Biomass allocation in a clonal vine: Effects of intraspecific competition and nutrient availability

Biomass allocation to roots, rhizomes, runners and climbing stems (i.e. twining axis and attached leaves) was studied inCalystegia sepium L., a clonal vine. In an experiment which took 2 months, nutrient availability (low and high) and intraspecific competition (none, shoot root and both shoot and root) were manipulated. Under low nutrients the highest biomass of climbing stems was found in plants with shoot competition; the lowest was found in plants with both shoot and root competition. Total biomass under high nutrients was also greatest in plants with shoot competition. Thus, plants benefited from climbing up a shared stake rather than separate stakes. Larger plants allocated a higher proportion of biomass to runners in the nutrient-poor environment than in the nutrient-rich environment. This behaviour may increase the chance of finding nutrient-rich patches in the neighbourhood of the mother plant in a heterogeneous environment.     

Small-scale disturbances spread along trophic chains: leaf-cutting ant nests, plants, aphids, and tending ants

Small-scale disturbances caused by animals often modify soil resource availability and may also affect plant attributes. Changes in the phenotype of plants growing on disturbed, nutrient-enriched microsites may influence the distribution and abundance of associated insects. We evaluated how the high nutrient availability generated by leaf-cutting ant nests in a Patagonian desert steppe may spread along the trophic chain, affecting the phenotype of two thistle species, the abundance of a specialist aphid and the composition of the associated assemblage of tending ants. Plants of the thistle species Carduus nutans and Onopordum acanthium growing in piles of waste material generated by leaf-cutting ant nests (i.e., refuse dumps) had more leaves, inflorescences and higher foliar nitrogen content than those in non-nest soils. Overall, plants in refuse dumps showed higher abundance of aphids than plants in non-nest soils, and aphid colonies were of greater size on O. acanthium plants than on C. nutans plants. However, only C. nutans plants showed an increase in aphid abundance when growing on refuse dumps. This resulted in a similar aphid load in both thistle species when growing on refuse dumps. Accordingly, only C. nutans showed an increase in the number of ant species attending aphids when growing on refuse dumps. The increase of soil fertility generated by leaf-cutting ant nests can affect aphid abundance and their tending ant assemblage through its effect on plant size and quality. However, the propagation of small-scale soil disturbances through the trophic chain may depend on the identity of the species involved.     

Ecological engineering by a native leaf-cutting ant increases the performance of exotic plant species

Numerous mechanisms are proposed to explain why exotic plants successfully invade natural communities. However, the positive effects of native engineers on exotic plant species have received less consideration. We tested whether the nutrient-rich soil patches created by a native ecological engineer (refuse dumps from the leaf-cutting ant Acromyrmex lobicornis) increase the performance of exotic more than native plants. In a greenhouse experiment, individuals from several native and exotic species were planted in pots with refuse dumps (RDs) and non-nest soils (NNSs). Total plant biomass and foliar nutrient content were measured at the end of the experiment. We also estimated the cover of exotic and native plant species in external RDs from 54 field ant nests and adjacent areas. Greenhouse plants showed more biomass and foliar nutrient content in RDs than in NNS pots. Nevertheless, differences in the final mean biomass among RD and NNS plants were especially great in exotics. Accordingly, the cover of exotic plants was higher in field RDs than in adjacent, non-nest soils. Our results demonstrated that plants can benefit from the enhanced nutrient content of ant RDs, and that A. lobicornis acts as an ecosystem engineer, creating a substrate that especially increases the performance of exotics. This supports the fluctuating resource hypothesis as a mechanism to promote biological invasions, and illustrates how this hypothesis may operate in nature. Since ant nests and exotic plants are more common in disturbed than in pristine environments, the role of ant nests in promoting biological invasions might be of particular interest. Proposals including the use of engineer species to restore disturbed habitats should be planned with caution because of their potential role in promoting invasions.     

Differences between European native and American invasive populations of Lythrum salicaria

Abstract. We compared the demographic characteristics of native central-European and invasive USA-populations of Lythrum salicaria growing in similar habitats. Based on the ‘Evolution of increased competitive ability’ (EICA) hypothesis, we predicted that shoot density, height, and biomass, fertility, and fecundity would be less in the European populations due to greater loss of plant material caused by exposure to control agents. Shoot density was significantly greater in most USA-populations, but shoot height and biomass were similar in invasive and native populations growing in similar habitats, especially in growing seasons with greater water availability. The number of fertile shoots was greater for invasive populations, except for those growing in sandy, nutrient-poor substrates, while percent fertility did not differ between populations growing under similar field conditions. Fecundity was also similar for populations growing in nutrient-poor and intermediate habitats, but was significantly greater in USA-populations (89–103 seeds per fruit), compared to European populations (58–64 seeds per fruit), growing in nutrient-rich habitats; seed predators were found in these European populations only. Log-linear analysis of transition frequency matrices showed that the growth of USA-populations is different from European populations, but that habitat effect was strong. Population dynamics were similar for populations growing in nutrient-poor habitats; location was of marginal importance only. USA-populations responded differently from European populations growing in intermediate and nutrient-rich habitats; differences were most pronounced between invasive (center of its North American distribution) and native populations growing in nutrient-rich habitats. The use of insect herbivores as biological control agents in North America will work best against L. salicaria populations growing in nutrient-rich habitats in the center of its invasive distribution, but will be less effective against populations growing in other habitats or portions of its range. Other factors, including nutrient- or water availability, and climate, may be as important as herbivory in affecting invasive populations; these factors interact to control L. salicaria in a more complex manner than thought previously.     

Soil heterogeneity affects ramet placement of Hydrocotyle vulgaris

Aims Soil heterogeneity is common in natural habitats. It may trigger foraging responses (placing more ramets and/or roots in nutrient-rich patches than in nutrient-poor patches) and further affect the growth of plants. However, the impact of soil heterogeneity on competitive interactions has been little tested.Methods We conducted a greenhouse experiment to investigate the effects of soil heterogeneity on intraspecific competition with a stoloniferous herb Hydrocotyle vulgaris. We grew one (without competition) or nine ramets (with competition) of H. vulgaris under a homogeneous environment and two heterogeneous environments differing in patch size (large or small patches). In the heterogeneous treatment, the soil consisted of the same number of nutrient-rich and nutrient-poor patches arranged in a chessboard manner, and in the homogeneous treatment, the soil was an even mixture of the same amount of the nutrient-rich and the nutrient-poor soil.Important findings Irrespective of intraspecific competition, H. vulgaris showed foraging responses to soil heterogeneity in the large patch treatment, e.g. it produced significantly more biomass, ramets, aboveground mass and root mass in the nutrient-rich patches than in the nutrient-poor patches. In the small patch treatment, foraging responses were observed when intraspecific competition was present, but responses were not observed when there was no competition. However, we find a significant effect of soil heterogeneity on neither overall growth nor competitive intensity of H. vulgaris. Our results suggest that foraging responses to soil heterogeneity may not necessarily be adaptive and intraspecific competition may not be influenced by soil heterogeneity.     

Interspecific competition in natural plant communities: mechanisms, trade-offs and plant-soil feedbacks

Interspecific competition in natural plant communities is highly dependent on nutrient availability. At high levels of nutrient availability, competition is mainly for light. As light is a unidirectional resource, high nutrient habitats are dominated by fast-growing perennials with a tall stature and a rather uniform vertical distribution of leaf area. Moreover, these species have high turnover rates of leaves and roots and a high morphological plasticity during the differentiation of leaves. There is less consensus, however, about the importance and intensity of interspecific competition in nutrient-poor environments. It is argued that selection in nutrient-poor habitats is not necessarily on a high competitive ability for nutrients and a high growth rate, but rather on traits which reduce nutrient losses (low tissue nutrient concentrations, slow tissue turnover rates, high nutrient resorption efficiency). Due to evolutionary trade-offs plants can not maximize both growth rate and nutrient retention. Thus, the low growth rate of species from nutrient-poor habitats should be considered as the consequence of nutrient retention rather than as a feature on which direct selection takes place. The contrasting traits of species from nutrient-poor and nutrient-rich habitats mutually exclude them from each others' habitats. Moreover, these traits have severe consequences for litter decomposability and thereby also for nutrient cycling. This leads both in nutrient-poor and nutrient-rich habitats to a positive feedback between plant species dominance and nutrient availability, thereby promoting ecosystem stability.     

Intraspecific competition favors ant–plant protective mutualism

The dependency of the anti-herbivore defense on ant–plant protective mutualism often varies depending on abiotic and biotic conditions. Although intraspecific competition is a primary interaction between neighboring plants, its effects on ant–plant mutualisms have yet to be sufficiently elucidated. In order to determine the effects of intraspecific competition and competitor genotype on ant–plant mutualisms, I conducted competition and ant-removal experiments and examined their effects on damage to the leaves of Urena lobata var. tomentosa plants. I found that larger numbers of worker ants visited the plants growing with non-siblings than plants growing alone and that plants growing with non-siblings had a higher shoot to root ratio and secreted greater volumes of extrafloral nectar than plants growing alone and/or with siblings. Under the presence of both sibling and non-sibling competitors, I observed that when ants were removed from plants, those grown with conspecific neighbors were characterized by a higher percentage of damaged leaf area than plants harboring ants. The effect of ant exclusion on leaf damage was more pronounced in plants grown with non-siblings than those grown near siblings. However, when the plants were grown alone, I detected no significant difference in percentage leaf damage between the ant-excluded and ant-harboring plants. The results indicate that neighboring plants can exert strong effects on ant–plant protective mutualisms, thereby highlighting the need to take into consideration plant–plant interactions in studies on these mutualistic associations.     

Bottom-up effects may not reach the top: the influence of ant-aphid interactions on the spread of soil disturbances through trophic chains

Soil disturbances that increase nutrient availability may trigger bottom-up cascading effects along trophic chains. However, the strength and sign of these effects may depend on attributes of the interacting species. Here, we studied the effects of nutrient-rich refuse dumps of the leaf-cutting ant, Acromyrmex lobicornis, on the food chain composed of thistles, aphids, tending ants and aphid natural enemies. Using stable isotopes tracers, we show that the nitrogen accumulated in refuse dumps propagates upward through the studied food chain. Thistles growing on refuse dumps had greater biomass and higher aphid density than those growing in adjacent soil. These modifications did not affect the structure of the tending ant assemblage, but were associated with increased ant activity. In contrast to the expectations under the typical bottom-up cascade effect, the increase in aphid abundance did not positively impact on aphid natural enemies. This pattern may be explained by both an increased activity of tending ants, which defend aphids against their natural enemies, and the low capacity of aphid natural enemies to show numerical or functional responses to increased aphid density. Our results illustrate how biotic interactions and the response capacity of top predators could disrupt bottom-up cascades triggered by disturbances that increase resource availability.     

The influence of harvester and Messor capensis nest-mounds on the productivity and distribution of some plant species in the southern Karoo,South Africa

Nest-mounds of the harvester ant Messor capensis occur on and around nutrient-rich patches, along minor drainage lines in nutrient-rich soils, and on the plains, generally in nutrient-poor soils. Nest-site selection is related to the presence of suitable deep soils, the presence of stones and the distance from the nearest neighbouring nest. Two plant species, Galenia fruticosa and Pteronia pallens, were significantly associated with Messor capensis nest-mounds, both in numbers of mounds occupied and in numbers of individuals. A third species, Drosanthemum montaguense, was also more common in numbers of individuals, while a fourth species, Rhinephyllum macradenium was negatively associated with these mounds. The analysis of species guilds by soil type shows that significantly more species of nutrient-rich soils are present on M. capensis nest-mounds. Also, significantly more taller, woody species occurred on nest-mounds than in inter-mound spaces. Two species, Pteronia pallens and Osteospermum sinuatum, growing on ant nest-mounds had significantly longer inter-nodes than the same species growing off mounds. However, two other species, Pteronia cf. empetrifolia and Galenia fruticosa showed no difference in inter-node lengths between plants growing on and off mounds. Six of the nine species of plants sampled on ant nest-mounds had significantly higher seed production than plants of the same species growing in inter-mound spaces. The other three species showed a tendency towards more seeds per plant on ant nest-mounds. The proportions of live and dead plants on mounds differed between species. Only Ruschia spinosa showed a significant difference between the numbers of dead plants in the population on and off mounds, with more dead plants occurring on mounds. Significantly more seeds set on individuals of Pteronia pallens growing on ant nest-mounds than those growing off nest-mounds, but no such difference occurred in P. cf. empetrifolia. There was no significant difference in the proportion of seeds parasitized by the tephritid fly Desmella anceps for individuals of P. pallens and P. cf. empetrifolia growing on and off mounds.     

How plants may benefit from their consumers: leaf-cutting ants indirectly improve anti-herbivore defenses in <Emphasis Type="Italic">Carduus nutans</Emphasis> L

Although herbivores often have a negative impact on plant fitness, sometimes plants may benefit from their consumers. However, these positive interactions usually occur as a result of plant damage (e.g., overcompensation, defense induction). I present evidence of a novel way by which plants may benefit from their consumers without being eaten. Plants of Carduus nutans increased their physical defenses when grown in external refuse dumps of the leaf-cutting ant Acromyrmex lobicornis. Seedlings planted in refuse exhibited longer spines and tougher leaves than those planted in control soils. Pick-up assays with entire leaves and leaf discs demonstrated that these enhanced physical defenses prevented leaf-cutting ant harvest. Additionally, plants established in refuse dumps showed fewer insect herbivory than those in non-nest soils. The nutrient-rich refuse dump appeared to reduce the stage at which leaves are tender and thus more vulnerable to herbivory. This is the first case where plants may benefit from insect herbivores via waste products without the cost of being eaten. This illustrates how plants may plastically respond to reliable cues of the risk of herbivory.     

The relationship between P,Fe and Mn uptakes by submersed rooted angiosperms

Three species of submersed rooted aquatic angiosperms (Lagarosiphon major, Myriophyllum triphyllum and Potamogeton ochreatus) were grown on nutrient-rich and nutrient-poor sediments in Lake Taupo, an oligotrophic New Zealand lake. Results indicate that plants grown on nutrient-rich sediments had significantly higher shoot tissue concentrations of P, Fe and Mn than those grown on nutrient-poor. High correlations between all three nutrients were recorded for all species. This suggested that the availabilities of P, Fe and Mn in these sediments to these aquatic plants were interrelated.     

Seedling recruitment in a semi‐arid Patagonian steppe: Facilitative effects of refuse dumps of leaf‐cutting ants

Question: What is the influence of refuse dumps of leaf‐cutting ants on seedling recruitment under contrasting moisture conditions in a semi‐arid steppe? Location: Northwestern Patagonia, Argentina. Methods: In a greenhouse experiment, we monitored seedling recruitment in soil samples from refuse dumps of nests of the leaf‐cutting ant Acromyrmex lobicornis and non‐nest sites, under contrasting moisture conditions simulating wet and dry growing seasons. Results: The mean number of seedling species and individuals were higher in wet than in dry plots, and higher in refuse dump plots than in non‐nest soil plots. The positive effect of refuse dumps on seedling recruitment was greater under low moisture conditions. Both the accumulation of discarded seeds by leaf‐cutting ants and the passive trapping of blowing‐seeds seems not explain the increased number of seeds in refuse dumps. Conversely, refuse dumps have higher water retention capacity and nutrient content than adjacent non‐nest soils, allowing the recruitment of a greater number of species and individual seedlings. Conclusions: Nests of A. lobicornis may play an important role in plant recruitment in the study area, allowing a greater number of seedlings and species to be present, hence resulting in a more diverse community. Moreover, leaf‐cutting ant nests may function as nurse elements, generating safe sites that enhance the performance of neighbouring seedlings mainly during the driest, stressful periods.     

Influence of sediment fertility on morphological variability of Vallisneria spiralis L.

One homogeneous and three heterogeneous nutrient enrichment treatments were imposed to investigate the growth responses of Vallisneria spiralis L. Morphological features of V. spiralis differed significantly between different nutrient patches. Roots elongated in nutrient-poor patches, and the specific root length (SRL) also increased significantly. Stolon length, diameter and leaf length and width increased significantly in nutrient-rich patches. Total plant biomass of V. spiralis grown in the homogeneous and three heterogeneous treatments on average were 2.9, 3.0, 3.9 and 2.3 fold higher than that grown in the control treatment. Number of ramets per clone was significantly higher in the heterogeneous treatments than in the homogeneous treatment. In three varying heterogeneous treatments, ramet biomass in nutrient-rich patches was 2.7, 4.3 and 3.0 fold higher than in nutrient-poor patches; however, ramet number was not affected by sediment nutrients, resulting in bigger ramets in nutrient-rich patches. The biomass allocation established adaptive plasticity to heterogeneous environments. The maximum value of biomass allocation to underground parts reached 16% in nutrient-rich patches, whereas the minimum value of underground parts reached 20% in nutrient-poor patches. Results demonstrate that clonal V. spiralis can maintain itself preferentially in favourable nutrient-rich sediments, whereas nutrient-poor conditions could be escaped by plastic biomass allocation.     

Competition Increases Sensitivity of Wheat (Triticum aestivum) to Biotic Plant-Soil Feedback

Plant-soil feedback (PSF) and plant competition play an important role in structuring vegetation composition, but their interaction remains unclear. Recent studies suggest that competing plants could dilute pathogenic effects, whereas the standing view is that competition may increase the sensitivity of the focal plant to PSF. In agro-ecosystems each of these two options would yield contrasting outcomes: reduced versus enhanced effects of weeds on crop biomass production. To test the effect of competition on sensitivity to PSF, we grew Triticum aestivum (Common wheat) with and without competition from a weed community composed of Vicia villosa, Chenopodium album and Myosotis arvensis. Plants were grown in sterilized soil, with or without living field inoculum from 4 farms in the UK. In the conditioning phase, field inocula had both positive and negative effects on T. aestivum shoot biomass, depending on farm. In the feedback phase the differences between shoot biomass in T. aestivum monoculture on non-inoculated and inoculated soils had mostly disappeared. However, T. aestivum plants growing in mixtures in the feedback phase were larger on non-inoculated soil than on inoculated soil. Hence, T. aestivum was more sensitive to competition when the field soil biota was present. This was supported by the statistically significant negative correlation between shoot biomass of weeds and T. aestivum, which was absent on sterilized soil. In conclusion, competition in cereal crop-weed systems appears to increase cereal crop sensitivity to soil biota.     

Effects of nitrogen availability on competition between Bromus tectorum and Bouteloua gracilis

Exotic plant invasions are a serious concern for land managers and conservationists. There is evidence that increased nitrogen availability favors exotic species and decreased nitrogen availability favors non-weedy native species. This study was conducted to test the effect of nitrogen availability on competition between two grass species with contrasting life histories, cheatgrass (Bromus tectorum), a North American exotic, and blue grama (Bouteloua gracilis), a North American native. We investigated the effects of nitrogen availability and competition on aboveground biomass, belowground biomass, height, and % nitrogen tissue concentrations by growing the two species in the greenhouse under five levels of nitrogen and six levels of competition. Nitrogen availability affected competition between Bromus tectorum and Bouteloua gracilis. At the lowest level of N availability, neither species was affected by competition. As N availability increased, aboveground biomass gain of Bromus was more negatively affected by intraspecific competition relative to interspecific competition while the opposite occurred for Bouteloua. At the competition level at which each species gained the most aboveground biomass, Bromus had a linear response to increasing N availability while the response of Bouteloua was asymptotic. Our results do provide some support for the theory that fast growing exotic species have a rapid response to nutrient enrichment while native non-weedy species do not, and that low N levels can reduce competitive pressure from the exotic on the native.     

EFFECTS OF COMPETITION AND DISTURBANCE ON THE RESPROUTING PERFORMANCE OF THE MEDITERRANEAN SHRUB ERICA MULTIFLORA L. (ERICACEAE)

Two field experiments were designed to evaluate the importance of competition, fire, repeated disturbance, and their interactions on the vegetative and reproductive performance of the Mediterranean shrub Erica multiflora over a 2.5-yr period. In a burn experiment, fire was applied to the ground-level stumps of previously clipped 13-yr-old plants with a propane torch and competition was diminished by removal of neighboring plants. Fire resulted in a reduction of sprout vigor and biomass of flowers; mature neighbors also reduced E. multiflora sprout vigor and flowering. The interaction between fire and competition was nonsignificant. In a stand burned by a wildfire we studied the effects of regenerating neighbors on target plants by removing all neighbors or only Quercus coccifera, the most dominant species in the burned stand. In this stand we also simulated herbivory by repeatedly clipping the sprouts of E. multiflora. Regenerating neighbors did not affect target plant sprout vigor after the wildfire, but did cause a decrease in the biomass of flowers per plant. Survival decreased after repeated clipping but was not affected by neighborhood treatment. The results suggest that the importance of competition on resprouting vigor was temporally variable. Variables related to plant size rather than species determined competitive superiority: resprouting neighbors did not affect resprouting performance of target plants, but mature neighbors did. In nature, fire may directly reduce vegetative and reproductive biomass by the heating effect. But it may have an indirect positive effect on biomass, by reducing competition among plants. Frequent disturbances that removed aboveground biomass of E. multiflora had a detrimental effect on target plant survival independent of neighborhood effect.     

Competitive relationships between two contrasting but coexisting grasses

Leymus chinensis (Trin.) Tzvelev and Phragmites communis Trin. are co-dominant grasses in both nutrient-poor and rich steppes in the Songnen Plains of northeastern China. A replacement series experiment was conducted along a five-level nutrient gradient to test for changes in the competitive relationships of the species with nutrient availability. There were significant effects of nutrient level and species proportion on mean plant height, biomass and rhizome length. Facilitation was found in these plant attributes for both species when grown in mixture compared with monoculture, especially at high nutrient levels. For example, L. chinensis grew taller and intercepted more light when in mixture. The aboveground dominance of P. communis did not decrease in mixture, possibly because it benefited from belowground interactions with L. chinensis. Biomass allocation was apparently influenced by both above and belowground competition. For example, L. chinensis allocated much more biomass to shoots and less to roots when neighbor number was increased, suggesting that aboveground competition occurred. Root: shoot ratios of L. chinensis in monoculture decreased gradually with increasing nutrient availability, whereas the ratio in mixture declined rapidly, implying intense competition for light at high levels. Our results support the hypothesis that nutrient competition is more important than light competition when soil nutrients are scarce. Although the interactions between these species benefit both, P. communis is more likely to displace L. chinensis at high nutrient availability.     

Effects of Grazing Regimes on Plant Traits and Soil Nutrients in an Alpine Steppe,Northern Tibetan Plateau

Understanding the impact of grazing intensity on grassland production and soil fertility is of fundamental importance for grassland conservation and management. We thus compared three types of alpine steppe management by studying vegetation traits and soil properties in response to three levels of grazing pressure: permanent grazing (M1), seasonal grazing (M2), and grazing exclusion (M3) in the alpine steppe in Xainza County, Tibetan Plateau. The results showed that community biomass allocation did not support the isometric hypothesis under different grassland management types. Plants in M1 had less aboveground biomass but more belowground biomass in the top soil layer than those in M2 and M3, which was largely due to that root/shoot ratios of dominant plants in M1 were far greater than those in M2 and M3. The interramet distance and the tiller size of the dominant clonal plants were greater in M3 than in M1 and M2, while the resprouting from rhizome buds did not differ significantly among the three greezing regimes. Both soil bulk density and soil available nitrogen in M3 were greater than in M1 at the 15–30 cm soil depth (P = 0.05). Soil organic carbon and soil total nitrogen were greater in M3 than in M1 and M2 (P = 0.05). We conclude that the isometric hypothesis is not supported in this study and fencing is a helpful grassland management in terms of plant growth and soil nutrient retention in alpine steppe. The extreme cold, scarce precipitation and short growing period may be the causation of the unique plant and soil responses to different management regimes.     

Effects of Spatial Scale of Soil Heterogeneity on the Growth of a Clonal Plant Producing Both Spreading and Clumping Ramets

Soil nutrients are commonly heterogeneously distributed at different spatial scales. Although numerous studies have tested the effects of soil nutrient heterogeneity on growth of clonal plants producing either spreading ramets or clumping ramets, no study has examined the effects on the growth of clonal plants producing both spreading and clumping ramets and how spatial scale affects such effects. To test these effects, clones of Buchloe dactyloides, a stoloniferous clonal plant that produces both clumping and spreading ramets, were grown in six heterogeneous environments with different patch sizes and one homogeneous environment containing the same quantity of nutrients. Total biomass, total number of ramets, number of clumping ramets, number of spreading ramets, spacer length, or root:shoot ratio of the whole plants did not differ significantly among the seven treatments. However, at the patch level there were significant effects of patch size by nutrient level on biomass, number of ramets, number of spreading ramets, and number of clumping ramets, and these four variables were significantly larger in the nutrient-rich patches than in the nutrient-poor patches in the heterogeneous treatment with the largest patch size, but not in the other five heterogeneous treatments with smaller patch sizes. Neither nutrient level nor patch size significantly affected spacer length or root:shoot ratio. Based on our results, we propose that B. dactyloides can efficiently exploit nutrient-rich patches by a plastic response of clumping ramets and spreading ramets at larger spatial scales of soil heterogeneity but not at smaller ones.     

氮素和水分添加对贝加尔针茅草原植物多样性及生物量的影响

为了解草原植物群落物种多样性和植物地上生物量对氮沉降增加和降水变化的响应,在内蒙古贝加尔针茅(Stipa baicalensis)草原,分别设置对照(N0)、1.5 g/m2(N15)、3.0 g/m2(N30)、5.0 g/m2(N50)、10.0 g/m2(N100)、15.0 g/m2(N150)、20.0g/m2(N200)和30.0 g/m2(N300)(不包括大气沉降的氮量)8个氮素(NH4NO3)添加梯度和模拟夏季增加降水100mm的水分添加交互试验,研究氮素和水分添加对草原群落植物物种多样性和几种常见植物地上生物量的影响。结果表明:(1)氮素和水分的添加降低了草原群落植物物种多样性,且氮素和水分有显著的互作效应。在水分添加的条件下,随着施氮水平的增加,群落植物物种多样性减小;在无水分添加的条件下,随着施氮水平的增加,群落植物物种多样性呈先增加后减小的"单峰"变化趋势。(2)不同植物对氮素和水分添加的响应不同,随着施氮水平的增加,羊草地上生物量显著增加;贝加尔针茅、羽茅、糙隐子草、寸草苔和冷蒿先增加后减少,呈单峰曲线;星毛委陵菜、牧马豆、扁蓄豆和线叶菊地上生物量则逐渐减少。而且氮素和水分对贝加尔针茅、羽茅、扁蓄豆地上生物量有显著的交互作用。