species(e.g. C₃ species flourishing in a cool spring and autumn whileC₄ species being more active in a hot summer) is essential forexploring the mechanism for their co-existence. Two parallelpot experiments were conducted, with one focusing on water andthe other on nitrogen (N), to examine growth responses to wateror nitrogen (N) seasonality and competition of two co-existingspecies Leymus chinensis (C₃ grass) and Chloris virgata (C₄grass) in a grassland. The two species were planted in eithermonoculture (two individuals of one species per pot) or a mixture(two individuals including one L. chinensis and one C. virgataper pot) under three different water or N seasonality regimes,i.e. the average model (AM) with water or N evenly distributedover the growing season, the one-peak model (OPM) with morewater or N in the summer than in the spring and autumn, andthe two-peak model (TPM) with more water or N in the springand autumn than in the summer. Seasonal water regimes significantlyaffected biomass in L. chinensis but not in C. virgata, whileN seasonality impacted biomass and relative growth rate of bothspecies over the growing season. L. chinensis accumulated morebiomass under the AM and TPM than OPM water or N treatments.Final biomass of C. virgata was less impacted by water and Nseasonality than that of L. chinensis. Interspecific competitionsignificantly decreased final biomass in L. chinensis but notin C. virgata, suggesting an asymmetric competition betweenthe two species. The magnitude of interspecific competitionvaried with water and N seasonality. Changes in productivityand competition balance of L. chinensis and C. virgata undershifting seasonal water and N availabilities suggest a contributionof seasonal variability in precipitation and N to the temporalniche separation between C₃ and C₄ species. Key words: Chloris virgata, competition, growth, Leymus chinensis, nitrogen seasonality, water seasonality Received 19 November 2007; Revised 29 January 2008 Accepted 4 February 2008     

Overstory Community Composition and Elevated Atmospheric CO2 and O3 Modify Understory Biomass Production and Nitrogen Acquisition

Elevated atmospheric CO₂ and O₃ have the potential to affect the primary productivity of the forest overstory, but little attention has been given to potential responses of understory vegetation. Our objective was to document the effects of elevated atmospheric CO₂ and O₃ on understory species composition and biomass and to quantify nitrogen (N) acquisition by the understory vegetation. The research took place at the aspen free-air CO₂ and O₃ enrichment (FACE) experiment, which has four treatments (control, elevated CO₂, elevated O₃, and elevated CO₂+O₃) and three tree communities: aspen, aspen/birch, and aspen/maple. In June 2003, each FACE ring was uniformly labeled with 15N applied as NH₄Cl. Understory biomass was harvested in June of 2004 for productivity, N, and 15N measurements, and photosynthetically active radiation (PAR) was measured below the canopy. The understory was divided into five species groups, which dominate in this young aggrading forest: Taraxacum officinale (dandelion), Solidago sp. (goldenrod), Trifolium repens and T. pretense (clover), various species from the Poaceae family (grass), and composited minor components (CMC). Understory species composition, total and individual species biomass, N content, and 15N recovery showed overstory community effects, but the direct effects of treatments was masked by the high variability of these data. Total understory biomass increased with increasing light, and thus was greatest under the open canopy of the aspen/maple community, as well as the more open canopy of the elevated O₃ treatments. Species were different from one another in terms of 15N recovery, with virtually no 15N recovered in clover and the greatest amount recovered in dandelion. Thus, understory species composition and biomass appear to be driven by the structure of the overstory community, which is determined by the tree species present and their response to the treatments. However, N acquisition by the understory does not appear to be affected by either the overstory community or the treatments at this point.     

Competition between the nymphs of two regionally co-occurring species of Notonecta (Hemiptera: Notonectidae)

1. Two species of freshwater invertebrate predator, Notonecta maculata and N. obliqua , showed a negative association in a series of small, man-made ponds in the Peak National Park, Derbyshire, U.K. The present study examines the potential role of interspecific interactions among nymphs on this regional distribution pattern.
2. The survival, development and feeding efficiency of nymphs were examined in laboratory and field mesocosm experiments with intra- and interspecific competition and contrasting environmental complexity.
3. Survival to adulthood and mean lifespan varied significantly in interspecific competition treatments in both laboratory and field experiments, with N . maculata showing higher survival in the simple environment and N. obliqua higher survival in the complex environment.
4. Variations in feeding efficiency were consistent with the survival trends: N. maculata had a higher efficiency in the simple environment, whereas N. obliqua had greater efficiency in the complex environment. There was evidence of a developmental response in feeding efficiency, with differences between species increasing with age.
5. These results suggest that the relative competitive abilities of the two species are affected by habitat complexity, and that competition between species may modify the species distribution where they co-occur.     

Species richness loss after nutrient addition as affected by N:C ratios and phytohormone GA3 contents in an alpine meadow community

Aims From the light-competition hypothesis, competition for light is asymmetric and the observed increases in plant-size variability with increasingly denser canopies are primarily due to competition for light. Greater plant height provides pre-emptive access to light and produces increased height differences among species. The question is what produces these differences in plant height or height growth response among species in response to fertilization.Methods In 2009, a field experiment of N, P and N + P enrichments at three levels each was initiated in an alpine meadow on the northeast Qinghai-Tibet Plateau. Effects of fertilization on species richness, aboveground net primary production (ANPP), relative light intensity and plant height of different plant functional groups were determined. Festuca ovina (grass), Kobresia humilis (sedge), Oxytropis ochrocephala (legume), Taraxacum lugubre (rosette forb) and Geranium pylzowianum (upright forb) were selected as exemplars of each of the indicated functional groups. The N:C ratios in aboveground biomass, gibberellic acid (GA 3) concentrations in leaves, plant heights and height relative growth rate (RGR) of these exemplar species were analyzed in detail.Important findings Species richness of grasses significantly increased with increasing N + P levels. Species richness of legumes and upright forbs decreased after N and N + P additions. P addition had no significant effect on species richness. The effects of N + P addition on species richness and ANPP were consistently stronger than those of the single N or P fertilization. Reductions in species richness caused by nutrient addition paralleled the increases in ANPP and decreases in light intensity under the canopies, indicating indirect effect of nutrient addition on species richness via ANPP-induced light competition. The exemplar species that responded most positively to fertilization in height and RGR also displayed stronger increases in their GA 3 content and N:C ratios. GA 3 concentrations and N:C ratios were positively correlated with height RGR when the data were pooled for all species. The tallest and the fastest-growing grass, F. ovina, had the largest increase in N:C ratios and the highest leaf GA 3 concentrations after nutrient addition. These results indicated that differential responses of GA 3 concentrations and N:C ratios to fertilization were related to the inequality in plant heights among species.     

Interspecific competition alters leaf stoichiometry in 20 grassland species

The extensive use of traits in ecological studies over the last few decades to predict community functions has revealed that plant traits are plastic and respond to various environmental factors. These plant traits are assumed to predict how plants compete and capture resources. Variation in stoichiometric ratios both within and across species reflects resource capture dynamics under competition. However, the impact of local plant diversity on species‐specific stoichiometry remains poorly studied. Here, we analyze how spatial and temporal diversity in resource‐acquisition traits affects leaf elemental stoichiometry of plants (i.e. the result of resource capture) and how flexible this stoichiometry is depending on the functional composition of the surrounding community. Therefore, we assessed inter‐ and intraspecific variations of leaf carbon (C), nitrogen (N), and phosphorus (P) (and their ratios) of 20 grassland species in a large trait‐based plant diversity experiment located in Jena (Germany) by measuring leaf elemental concentrations at the species‐level along a gradient in plant trait dissimilarity. Our results show that plants showed large intra‐ and interspecific variation in leaf stoichiometry, which was only partly explained by the functional group identity (grass or herb) of the species. Elemental concentrations (N, P, but not C) decreased with plant species richness, and species tended to become more deviant from their monoculture stoichiometry with increasing trait dissimilarity in the community. These responses differed among species, some consistently increased or decreased in P and N concentrations; for other species, the negative or positive change in P and N concentrations increased with increasing trait difference between the target species and the remaining community. The strength of this relationship was significantly associated to the relative position of the species along trait gradients related to resource acquisition. Trait‐difference and trait‐diversity thus were important predictors of how species’ resource capture changed in competitive neighbourhoods.     

Direct and feeding‐induced interactions between two rice planthoppers, Sogatella furcifera and Nilaparvata lugens: effects on dispersal capability and performance

Abstract. 1. A series of laboratory experiments was conducted to explore the effects of inter‐specific interactions, both direct interactive effects and those induced through previous feeding, on the dispersal capability (proportion of macropterous adults) and performance (development time and survival) of two wing‐dimorphic planthoppers, the whitebacked planthopper Sogatella furcifera and the brown planthopper Nilaparvata lugens, two pests of rice throughout Asia. 2. An asymmetric effect of inter‐specific crowding on dispersal capability was detected between the two planthoppers. With density controlled, the proportion of macropterous adults in N. lugens was higher when raised in mixed populations with S. furcifera than in pure cultures comprised of conspecifics, suggesting that interspecific effects on dispersal capability are stronger than intraspecific effects. In contrast, interspecific effects on macroptery (%) were weaker than intraspecific impacts for S. furcifera. 3. This trend was parallelled by adverse competitive effects on survival, with interspecific effects stronger than intraspecific impacts for N. lugens and intraspecific effects stronger for S. furcifera. 4. Crowding also affected the incidence of macroptery indirectly through feeding‐induced changes in plant physiology. Rearing N. lugens on plants fed on previously by heterospecifics resulted in a higher incidence of macroptery in females and protracted development than for N. lugens raised on plants fed on previously by conspecifics. By contrast, the intraspecific effect of previous feeding was stronger than the interspecific effect in S. furcifera, with higher macropter production and prolonged development occurring on plants exposed previously to conspecifics. 5. The results suggest that interspecific interactions between herbivorous insects, both direct and via feeding‐induced changes in plant physiology, can have negative consequences for performance and survival and promote the production of flight‐capable adults that can disperse.     

Why plants bother: root proliferation results in increased nitrogen capture from an organic patch when two grasses compete

We investigated whether the capacities of Lolium perenne L. and Poa pratensis L. roots to proliferate locally and to alter local nitrogen (N) inflows in a decomposing organic matter patch were important in their capture of N when grown together. In the presence of a patch, plants of both species were significantly heavier and contained more N. Root length and weight densities increased in the patch, but specific root length was unaltered. Although both species proliferated roots in the patch, L. perenne produced greater root length densities than P. pratensis , and also captured more N from the patch. Indeed, total N uptake from the patch was related to root length density within the patch. N inflows (rate of N uptake per unit root length) in the patch were no faster than in the whole root system for both species. Under the conditions of this study, root proliferation in an organic patch was more important for N capture from the patch than alterations in N inflows. Local proliferation of roots may be a key factor in interspecific competition for non-uniformly distributed supplies of N in natural habitats, so resolving the previous uncertainty as to the 'adaptive' nature of root proliferation.