On periodic algal cyclostat populations

A number of useful relations for population growth rate, uptake of limiting nutrient, amount of intracellular growth limiting nutrient and extracellular limiting nutrient concentration are presented in terms of their period averaged functions. These period averages may provide “species specific” information. The period length is shown to be unique for a given dilution rate and amount of intracellular growth limiting nutrient. A plot of residual limiting nutrient concentration versus cell density describes a closed trajectory (or “phase” loop). The area of this loop may be evaluated in terms of certain period averages of the parameters of the cyclostat population. This area is characteristic for a species at any given dilution rate and growth limiting nutrient concentration entering the cyclostat vessel. Equations for describing multispecies cyclostats are presented. Also, some necessary conditions for stable coexistence in such a system are described.     

Herbivores, the Functional Diversity of Plants Species, and the Cycling of Nutrients in Ecosystems

Numerous investigators have suggested that herbivores almost always increase rates of nutrient and energy flow through terrestrial ecosystems by returning to the soil fecal material and urine with faster turnover rate than shed plant litter. These previous theories and models always treat the producer compartment as a homogenous pool. Essentially, they assume that consumers feed through a pureed cream of vegetable soup. However, many field observations and experiments have shown that consumers feed selectively (i.e., in a cafeteria) and that consumer choice is made on the same chemical basis that determines decomposition rates. Plants that are preferred food sources often have higher nutrient content, higher growth rates, and faster decomposition rates. As consumption reduces dominance of these species in favor of unpreferred species with slower decomposition, rates of nutrient cycling and energy flow should therefore decline. We analyze a model in which the consumer is given a choice among producers that vary in nutrient uptake rates, rates of nutrient return to decomposers, and consumer preference, and which is parameterized for plants and consumers characteristic of boreal regions. In this model, in an open, well-mixed system with one consumer and two such producers, the nutrient/energy flow will not exceed that of a system without the consumer. If the consumer has a choice between two such producers, it must choose one plant over the other at a greater ratio than that between the two plants in uptake and decay rates. In contrast, in a closed system the consumer must be less selective to coexist with the two plants. The system behavior is determined by the level of nutrient return through the consumer and the differences between the plants in nutrient uptake rates and consumer preference. Species richness affects properties of this model system to the extent that species are functionally distinct (i.e., have different rate constants) in a multivariate space of life history traits (i.e., nutrient uptake and palatability). We suggest that the biochemical variability of plant tissues that simultaneously determines both consumer preference and decomposition rates is an essential feature of food webs that cannot be ignored. Thus, ecosystem models should, at minimum, consider more than one producer type with consumer preference.     

Mineral Nutrient Requirements of Vaccinium vitis idaea and V. myrtillus

The mineral nutrient requirements of Vaccinium vitis idaea L. and V. myrtillus L. have been studied. It is found that the optimum nutrient proportions are similar to those of birch seedlings except for a relatively low potassium requirement. The nitrogen sources ammonium and nitrate are both usable at low concentrations. A system with both sources present seems preferable. The ammonium uptake is then much more rapid than the nitrate uptake. The optimum total concentration falls within the low salt range and is about the same as found for birch seedlings, but the sensitivity to high salt concentrations is more pronounced in the Vaccinium species. A very marked characteristic of the nutrient absorption is the rapid calcium uptake. The results are discussed in relation to similar experiments with birch and cucumber seedlings, representing species with a wide or calcareous ecological adaptation.     

The involvement of mycorrhizas in assessment of genetically dependent efficiency of nutrient uptake and use

This article summarises the way in which mycorrhizal infection of roots affects the mineral nutrition of plants and how the symbiosis may interact with the evaluation of efficiency of nutrient uptake and use by plants. A brief account of the processes of infection and the way they are affected by host genotype and environmental conditions is given and the relationships between this and mineral nutrition (especially phosphate nutrition) are outlined.The interactions between mycorrhizal infection and P efficiency are considered at two levels. Mycorrhizas may act as general modifiers of efficiency regardless of the extent to which the plants are infected and in some mycorrhiza-dependent plants infection may change the ranking of genotypes. The extent of infection is also under genetic control and shows considerable variability between genotypes in some species. This variation could be used in programs to select varieties in which infection is rapid and nutrient uptake from nutrient deficient or low input systems is, in consequence, increased.     

Nutrient uptake in eastern deciduous tree seedlings

Tree seedlings that colonize large treefall gaps are generally shade-intolerant species with high potential relative growth rates. Nutrient availability may be significantly elevated in disturbance-induced gaps, however, little is known about the role of differences in nutrient uptake capacities of different species in structuring the community response to gap openings in eastern North American deciduous forests. Seven tree species were grown from seed under both a high and a low nutrient regime, and uptake kinetics of phosphate, ammonium, and nitrate were studied. Yellow birch, a species with intermediate shade tolerance and relative growth rate, had the highest maximum rates of uptake of all ions, while tulip tree, a gap-colonizing species with high relative growth rate, had the lowest rate of phosphate uptake and intermediate rates of ammonium and nitrate uptake. Beech and hickory, which have low relative growth rates and are not gap-colonizing species, had intermediate levels of nutrient uptake. There was no evidence that species with the highest maximum uptake rates measured at high supply concentrations had relatively low uptake at low nutrient supply concentrations. Although birch increased phosphate absorption capacity when grown under a low nutrient regime, this pattern did not hold for nitrate or ammonium uptake, and other species showed no change in nutrient uptake capacity according to nutrient growth regime. Clearly, factors other than nutrient absorption capacity, such as nutrient use efficiency or allocation to root vs. shoot biomass, underlie differences in species' capacities to colonize and maintain a high relative growth rate in canopy gaps.     

Ecosystem Nutrient Use Efficiency, Productivity, and Nutrient Accrual in Model Tropical Communities

Ecosystem nutrient use efficiency–the ratio of net primary productivity to soil nutrient supply–is an integrative measure of ecosystem functioning. High productivity and nutrient retention in natural systems are frequently attributed to high species diversity, even though some single-species systems can be highly productive and effective at resource capture. We investigated the effects of both individual species and life-form diversity on ecosystem nutrient use efficiency using model tropical ecosystems comprised of monocultures of three tree species and polycultures in which each of the tree species was coplanted with species of two additional life forms. Tree species significantly influenced nutrient use efficiency by whole ecosystems in monocultures; however, in polycultures, the additional life forms interacted with the influence exerted by the dominant tree. Furthermore, the presence of the additional life forms significantly increased nutrient uptake and uptake efficiency, but in only two of the three systems and 2 of the 4 years of the study period. These results indicate that the effect of life-form diversity on ecosystem functioning is not constant and that there may be temporal shifts in the influence exerted by different components of the community. Furthermore, although species (and life forms) exerted considerable influence on ecosystem nutrient use efficiency, this efficiency was most closely related to soil nutrient availability. These findings demonstrate that ecosystem nutrient use efficiency is an outcome not only of the characteristics of the species or life forms that comprise the system but also of factors that affect soil nutrient supply. The results argue against the simple upward scaling of nutrient use efficiency from leaves and plants to ecosystems. Received 29 March 2000; accepted 27 April 2001.     

Analysis of a lottery competition model with limited nutrient availability

How a plant species utilizes a limited nutrient is important for its survival. The purpose of this work is to examine how nutrient utilization mechanisms (for seed production) affect the coexistence of competing plant species. We construct a revised lottery model that uses one of three possible kinds of nutrient utilization functions. In all cases the models suggest that two species can coexist under certain circumstances, but that three species cannot coexist, at least when the nutrient utilization functions are continuous functions of nutrient uptake. However, in the discontinuous case three species can coexist in a state of sustained oscillations. The results suggest that one need pay close attention to the differences in the nutrient utilization mechanisms among competing plant species in order to ascertain the competitive outcome.     

Analysis of a lottery competition model with limited nutrient availability

How a plant species utilizes a limited nutrient is important for its survival. The purpose of this work is to examine how nutrient utilization mechanisms (for seed production) affect the coexistence of competing plant species. We construct a revised lottery model that uses one of three possible kinds of nutrient utilization functions. In all cases the models suggest that two species can coexist under certain circumstances, but that three species cannot coexist, at least when the nutrient utilization functions are continuous functions of nutrient uptake. However, in the discontinuous case three species can coexist in a state of sustained oscillations. The results suggest that one need pay close attention to the differences in the nutrient utilization mechanisms among competing plant species in order to ascertain the competitive outcome.     

土壤养分空间异质性与植物根系的觅食反应

植物在长期进化过程中,为了最大限度地获取土壤资源,对养分的空间异质性产生各种可塑性反应．包括形态可塑性、生理可塑性、菌根可塑性等．许多植物种的根系在养分丰富的斑块中大量增生,增生程度种间差异较大,并受斑块属性(斑块大小、养分浓度)、营养元素种类和养分总体供应状况的影响．植物还通过调整富养斑块中细根的直径、分枝角、节问距以及空间构型来实现斑块养分的高效利用．根系的生理可塑性及菌根可塑性可能在一定程度上影响其形态可塑性．生理可塑性表现为处于不同养分斑块上的根系迅速调整其养分吸收速率,从而增加单位根系的养分吸收,对在时间上和空间上变化频繁的空间异质性土壤养分的利用具有重要意义,可在一定程度上弥补根系增生反应的不足．菌根可塑性目前研究较少,一些植物种的菌根代替细根实现在富养斑块中的增生．菌根增生的碳投入养分吸收效率较高、根系增生对增加养分吸收的作用较复杂,取决于养分离子在土壤中的移动性能以及是否存在竞争植物;对植物生长(竞争能力)的作用因种而异,一些敏感种由此获得生长效益,而其它一些植物种受影响较小．植物个体对土壤养分空间异质性反应能力和生长差异,影响其在群落中的地位和命运,最终影响群落组成及其结构．     

Aerial roots of epiphytic orchids: the velamen radicum and its role in water and nutrient uptake

The velamen radicum, a spongy, usually multiple epidermis of the roots, which at maturity consists of dead cells, is frequently described as an important adaptation of epiphytic orchids. Yet, quantitative evidence for the alleged functions, e.g., efficient water and nutrient uptake, nutrient retention, reduction of water loss, mechanical protection, or the avoidance of overheating, is rare or missing. We tested the notion originally put forward by Went in 1940 that the velamen allows plants to capture and immobilize the first solutions arriving in a rainfall, which are the most heavily charged with nutrients. In a series of experiments, we examined whether all necessary functional characteristics are given for this scenario to be realistic under ecological conditions. First, we show that the velamen of a large number of orchid species takes up solutions within seconds, while evaporation from the velamen takes several hours. Charged ions are retained in the velamen probably due to positive and negative charges in the cell walls, while uncharged compounds are lost to the external medium. Finally, we demonstrate that nutrient uptake follows biphasic kinetics with a highly efficient, active transport system at low external concentrations. Thus, our results lend strong support to Went’s hypothesis: the velamen fulfills an important function in nutrient uptake in the epiphytic habitat. Most of the other functions outlined above still await similar experimental scrutiny.     

亚热带草地中植物优势种与从属种对AM真菌的差异性生长反应

AM真菌能够影响植物生态系统的群落结构.以亚热带草地生态系统为研究对象,调查了两块草地中优势种和从属种的菌根,并在盆栽试验中比较了优势种和从属种对AM真菌的土著菌种和外源菌种Glomus mosseae的生长反应、养分吸收.结果表明,两块草地各自的优势种藿香蓟和两耳草对土著菌种的菌根依赖性分别是41.5%和77.4%,远远高于从属种莎草和毛蓼（16.0%和7.9%）;但是它们对Glomus mosseae的菌根依赖性有所变化,分别是79.6%、44.2%、74.1%和24.9%.这表明,土著菌种是优势种和从属种的形成机制之一,而外源菌种可能改变基于土著菌种而形成的植物群落结构.植株磷营养的分析结果表明,AM真菌对优势种和从属种生长的促进与对磷吸收的促进高度相关,表明AM真菌促进养分吸收是其影响植物群落结构的机制之一.     

Stoichiometry, herbivory and competition for nutrients: simple models based on planktonic ecosystems

Models are examined in which two prey species compete for two nutrient resources, and are preyed upon by a predator that recycles both nutrients. Two factors determine the effective relative supply of the nutrients, hence competitive outcomes: the external nutrient supply ratio, and the relative recycling of the two nutrients within the system. This second factor is governed by predator stoichiometry--its relative requirements for nutrients in its own biomass. A model with nutrient resources that are essential for the competing prey is detailed. Criteria are given to identify the limiting nutrient for a food chain of one competitor with the predator. Increased supply of this limiting nutrient increases predator density and concentration of this nutrient at equilibrium, while decreasing the concentration of a non-limiting nutrient. Changes in supply or recycling of a non-limiting nutrient affect only the concentration of that nutrient. Criteria for the invasion of a second prey competitor are presented. When different nutrients limit growth of the resident prey and the invader, increased supply or recycling of the invader's limiting nutrient assists invasion, while increased supply or recycling of the resident's limiting nutrient hinders invasion. If the same nutrient limits both resident and invader, then changes in supply and recycling have complex effects on invasion, depending on species properties. In a parameterized model of a planktonic ecosystem, green algae and cyanobacteria coexist over a wide range of nitrogen:phosphorus supply ratios, without predators. When the herbivore Daphnia is added, coexistence is eliminated or greatly restricted, and green algae dominate over a wide range of supply conditions, because the effective supply of P is greatly reduced as Daphnia rapidly recycles N.     

A review of the evolution of viviparity in lizards: structure,function and physiology of the placenta

The aim of this review is to collate data relevant to understanding the evolution of viviparity in general, and complex placentae in particular. The wide range of reproductive modes exhibited by lizards provides a solid model system for investigating the evolution of viviparity. Within the lizards are oviparous species, viviparous species that have a very simple placenta and little nutrient uptake from the mother during pregnancy (lecithotrophic viviparity), through a range of species that have intermediate placental complexities and placental nutrient provision, to species that lay microlecithal eggs and most nutrients are provided across the placenta during development (obligate placentotrophy). In its commonest form, lecithotrophic viviparity, some uptake of water, inorganic ions and oxygen occurs from the mother to the embryo during pregnancy. In contrast, the evolution of complex placentae is rare, but has evolved at least five times. Where there is still predominantly a reliance on egg yolk, the omphaloplacenta seems to be paramount in the provision of nutrition to the embryo via histotrophy, whereas the chorioallantoic placenta is more likely involved in gas exchange. Reliance on provision of substantial organic nutrient is correlated with the regional specialisation of the chorioallantoic placenta to form a placentome for nutrient uptake, particularly lipids, and the further development of the gas exchange capabilities of the other parts of the chorioallantois.     

Measuring Nitrogen and Phosphorus Uptake by Intact Roots of Mature Acer saccharum Marsh., Pinus resinosa Ait., and Picea abies (L.) Karst

A common method for measuring uptake by intact roots in situ is the depletion method, wherein intact fine roots are separated from soil and placed in nutrient solution. The difference between initial and final amounts of nutrient in solution is attributed to root uptake. Variations on this method include applying pretreatment solutions, training roots to grow into bags or trays, and varying concentrations of nutrient solution. We tested whether variations in methods affected measured net uptake rates of NH ₄ ⁺ , NO ₃ ⁻ , and PO ₄ ³⁻ . Intact roots of 60 year-old sugar maple (Acer saccharum Marsh.), red pine (Pinus resinosa Ait.), and Norway spruce (Picea abies (L.) Karst.) were given one of four treatments prior to measuring net uptake. “Trained” roots were grown in a sand-soil mixture. “Recovered” roots were excavated and allowed to recover in nutrient solution for two or four days (“two-day recovery” and “four-day recovery”, respectively). “No recovery” roots were excavated and used immediately in experiments. We exposed roots to three concentrations of nutrient solutions to observe the effects of initial nutrient solution concentration. Initial nutrient solution concentration was an important source of variation in measured uptake rates, and N uptake was stimulated by low antecedent concentrations. We found no significant differences in net uptake rates between pretreatments for any of the species studied, indicating that our pretreatments were not effective in improving measurement of uptake. Such pretreatments may not be necessary for measuring net uptake and may not hinder the comparison of rates measured using variations of the depletion method.     

Effects of spatial variation of cells and nutrient and product concentrations coupled with product inhibition on cell growth in a polymer scaffold.

The effects of spatial variation of cells and nutrient and product concentration, in combination with product inhibition in cell growth kinetics on chondrocyte generation in a polymer scaffold, are analyzed. Experimental studies reported previously have demonstrated spatial dependence in the cultivation of chondrocytes. In the present study, the cell-polymer system is assumed to consist of two distinct phases. The cells, fluid, polymer matrix, and extracellular matrix comprise one phase, and the other phase consists of a fluid and polymer matrix. The only two species in the fluid considered to affect cell growth are the nutrient and product. The multiphase transport process of these two species in the cell-polymer system is described by the species continuity equations and corresponding boundary conditions for each individual phase. A volume-averaging approach is utilized for this system to derive averaged species continuity equations for the nutrient and product concentrations. The volume-averaging approach allows for a single species in a two-phase system to be represented by a single averaged continuity equation. Competitive product inhibition, saturation kinetics of substrate, and cell population control are assumed to affect the cell growth kinetics. A modified Contois growth kinetic model is used to represent the three factors that affect cell growth. A parameter analysis is performed and the results are compared qualitatively with experimental data found in the literature.     

Long-term grazing affects relationships between nitrogen form uptake and biomass of alpine meadow plants

Trade-off between nutrient uptake rate and product accumulation has been found among species characterized as acquisitive and conservative strategies in resource utilization. However, long-term grazing causes changes in soil nutrient availability and plant species abundance by selective foraging and resource allocation between above- and belowground organs, which may cover up such trade-off. However, little is known whether the trade-off can be observed among species in community without grazing disturbance, and how grazing influences the trade-off. We conducted a ¹⁵N labelling experiment in winter grazing and grazing release alpine meadow communities on the Tibet Plateau. We examined changes in N form uptake of 11 common species and relationship of N chemical uptake rate with aboveground biomass. Grazing release increased soil \({\text{NH}}_{4}^{ + }\)–N and \({\text{NO}}_{3}^{ - }\)–N, and increased \({\text{NO}}_{3}^{ - }\)–N uptake rate in two species and \({\text{NH}}_{4}^{ + }\)–N uptake rate in three species. Meanwhile, grazing release decreased aboveground biomass of three species and two of them belong to those species’ increased N uptake rate. Contrarily, grazing release increased aboveground biomass of four species and none belongs to the changed N uptake rate. Thus, grazing release caused changes of plant nutrient uptake rate and aboveground production in different directions, which explains the negative relationship of N uptake rate with aboveground biomass in ungrazed community. Our results indicate that the increase in nutrient uptake is probably one of the mechanisms for acquisitive species to cope with the raising nutrient availability and/or competition from the conservative dominant grasses after grazing release.     

ROLE OF MYCORRHIZAL FUNGUS ORIGIN IN GROWTH AND NUTRIENT UPTAKE BY GERANIUM ROBERTIANUM

Prior field studies have shown that populations of forest herbs on relatively nutrient poor soils have higher vesicular-arbuscular mycorrhizal (VAM) infection intensity than plants on rich soils. However, the growth responses and ability to take up P against the soil nutrient gradient are often not linearly related to infection intensity. To determine if intraspecific differences among populations of the common VAM fungus Glomus occultum could differentially affect growth and nutrient uptake, Geranium robertianum seedlings were inoculated with Glomus occultum isolated from four forest types along a gradient of soil fertility, and grown in a greenhouse at P levels typical of the extremes of that gradient. Plants given inoculum from relatively infertile forest sites generally produced greater root, shoot, and total mass than plants given inoculum from fertile sites or uninoculated plants, especially at the low P supply rate. Total P uptake and both P and N uptake efficiency were also highest in plants given inocula from low fertility sites. These results indicate that local adaptation and intraspecific variations in the ability of VAM fungi to induce growth and nutrient uptake effects on host plants may be as important as interspecific differences among VAM fungus species.     

Analysis of energy and friction coefficient fluctuations of a Lennard-Jones liquid coupled to the Nosé–Hoover thermostat

Thermal fluctuations of a many-body system coupled to a Nosé–Hoover thermostat depend on the strength of the coupling parameter τ*. A wrong choice may bring non-ergodic features and non-canonical fluctuations. Here, we analyse by means of molecular dynamics simulations both the energy fluctuations and the spectrum of the friction coefficient ζ^* of an extended Lennard-Jones system in a wide range of τ* at liquid density. Three ranges of τ* are identified – small, intermediate and large, plus their transitions. For τ* values in the intermediate range, ζ^* shows chaotic behaviour, and the particle system requires reasonable computing time for its thermalisation. As a result, the extended system is ergodic and energy fluctuations are canonical and stable in time. On the contrary, small and large ranges of τ* reveal clear evidence of periodicity in the thermostat variable, for instance by propagating the initial temperature condition. For these two ranges, the extended system shows non-ergodic features and energy fluctuations are non-canonical. For large τ*, micro-canonical fluctuations are occasionally obtained. For small to intermediate and intermediate to large ranges of τ*, the thermostat variable exhibits beat waves and is thus unable to reach equilibrium no matter how extended in time the simulations are. Here, we compare our results with previous work and explain the differences.     

Root traits associated with nutrient exploitation following defoliation in three coexisting perennial grasses in a semi-arid savanna

Experiments were conducted to evaluate root traits associated with nutrient exploitation following defoliation in three coexisting perennial grasses in a semi‐arid savanna. Root length density was determined within soil cores directly beneath plants, nitrogen uptake was evaluated by excised‐root assay with (¹⁵NH₄)₂SO₄, and mycorrhizal root colonization was estimated by observation of root segments. Root length density was lowest for Bouteloua curtipendula, intermediate for Eriochloa sericea, and highest for Aristida purpurea indicating that root length density was a more important trait for the mid‐seral than the late‐seral species. Rates of ¹⁵N uptake were greatest in the least grazing tolerant late‐seral species, E. sericea, intermediate in the mid‐seral species, A. purpurea, and lowest in the most grazing tolerant late‐seral species, B. curtipendula. Two successive defoliations reduced ¹⁵N uptake 60% in the late‐seral species with the greatest uptake rate (E. sericea), but not in species with lowest uptake rates (B. curtipendula). Root length colonization was consistently high (33–61%) in all three species suggesting that these C₄ perennial grasses may function as obligate mycotrophs. Contrasting responses among the two late‐seral species indicate that the least grazing tolerant species, E. sericea, appears best adapted for nutrient exploitation while the most grazing tolerant species, B. curtipendula, appears best adapted for efficient nutrient retention. Contrasting responses of nitrogen uptake to short‐term defoliation parallel the population responses of these two coexisting late‐seral species to long‐term herbivory. These data indicate that herbivory may shift interspecific competitive interactions by mediating nutrient exploitation and that a trade‐off may exist between nutrient exploitation and herbivory tolerance in these species.     

Sources of nutrients to rooted submerged macrophytes growing in a nutrient-rich stream

1. The relative contribution of roots and leaves to nutrient uptake by submerged stream macrophytes was tested in experiments where plants were grown in an outdoor flow-channel system. Water was supplied from a nutrient-rich stream with inorganic nitrogen and phosphorus concentrations typical of Danish streams.
2. Four submerged macrophyte species were tested, Elodea canadensis , Callitriche cophocarpa , Ranunculus aquatilis and Potamogeton crispus, and all species were able to satisfy their demand for mineral nutrients by leaf nutrient uptake alone. This was evident from manipulative experiments showing that removal of the roots had no negative impact on the relative growth rate of the plants. Further, the organic N and P concentrations of the plant tissue was constant with time for the de-rooted plants.
3. Enrichment of water and/or sediment had no effect on the relative growth rate of two species, E. canadensis and C. cophocarpa , indicating that in situ nutrient availability was sufficient to cover the needs for growth. Despite the lack of a response in growth rate, a reduced root/shoot biomass ratio was observed with nutrient enrichment of water and/or sediment, and an increased tissue-P concentration in response to open-water enrichment.
4. The open-water nutrient concentrations of the stream in which the experiments were performed are in the upper part of the range found for Danish farmland streams (the majority of Danish streams). Still, however, the negligible effect of nutrient enrichment on the growth of submerged macrophytes observed suggests that mineral nutrient availability might play a minor role in controlling macrophyte growth in most Danish streams.