Plant traits and ecosystem effects of clonality: a new research agenda

Background

Clonal plants spread laterally by spacers between their ramets (shoot–root units); these spacers can transport and store resources. While much is known about how clonality promotes plant fitness, we know little about how different clonal plants influence ecosystem functions related to carbon, nutrient and water cycling.

Approach

The response–effect trait framework is used to formulate hypotheses about the impact of clonality on ecosystems. Central to this framework is the degree of correspondence between interspecific variation in clonal ‘response traits’ that promote plant fitness and interspecific variation in ‘effect traits’, which define a plant''s potential effect on ecosystem functions. The main example presented to illustrate this concept concerns clonal traits of vascular plant species that determine their lateral extension patterns. In combination with the different degrees of decomposability of litter derived from their spacers, leaves, roots and stems, these clonal traits should determine associated spatial and temporal patterns in soil organic matter accumulation, nutrient availability and water retention.

Conclusions

This review gives some concrete pointers as to how to implement this new research agenda through a combination of (1) standardized screening of predominant species in ecosystems for clonal response traits and for effect traits related to carbon, nutrient and water cycling; (2) analysing the overlap between variation in these response traits and effect traits across species; (3) linking spatial and temporal patterns of clonal species in the field to those for soil properties related to carbon, nutrient and water stocks and dynamics; and (4) studying the effects of biotic interactions and feedbacks between resource heterogeneity and clonality. Linking these to environmental changes may help us to better understand and predict the role of clonal plants in modulating impacts of climate change and human activities on ecosystem functions.     

Vegetation pattern formation in a fog-dependent ecosystem

Vegetation pattern formation is a striking characteristic of several water-limited ecosystems around the world. Typically, they have been described on runoff-based ecosystems emphasizing local interactions between water, biomass interception, growth and dispersal. Here, we show that this situation is by no means general, as banded patterns in vegetation can emerge in areas without rainfall and in plants without functional root (the Bromeliad Tillandsia landbeckii) and where fog is the principal source of moisture. We show that a simple model based on the advection of fog-water by wind and its interception by the vegetation can reproduce banded patterns which agree with empirical patterns observed in the Coastal Atacama Desert. Our model predicts how the parameters may affect the conditions to form the banded pattern, showing a transition from a uniform vegetated state, at high water input or terrain slope to a desert state throughout intermediate banded states. Moreover, the model predicts that the pattern wavelength is a decreasing non-linear function of fog-water input and slope, and an increasing function of plant loss and fog-water flow speed. Finally, we show that the vegetation density is increased by the formation of the regular pattern compared to the density expected by the spatially homogeneous model emphasizing the importance of self-organization in arid ecosystems.     

The influence of non-sorted circles on species diversity of vascular plants, bryophytes and lichens in Sub-Arctic Tundra

Non-sorted circles (NSCs), also known as frost boils, are common soil frost features that create a small-scale mosaic of vegetation zones in periglacial landscapes. The causes of variation in plant diversity within NSCs are poorly understood. This lack of understanding hampers our ability to predict how arctic plant communities respond to changing soil frost conditions. We hypothesised that plant communities of different ages develop at a micro-site scale within NSCs as soil frost periodically exposes uncolonised soil or fatally offsets plant succession. To test this hypothesis, we investigated the species diversity of plant communities (vascular plants, bryophytes and lichens) from the sparsely vegetated centre of the circles to the densely vegetated outer domain in conjunction with estimates of the age of the plant communities (inferred using lichenometry). Our results suggest that the variation in species diversity and density can largely be explained by the occurrence of progressively older plant communities from the centre towards the vegetated rim. Here, the high species diversity was observed to occur in communities having the ages approximately around 150?years. Our findings suggest that soil frost disturbances are important for maintaining successional gradients several centuries long within the arctic landscape at a small spatial scale (<3?m). The termination of soil frost activity as a result of a warmer future winter climate is therefore most likely to result in a loss of micro-sites having young vegetation communities with high plant diversities and a subsequent establishment of mature shrub-dominated plant communities.     

Effects of Nutrient Heterogeneity and Competition on Root Architecture of Spruce Seedlings: Implications for an Essential Feature of Root Foraging

Background

We have limited understanding of root foraging responses when plants were simultaneously exposed to nutrient heterogeneity and competition, and our goal was to determine whether and how plants integrate information about nutrients and neighbors in root foraging processes.

Methodology/Principal Findings

The experiment was conducted in split-containers, wherein half of the roots of spruce (Picea asperata) seedlings were subjected to intraspecific root competition (the vegetated half), while the other half experienced no competition (the non-vegetated half). Experimental treatments included fertilization in the vegetated half (FV), the non-vegetated half (FNV), and both compartments (F), as well as no fertilization (NF). The root architecture indicators consisted of the number of root tips over the root surface (RTRS), the length percentage of diameter-based fine root subclasses to total fine root (SRLP), and the length percentage of each root order to total fine root (ROLP). The target plants used novel root foraging behaviors under different combinations of neighboring plant and localized fertilization. In addition, the significant increase in the RTRS of 0–0.2 mm fine roots after fertilization of the vegetated half alone and its significant decrease in fertilizer was applied throughout the plant clearly showed that plant root foraging behavior was regulated by local responses coupled with systemic control mechanisms.

Conclusions/Significance

We measured the root foraging ability for woody plants by means of root architecture indicators constructed by the roots possessing essential nutrient uptake ability (i.e., the first three root orders), and provided new evidence that plants integrate multiple forms of environmental information, such as nutrient status and neighboring competitors, in a non-additive manner during the root foraging process. The interplay between the responses of individual root modules (repetitive root units) to localized environmental signals and the systemic control of these responses may well account for the non-additive features of the root foraging process.     

Explore less to control more: why and when should plants limit the horizontal exploration of soil by their roots?

In ecosystems limited by soil nutrients, some plants show a restricted horizontal distribution of their roots. We explored the hypothesis that this particular pattern is a foraging strategy emerging from tradeoffs between soil exploration (that increases the pool of nutrients available for plants) and the local control of nutrient cycling within the soil that we call soil occupation. We developed two general analytical models of the cycling of a limiting nutrient in a plant population that is not limited by water. They allowed to explore how plant productivity is affected when roots do not exploit the whole soil available and to determine the conditions for which plant nutrient stock is maximized when plants limit their exploration of soil. We predict that a restricted exploration strategy can be beneficial when 1) there is at least one tradeoff between a nutrient cycling parameter and soil exploration, 2) nutrient availability in the unexplored soil is poor and 3) the area of soil explored by plants is stable over time. The exploration limitation strategy results in spatially heterogeneous and nutrient‐conservative ecosystems. Our results should apply well to perennial tussock grasses within tropical nutrient‐limited ecosystems and raises interesting cues for the construction of more sustainable agro‐ecosystems. Overall, our study underlines the importance of considering the multiplicity of root–soil interactions and of their scales when considering root foraging strategies.     

Linking habitat modification to catastrophic shifts and vegetation patterns in bogs

Paleoecological studies indicate that peatland ecosystems may exhibit bistability. This would mean that these systems are resilient to gradual changes in climate, until environmental thresholds are passed. Then, ecosystem stability is lost and rapid shifts in surface and vegetation structure at landscape scale occur. Another remarkable feature is the commonly observed self-organized spatial vegetation patterning, such as string-flark and maze patterns. Bistability and spatial self-organization may be mechanistically linked, the crucial mechanism being scale-dependent (locally positive and longer-range negative) feedback between vegetation and the peatland environment. Focusing on bogs, a previous model study shows that nutrient accumulation by vascular plants can induce such scale-dependent feedback driving pattern formation. However, stability of bog microforms such as hummocks and hollows has been attributed to different local interactions between Sphagnum, vascular plants, and the bog environment. Here we analyze both local and longer-range interactions in bogs to investigate the possible contribution of these different interactions to vegetation patterning and stability. This is done by a literature review, and subsequently these findings are incorporated in the original model. When Sphagnum and encompassing local interactions are included in this model, the boundaries between vegetation types become sharper and also the parameter region of bistability drastically increases. These results imply that vegetation patterning and stability of bogs could be synergistically governed by local and longer-range interactions. Studying the relative effect of these interactions is therefore suggested to be an important component of future predictions on the response of peatland ecosystems to climatic changes.     

Allocation,stress tolerance and carbon transport in plants: how does phloem physiology affect plant ecology?

Despite the crucial role of carbon transport in whole plant physiology and its impact on plant–environment interactions and ecosystem function, relatively little research has tried to examine how phloem physiology impacts plant ecology. In this review, we highlight several areas of active research where inquiry into phloem physiology has increased our understanding of whole plant function and ecological processes. We consider how xylem–phloem interactions impact plant drought tolerance and reproduction, how phloem transport influences carbon allocation in trees and carbon cycling in ecosystems and how phloem function mediates plant relations with insects, pests, microbes and symbiotes. We argue that in spite of challenges that exist in studying phloem physiology, it is critical that we consider the role of this dynamic vascular system when examining the relationship between plants and their biotic and abiotic environment.     

Plant community dynamics, nutrient cycling, and alternative stable equilibria in peatlands

Although observational data and experiments suggest that carbon flux and storage in peatlands are controlled by hydrology and/or nutrient availability, we lack a rigorous theory to account for the roles that different plant species or life-forms, particularly mosses, play in carbon and nutrient flux and storage and how they interact with different hydrologic sources of nutrients. We construct and analyze a model of peatlands that sheds some light on this problem. The model is a set of six coupled differential equations that define the flow of nutrients from moss and vascular plants to their litters, then to peat, and finally to an inorganic nutrient resource pool. We first analyze a simple version of this model (model 1) in which all nutrient input is from precipitation and enters the moss compartment directly, mimicking the dynamics of ombrotrophic bogs. There is a transcritical bifurcation that results in a switch of stability between two equilibrium bog communities: a moss monoculture and a community where mosses and vascular plants coexist. The bifurcation depends on the magnitudes of the input/output budget of the peatland and the life-history traits of the plants. We generalize model 1 to model 2 by dividing nutrient inputs between precipitation and groundwater, thus also allowing the development of minerotrophic fens that receive nutrient subsidies from both groundwater and precipitation and adding intraspecific competition (self-limitation) terms for both moss and vascular plants. Partitioning precipitation inputs between moss and the nutrient pool resulted in the greatest changes in model behavior, including the appearance of a lake and a vascular plant monoculture as well as the moss monoculture and coexistence equilibrium. As with model 1, these solutions are separated by transcritical bifurcations depending on critical combinations of parameters determining the input-output budget of the peatland as well as the life-history characteristics of the plant species. Model 2 also allowed for an early transient spike in vascular plant dominance followed by approach to near moss monoculture and then eventual approach to coexistence equilibrium. This generalized model mimics the broad features of successional development of peatlands from fens to bogs often found in the paleorecords of peat cores.     

Variation in terricolous bryophyte and macrolichen vegetation along primary gradients in Canadian boreal forests

Abstract. Compositional variation among 172 boreal forests stands, based upon the understorey terricolous bryophyte and macrolichen vegetation, is compared with that based upon vascular vegetation. Detrended correspondence analysis (DCA) of the two datasets yielded stand ordinations each indicating a single dominant upland-bottomland gradient. This was most clear among 138 stands in the cryptogam dataset. Canonical correspondence analysis (CCA) of the 138 stand subset yielded a pronounced first axis gradient from dry, nutrient poor pine dominated forest sites to moist, nutrient rich bottomland sites dominated by balsam poplar woodland. Individual species response curves, by the method of log-linear least squares regression, yielded three different respective patterns among the most abundant tree, understorey vascular and cryptogamic plant species. Whereas curves for the tree species were ‘Gaussian’ in shape and displaced at somewhat regular intervals along the gradient, the abundant understorey vascular plant species curves clustered in the mesic, mixed wood region two thirds of the way along. Curves for the abundant cryptogams were mostly linear in shape and absent from the mixed wood zone, showing concentrations at one end of the gradient or the other. Two interpretations of the understorey pattern are considered. One proposes competitive exclusion of cryptogams from the mesic mixed wood region by the vascular understorey plants. Another proposes that the cryptogams are adapted to the sharply contrasting ecosystems encountered at one end of the gradient or another but that none can effectively cope with the intermediate, mixed wood ecosystems. Evidence from the response curves favours this second interpretation.     

温带阔叶林中氮的保留机制:"春坝"假设及研究实例

在过去的半个世纪中,森林生态系统物质能量流动模型的发展使得生态学者对矿物元素的来源、去处、转换速率及其干扰破坏的影响有了进一步的认识.其中"春坝"假设提出在早春来临之际,林冠树木仍处于冬眠之时,林下地被植物先于林冠树木发芽开花,吸收土壤养分于植物体内,使得养分免于因雨水或雪水而造成的淋失而得以保留于森林生态系统中.本文对"春坝"假设的研究作一小结.现有的研究结果显示,早春地被植物对土壤氮的吸收量因不同生态系统而异;而土壤微生物对氮的吸收量远高于过去人们所认识的水平.早春地被物与土壤微生物都能在季节的变化中吸收土壤养分、共同起着"春坝"作用.两者所起作用大小因气候及生态系统的不同而有所改变.     

Ecosystem-phase interactions: aquatic eutrophication decreases terrestrial plant diversity in California vernal pools

Eutrophication has long been known to negatively affect aquatic and terrestrial ecosystems worldwide. In freshwater ecosystems, excessive nutrient input results in a shift from vascular plant dominance to algal dominance, while the nutrient-species richness relationship is found to be unimodal. Eutrophication studies are usually conducted in continuously aquatic or terrestrial habitats, but it is unclear how these patterns may be altered by temporal heterogeneity driven by precipitation and temperature variation. The California vernal pool (CVP) ecosystem consists of three distinct phases (aquatic, terrestrial, and dry) caused by variation in climatic conditions. The purpose of this study was to test the hypothesis that resource addition during the aquatic phase results in increased algal abundance, which reduces vascular plant cover and richness of the terrestrial phase upon desiccation. We used mesocosms layered with CVP soil, in which treatments consisted of five levels of nitrogen and phosphorous added every 2 weeks. Resource addition increased available phosphorus levels and algae cover during the aquatic phase. Increased algal crusts resulted in decreased vascular plant percent cover and species richness. Few significant patterns were observed with individual plant species and total biomass. The phosphorus-plant richness relationship was not significant, but species composition was significantly different among the low and high treatment comparisons. These results highlight a neglected effect of eutrophication in seasonal habitats. Interactions among ecosystem phases clearly require more attention empirically and theoretically. Management and restoration of temporally heterogeneous habitat, such as the endemic-rich CVP, need to consider the extensive effects of increased nutrient input.     

Roots,nutrients and their relationship to spatial patterns

Ecosystem sustainability and resilience after a disturbance may be regulated by processes occurring at smaller spatial scales. The matrix of different spatial environments are created by (1) individual plants that accumulate higher concentrations of specific nutrients, trace elements or defensive plant secondary chemicals and thereby modify the chemistry of their ecological space and/or rates of processes, (2) the presence of structures (e.g., coarse woody debris) that may buffer some micro-environments from disturbances by functioning as a hospitable environment or as a reservoir for mycorrhizal fungi to sustain them into the next phase of stand development, and (3) chemical changes in soils during soil development which may result in distinct soil chemical environments. The response of the plants or change in the sustainability of carbon and nutrient cycles may be expressed more strongly at this smaller ecological space of an individual plant and furthermore must be frequently examined separately by the above- and belowground space of that individual.This paper will present three case studies from temperate and tropical forest ecosystems which suggest the importance of studying plant growth and nutrient and trace element cycling by stratifying sampling to encompass the mosaic patterns of existing spatial variability within the ecosystem. The examples show how individual plant species are able to create ecologically distinct spatial environments because of their distribution patterns within the landscape, how nutrient transfers in roots respond to the chemical variations in the soil, and how roots and mycorrhizal fungi are able to maintain themselves in the mosaic of coarse woody debris remaining on a site after the elimination of aboveground tree biomass.     

The distribution of soil nutrients with depth: Global patterns and the imprint of plants

To understand the importance of plants in structuring the vertical distributions of soil nutrients, we explored nutrient distributions in the top meter of soil for more than 10,000 profiles across a range of ecological conditions. Hypothesizing that vertical nutrient distributions are dominated by plant cycling relative to leaching, weathering dissolution, and atmospheric deposition, we examined three predictions: (1) that the nutrients that are most limiting for plants would have the shallowest average distributions across ecosystems, (2) that the vertical distribution of a limiting nutrient would be shallower as the nutrient became more scarce, and (3) that along a gradient of soil types with increasing weathering-leaching intensity, limiting nutrients would be relatively more abundant due to preferential cycling by plants. Globally, the ranking of vertical distributions among nutrients was shallowest to deepest in the following order: P > K > Ca > Mg > Na = Cl = SO₄. Nutrients strongly cycled by plants, such as P and K, were more concentrated in the topsoil (upper 20 cm) than were nutrients usually less limiting for plants such as Na and Cl. The topsoil concentrations of all nutrients except Na were higher in the soil profiles where the elements were more scarce. Along a gradient of weathering-leaching intensity (Aridisols to Mollisols to Ultisols), total base saturation decreased but the relative contribution of exchangeable K⁺ to base saturation increased. These patterns are difficult to explain without considering the upward transport of nutrients by plant uptake and cycling. Shallower distributions for P and K, together with negative associations between abundance and topsoil accumulation, support the idea that plant cycling exerts a dominant control on the vertical distribution of the most limiting elements for plants (those required in high amounts in relation to soil supply). Plant characteristics like tissue stoichiometry, biomass cycling rates, above- and belowground allocation, root distributions, and maximum rooting depth may all play an important role in shaping nutrient profiles. Such vertical patterns yield insight into the patterns and processes of nutrient cycling through time.     

Successional patterns of woody plants in catchment areas in a semi-arid region

Patterns of woody plants dispersal in a semi-arid nature reserve situated in Eastern Transvaal, South Africa, revealed that trees have spread from core areas and converted previously open grasslands to densely vegetated woodlands. These patterns were found in catchment areas of the gently undulating terrain which characterizes the region.Two plant communities dominated by Acacia senegal-Acacia tortilis and Euclea divinorum-Acacia nilotica were distinguished. Analysis of nearest-neighbour distances, dispersal patterns of seedlings and mature woody plants identified successional processes. These were manifested through intra and interspecific competition among the dominant tree species. Within each plant community, a transition of relative abundance was occurring namely, A. senegal became dominant in areas previously dominated by A. tortilis while E. divinorum was replacing previous A. nilotica dominance.Comparative assessment of the two plant communities was facilitated by the summary of competition and seedling dispersal indices in a multivariate analysis. Results indicated that tree species had characteristic dispersal strategies. Identification the patterns of woody plants establishment could advance the evaluation of successional dynamics and management of savannas in areas prone to bush encroachment.     

温带阔叶林中氮的保留机制：

 在过去的半个世纪中,森林生态系统物质能量流动模型的发展使得生态学者对矿物元素的来源、去处、转换速率及其干扰破坏的影响有了进一步的认识。其中“春坝”假设提出：在早春来临之际,林冠树木仍处于冬眠之时,林下地被植物先于林冠树木发芽开花,吸收土壤养分于植物体内,使得养分免于因雨水或雪水而造成的淋失而得以保留于森林生态系统中。本文对“春坝”假设的研究作一小结。现有的研究结果显示,早春地被植物对土壤氮的吸收量因不同生态系统而异;而土壤微生物对氮的吸收量远高于过去人们所认识的水平。早春地被物与土壤微生物都能在季节的变化中吸收土壤养分、共同起着“春坝”作用。两者所起作用大小因气候及生态系统的不同而有所改变。      

Herbivory and plant tolerance: experimental tests of alternative hypotheses involving non‐substitutable resources

A mechanistic understanding of the highly variable effects of herbivores on plant production in different ecosystems remains a major challenge. To explain these patterns, the compensatory continuum hypothesis (CCH) predicts plants to compensate for defoliation when resources are abundant, whereas the growth rate hypothesis (GRH) makes the opposite claim of high herbivory tolerance under resource‐poor conditions. The limiting resource model (LRM) tries to reconcile this dichotomy by incorporating the indirect effects of herbivores on plant resources and predicts that the potential for plant compensation is dependent upon whether, and how, herbivory influences limiting resources. Although extensively evaluated in laboratory monocultures, it remains uncertain whether these models can also explain the response of heterogeneous and multi‐species natural plant communities to defoliation. Here we investigate community‐wide plant response to defoliation and report data from a field experiment in the arid and primarily water‐limited Trans‐Himalayan grazing ecosystem in northern India involving clipping, irrigation and nutrient‐feedback with herbivore dung. Without nutrient‐feedback, plants compensated for defoliation in absence of irrigation but failed to compensate under irrigation. Whereas, in the presence of nutrient‐feedback plants compensated for defoliation when irrigated. This divergent pattern is not consistent with the CCH and GRH, and is only partially explained by the LRM. Instead, these pluralistic results are consistent with the hypothesis that herbivory may alter the relative strengths of water and nutrient limitation since irrigation increased root:shoot ratio in absence of fertilization in unclipped plots, but not in the corresponding clipped plots. So, herbivory appears to increase relative strength of nutrient‐limitation for plants that otherwise seem to be primarily water‐limited. Extending the LRM framework to include herbivore‐mediated transitions between water and nutrient‐limitation may clarify the underlying mechanisms that modulate herbivory‐tolerance under different environmental conditions.     

Bromeliad growth and stoichiometry: responses to atmospheric nutrient supply in fog-dependent ecosystems of the hyper-arid Atacama Desert, Chile

Carbon, nitrogen, and phosphorus (C, N, P) stoichiometry influences the growth of plants and nutrient cycling within ecosystems. Indeed, elemental ratios are used as an index for functional differences between plants and their responses to natural or anthropogenic variations in nutrient supply. We investigated the variation in growth and elemental content of the rootless terrestrial bromeliad Tillandsia landbeckii, which obtains its moisture, and likely its nutrients, from coastal fogs in the Atacama Desert. We assessed (1) how fog nutrient supply influences plant growth and stoichiometry and (2) the response of plant growth and stoichiometry to variations in nutrient supply by using reciprocal transplants. We hypothesized that T. landbeckii should exhibit physiological and biochemical plastic responses commensurate with nutrient supply from atmospheric deposition. In the case of the Atacama Desert, nutrient supply from fog is variable over space and time, which suggests a relatively high variation in the growth and elemental content of atmospheric bromeliads. We found that the nutrient content of T. landbeckii showed high spatio-temporal variability, driven partially by fog nutrient deposition but also by plant growth rates. Reciprocal transplant experiments showed that transplanted individuals converged to similar nutrient content, growth rates, and leaf production of resident plants at each site, reflecting local nutrient availability. Although plant nutrient content did not exactly match the relative supply of N and P, our results suggest that atmospheric nutrient supply is a dominant driver of plant growth and stoichiometry. In fact, our results indicate that N uptake by T. landbeckii plants depends more on N supplied by fog, whereas P uptake is mainly regulated by within-plant nutrient demand for growth. Overall, these findings indicate that variation in fog nutrient supply exerts a strong control over growth and nutrient dynamics of atmospheric plants, which are ubiquitous across fog-dominated ecosystems.     

When microbes and consumers determine the limiting nutrient of autotrophs: a theoretical analysis

Ecological stoichiometry postulates that differential nutrient recycling of elements such as nitrogen and phosphorus by consumers can shift the element that limits plant growth. However, this hypothesis has so far considered the effect of consumers, mostly herbivores, out of their food-web context. Microbial decomposers are important components of food webs, and might prove as important as consumers in changing the availability of elements for plants. In this theoretical study, we investigate how decomposers determine the nutrient that limits plants, both by feeding on nutrients and organic carbon released by plants and consumers, and by being fed upon by omnivorous consumers. We show that decomposers can greatly alter the relative availability of nutrients for plants. The type of limiting nutrient promoted by decomposers depends on their own elemental composition and, when applicable, on their ingestion by consumers. Our results highlight the limitations of previous stoichiometric theories of plant nutrient limitation control, which often ignored trophic levels other than plants and herbivores. They also suggest that detrital chains play an important role in determining plant nutrient limitation in many ecosystems.     

Convergent responses of nitrogen and phosphorus resorption to nitrogen inputs in a semiarid grassland

Human activities have significantly altered nitrogen (N) availability in most terrestrial ecosystems, with consequences for community composition and ecosystem functioning. Although studies of how changes in N availability affect biodiversity and community composition are relatively common, much less remains known about the effects of N inputs on the coupled biogeochemical cycling of N and phosphorus (P), and still fewer data exist regarding how increased N inputs affect the internal cycling of these two elements in plants. Nutrient resorption is an important driver of plant nutrient economies and of the quality of litter plants produce. Accordingly, resorption patterns have marked ecological implications for plant population and community fitness, as well as for ecosystem nutrient cycling. In a semiarid grassland in northern China, we studied the effects of a wide range of N inputs on foliar nutrient resorption of two dominant grasses, Leymus chinensis and Stipa grandis. After 4 years of treatments, N and P availability in soil and N and P concentrations in green and senesced grass leaves increased with increasing rates of N addition. Foliar N and P resorption significantly decreased along the N addition gradient, implying a resorption‐mediated, positive plant–soil feedback induced by N inputs. Furthermore, N : P resorption ratios were negatively correlated with the rates of N addition, indicating the sensitivity of plant N and P stoichiometry to N inputs. Taken together, the results demonstrate that N additions accelerate ecosystem uptake and turnover of both N and P in the temperate steppe and that N and P cycles are coupled in dynamic ways. The convergence of N and P resorption in response to N inputs emphasizes the importance of nutrient resorption as a pathway by which plants and ecosystems adjust in the face of increasing N availability.     

松嫩盐碱草地几种植物的土壤营养位分析

针对松嫩盐碱草地上的几种主要植物,以植物地上生物量和总和优势度（SDR）为土壤营养位效能指标,分别从多项土壤因子综合、可溶盐含量、总碱度、Na^ 含量、有机质含量、全氮含量角度进行了土壤营养位分析。讨论了松嫩盐碱草地植物种群分布格局与土壤营养的关系,指出松嫩盐碱草地植物种群分布格局的形成是可溶盐含量、总碱度、Na^ 含量、有机质含量、全氮含量5项主要土壤因子综合作用的结果,对于各植物种群土壤营养位的分化结果而言,没有一项土壤因子是起决定性作用的。