The interaction of density and resource levels in monospecific stands of plants: a review of hypotheses and evidence

The series of asymptotic yield-density curves traversed by even-aged monospecific stands of plants at the same resource level is described by the function f(w_max y _max), where w_max= spaced plant weight and y_max= maximum yield possible at high densities. If the resource level at which the stands are grown is reduced, two sets of outcomes are possible. The stands at the decreased resource level may show essentially the same crowding effects as the control stands, but at a slower rate (altered-speed hypothesis). This hypothesis implies a common family of yield-density curves and a common thinning line for stands at different resource level may occur not only at a different rate at a harvest in the stands of higher growth. Alternatively, crowding effects at a reduced resource level may not only occur at a different rate to the controls but take a different course, leading to more intense crowding effects at a harvest being observed in the stands of lower growth (altered-form hypothesis). Thus a different family of yield-density curves and different thinning lines would apply at each resource level. For yield-density curves, the altered-speed hypothesis applied if f(w_max, y_max) is the same at each resource level. If this function differs with resource level, the altered-form hypothesis applies at a harvest provided the ratio y_max/w_max is less at the low resource level. Cases intermediate between these two hypotheses may occur. For thinning lines differences in the position of the non-thinning/self-thinning boundary and in f(w_max, y_max) can affect the intercept and slope of thinning lines. The theoretical magnitude of these differences are discussed. Examples of both hypotheses are reviewed. For the altered form cases differences in non-thinning crowding effects between resource levels are capable of causing significant differences in thinning lines.     

Yield-density Relationships: the Influence of Resource Availability on Growth and Self-thinning in Populations of Vulpia fasciculata

Monocultures of Vulpia fasciculata were grown over a wide rangeof densities to investigate the influence of crowding and nutrientsupply on growth and self-thinning. For a given time and densityseries the relationship between mean yield per plant (w) andthe density of survivors (N) could be described by the equation w= w_m (1+aN)^–b. where w_m is the yield of an isolated plant, a is the area requiredto achieve a yield of w_m and b describes the effectiveness withwhich resources are taken up from the area. All three parametersincreased with time. Adding nutrients changed not only the rate at which the effectsof crowding occurred but also the intensity of crowding since w_m = C(a^b)^D. where C and D are constants. The addition of nutrients resultedin an increase in the value of C. Such an increase means thata larger weight can be supported by a given area because theresources within that area are greater. During the early phases of growth, populations of V. fasciculataconformed to the –3/2 power law, w = cN^–3/2, butonly at very high densities with a plentiful supply of nutrients.However, once the maximum standing crop had been reached thetrajectory of the thinning line switched to a slope of justless than –1 when weight was ploted against density onlogarithmic scales. The intercept of the –3/2 thinningline was considerably higher (log c = 5.74) than those for mosttrees and forbs but was similar to those of a number of othergrasses. Vulpia fasciculata, dune fescue, yield-density models, self-thinning, density-dependence, nutrient supply     

Biomass and photosynthetic productivity of water hyacinth (Eichhornia crassipes) as affected by nutrient supply and mirid (Eccritotarus catarinensis) biocontrol

The biological control of water hyacinth is affected by water nitrogen and phosphorus content and this was investigated experimentally at five levels of nutrient supply by measuring plant photosynthetic and growth responses, and mirid reproduction and herbivory of nutrient treated plants. Low nitrogen (2–0.2 mg L⁻¹) and phosphorus (0.2–0.01 mg L⁻¹) supply decreased hyacinth photosynthesis, growth and biomass accumulation relative to plants supplied 200 mg L⁻¹ N and 20 mg L⁻¹ P. This effect depended more on nitrogen supply than phosphorus supply. Chlorophyll fluorescence showed that the photosynthetic light reactions of low nutrient plants were affected and leaves had decreased chlorophyll content, density of functional photosystems II and dissipated a greater proportion of absorbed energy as heat. Gas exchange parameters showed reduced carboxylation efficiency, rates of RuBP regeneration and light saturated photosynthetic rates, but not quantum yields. Effects on photosynthesis translated into lower plant dry biomass. Mirid herbivory exacerbated the effects of low nutrients noted for chlorophyll fluorescence, gas exchange parameters and biomass accumulation, however, these effects were not always significant and there was no obvious correlation between the level of nutrients supplied and the effect of mirid herbivory. Low nutrient supply did, however, affect mirid performance reducing the number of adult insects, nymphs and herbivory intensity suggesting that in the long-term mirid populations would be significantly affected by water nutrient status.     

Supply and uptake of potassium,calcium and magnesium of spray carnations (Dianthus caryophyllus) grown in rockwool

Summary In two experiments with carnations grown in rockwool the effects of different cation ratios in the nutrient solution were studied. The results showed that carnations need a high calcium supply. The crop did not appear to be sensitive to different potassium-magnesium ratios in the nutrient solution. In the nutrient solution added mole ratios K∶Ca∶Mg=55∶35∶10 seemed to be optimal. Such ratios in addition led to ratios of 55∶30∶15 in the root environment. Tissue analysis showed that in younger leaves of peduncles harvested a potassium content of 900 mmol per kg dry matter was optimal. For calcium a content of 350 and for magnesium 100–150 mmol per kg was needed. Analytical data of plant-sap analyses were closely correlated with data gained by digestion of dried material. For potassium and magnesium the relationships were linear. However, for calcium a curvilinear relationship was found. In the experiments indications were obtained that a sufficient calcium supply suppressed foot rot in carnations.     

Local structural changes in tropomyosin detected by a trypsin-probe method

Structural changes in tropomyosin from rabbit skeletal muscle were studied by the tryptic digestion method, which is an application of the quantitative enzyme-probe method recently developed by Ueno and Harrington [Ueno, H., & Harrington, W.F. (1984) J. Mol. Biol. 173, 35-61]. Effects of ionic strength, temperature, and an interchain disulfide bond at Cys-190 on the structure of tropomyosin were examined. A region of high susceptibility to trypsin was found to be localized in the middle portion of the molecule, and its susceptibility increased on lowering ionic strength and/or raising temperature. With the introduction of a disulfide bond at Cys-190, cleavage on the N-terminal side of Cys-190 was accelerated. The results suggest that skeletal muscle tropomyosin is flexible in the middle of the molecule in contrast to the flanking N- and C-terminal trypsin-resistant segments.     

Effects of nutrient enhancement on the fecundity of a coral reef macroalga

On coral reefs, there is concern that increased nutrient supply (e.g. due to eutrophication) causes increased algal growth and hence increased algal abundance, in turn causing colonisation and invasions of coral populations, resulting in reef degradation, or a coral-algal phase shift. For example, species of Sargassum, a highly seasonal, large, brown seaweed, are suggested to be colonising corals on inshore coral reefs of the Great Barrier Reef, as a result of anthropogenic increases in terrestrial runoff of sediments and nutrients. However, implicit in this argument is the assumption that nutrient-related increases in growth will lead to increased fecundity (and/or propagule success), since without such changes, increased abundance can only occur by vegetative means. Whilst plausible, there is no experimental evidence for this assumption in coral reef algae.We here present an initial study in which experimental increases in nutrient supply apparently did not lead to increased fecundity of Sargassum siliquosum; rather, density and biomass of receptacles were reduced in nutrient-enhanced algae. There was little effect of nutrient treatments on the proportional allocation of biomass to reproductive and vegetative structures: nutrient enhancement led to similar decreases in biomass of reproductive and vegetative tissue. Tissue nutrient levels indicated effective enhancement of nitrogen supply, although phosphorus levels were not different across nutrient treatments.The reduced fecundity with increased nutrients may reflect either a genuine inhibition by higher nutrient levels, as found in previous studies, or accelerated maturation, causing increased tissue losses due to more advanced seasonal senescence. Either way, this exploratory experiment provides initial evidence that nutrient effects on tropical coral reef macroalgae may be complex, and does not support the assumption that increased nutrient supply will result in a numeric increase in populations of Sargassum spp. Our results should not be taken as an unequivocal demonstration that nutrients inhibit fecundity overall, but illustrate the need to distinguish between effects on different life-history processes (e.g. growth and reproduction). For increased growth of pre-existing individuals to contribute to algal invasions and phase shifts, that growth must result in either increased fecundity, or increased propagule success.     

Growth of cells on solid culture medium

Summary For precise experiments with yeast (or other) cells stationary populations are produced by growth on the surface of a solid nutrient medium. The energy supply to these cells is well known from a former publication. The oxygen supply during growth is analysed here in detail. Different types of cell populations can be produced in this way dependent on the thickness of nutrient medium.If such cells are transferred into a liquid buffer solution cell multiplication can be initiated without any nutrient flux into the cell. This new type of initiation of the cell cycle of G₁-cells has to be distinguished from the usual initiation by nutrient supply and from the mechanism of meiotic cell division. The dependence of this cell growth on cell volume, pH-value, oxygen concentration and osmotic pressure is analysed and possibilities to avoid this kind of cell multiplication reaction are discussed.     

Is the negative effect of crowding on ingestion rate in <Emphasis Type="Italic">Daphnia pulex</Emphasis> induced physically or chemically?

The effect of physical interference and accumulation of auto-metabolites due to crowding on ingestion rates in Daphnia pulex were examined under an excess food supply. Ingestion rates of D. pulex decreased to 40–80% with an increasing number of animals in an experimental container irrespective of the volume of the container. Conditioned water from the crowded culture of D. pulex did not affect the ingestion rate. These results suggest that physical interference may play a more influential role in the detection of crowding rather than the accumulation of auto-metabolites. Since the daphniids did not visually respond to crowding, the effect of crowding on daphniid feeding shown in this study is considered to be caused by some physical interaction, e.g. physical contact between neighbouring individuals or pulses generated by the beating of thoracic limbs of other individuals.     

Density-dependent natural selection in Drosophila: Adaptation to adult crowding

The effects of adult crowding on two components of fitness were studied in three sets of Drosophila melanogaster populations, subjected to life-stage-specific, density-dependent natural selection in the laboratory for over 50 generations. Three days of crowding, early in adult life, were observed to increase mortality significantly during the episode of crowding, as well as decrease subsequent fecundity. Populations selected for adaptation to high adult densities suffered significantly lower mortality during episodes of adult crowding, as compared to populations selected specifically for adaptation to larval crowding, as well as control populations typically maintained at low larval and adult densities. Moreover, populations adapted to larval crowding seemed to be adversely affected by adult crowding to a greater extent than the controls, raising the possibility of trade-offs between adaptations to larval and adult crowding, respectively. Preliminary evidence suggests that the populations adapted to adult crowding may have evolved a propensity to stay away from the food medium, which is where most deaths occur when adults are crowded in culture vials. This revised version was published online in July 2006 with corrections to the Cover Date.     

Some effects of density on the juveniles of two species of tropical,territorial damselfish

Small juveniles of Pomacentrus wardi Whitley and P. flavicauda Whitley were transplanted onto replicate reefs, at various densities, to assess the importance of intraspecific and interspecific competition among fishes of the same size, age, and experience. Although P. flavicauda disappeared from the reefs faster than did P. wardi, survival of neither species was affected by the density of conspecifics or the presence of its congener (during a 1-yr period). Thus densities at the end of the experiment reflected the initial stocking densities which simulated a range of recruitment densities exceeding those observed in wild populations. Crowding caused both species to change their intraspecific behaviours from territorialism to participation in dominance hierarchies. For P. wardi, this was correlated with unequal growth among the individuals on each patch, which partially disguised a trend for the average size of all fish to decrease with increasing density. Over the range of crowding and the time scales explored, total biomass ( = production) did not reach an asymptotic value which might have indicated an absolute shortage of food for these small fishes. Variations in the survival and biomass of P. wardi among replicate reefs were not correlated with the distribution and abundance of any other species also resident on these reefs. The highest densities of damselfishes used in this experiment were more than 50 times the average year-class strength observed at this site. Yet proportional losses from the experimental and natural cohorts were not distinguishable. The lack of compensatory mortality over this range of densities suggests that competition within a year-class does not control the density of young fish in wild populations. This result is consistent with other evidence that recruit densities are extrinsically controlled by a limiting supply of pelagic larvae.     

Between- and within-population differences in Phragmites australis

Phragmites australis (common reed) is a dominant clonal species in the interface between land and water in many European wetlands. Along the land-water gradient, strong consistently different selective forces might operate to give rise to genetic substructuring. I have investigated the occurrence of genetic substructuring in European P. australis populations. The present paper examines whether seedlings, from seeds collected at both ends of the land-water gradient, showed differences in response to nutrient supply. Under controlled conditions, the relative growth rate (RGR) in the exponential growth phase, and growth characters of 10-week old seedlings were assessed. Among populations, no differences in response to nutrient supply were found. Although total dry weight was not related to the geographic origin of the populations, northern/western compared to southern/eastern European populations (1) formed more but shorter shoots, (2) formed thinner but longer rhizomes, and (3) invested more dry matter in leaves at the expense of stems. It was concluded that these trait differences are likely to originate from differences in the length of the growing season in the native habitat. Within populations, ’water-side’ seedlings had a higher RGR under sub-optimal while for ’land-side’ seedlings this was under optimal nutrient conditions. Ten-week-old ’water-side’ seedlings had a higher total dry weight than ’land-side’ ones, irrespective of nutrient loading. Differences in growth could not clearly be related to differences in single biomass allocation and morphological traits. A discriminant analysis on these traits, however, revealed that ’water-side’ seedlings showed higher plasticity in discriminant scores than ’land-side’ seedlings in response to nutrient supply. Discriminant scores also pointed towards a subtle trade-off between height versus expansion growth of seedlings, from the water to landward side. In the Romanian population, this could be related to morphological differences between ploidy levels. Overall, it was concluded that within populations, selection on growth form rather than on adaptations to the nutrient status of the habitat might have taken place. Received: 20 August 1998 / Accepted: 29 July 1999     

Food web structure provides biotic resistance against plankton invasion attempts

It is generally accepted that native communities provide resistance against invaders through biotic interactions. However, much remains uncertain about the types of ecological processes and community attributes that contribute to biotic resistance. We used experimental mesocosms to examine how zooplankton community structure, invertebrate predation, and nutrient supply jointly affected the establishment of the exotic Daphnia lumholtzi. We predicted that establishment would increase with declining biomass and diversity of native zooplankton communities and that an invertebrate predator (IP) would indirectly facilitate the establishment of D. lumholtzi due to its relatively long predator-deterring spines. Furthermore, we hypothesized that elevated nutrient supply would increase algal food availability and facilitate establishment. Only when the biomass and diversity of native zooplankton were significantly reduced, was D.␣lumholtzi able to successfully invade mesocosms. Although invertebrate predation and resource supply modified attributes of native zooplankton communities, they did not influence the establishment of D. lumholtzi. Overall, our␣results are consistent with observed population dynamics in invaded reservoirs where D.␣lumholtzi tends to be present only during the late summer, coinciding with historic mid-summer declines in native zooplankton populations. Lakes and reservoirs may be more susceptible to invasion not only by D. lumholtzi, but also by other planktonic species, in the late summer when native communities exhibit characteristics associated with lower levels of biotic resistance.     

Differences in performance of Aedes aegypti larvae raised at different densities in tires and ovitraps under field conditions in Argentina.

Alteration of fitness components was assessed in the yellow fever mosquito, Aedes aegypti, in automobile tires and vases (ovitraps) under field conditions. Larval numbers were manipulated in both kinds of containers to compare low, high, and control (natural) densities. Densities were set from a census of a wild population, then doubling and reducing to half the mean crowding, m*. Artificially altered densities were not high or low enough to produce differences among treatments. Tires generated more vigorous larval populations and females with higher fecundity than did small containers, although the mortality was more intense.     

增加水分与养分对克隆植物羊柴自然种群繁殖权衡的影响

 羊柴(Hedysarum laeve)是豆科多年生半灌木,在自然条件下可以同时进行有性繁殖和克隆繁殖。该文在野外条件下研究了不同水平的水分和养 分处理对羊柴种群的繁殖权衡的影响。结果表明,与对照相比,增加一定量的水分处理显著减少了花和荚果的生物量;显著增加了克隆分株枝 的生物量,显著减少了分株根茎的生物量, 但没有影响其它部分的生物量。增加一定量的水分会抑制有性繁殖,改变生物量对克隆繁殖分株各 部分的分配比例。与对照相比,增加一定量的养分能够促进有性繁殖,抑制克隆繁殖。     

Genotype-environment interactions in determining fitness in dense,artificial populations of Phlox drummondii

Summary Survivorship and growth of individuals of the annual plant Phlox drummondii were examined in artificial, density-stressed populations composed of five genetically differentiated varieties sown in monocultures and in five pairwise mixtures. All experiments were replicated under two treatments of nutrient availability.Varieties differed significantly in competitive ability, and relative competitive success was shown to be highly habitat (treatment) dependent. In 19 of 20 mixed populations, the relative rankings of pairs of competitors were consistent with a priori expectations based on the morphological and/or historical differences between seed sources, but were not predictable from relative tolerances to density stress in monoculture.Estimated selection coefficients based on relative competitive abilities ranged from 0.04 to 0.94. Despite these large fitness differences among competing varieties, most of the variance in the absolute fitness of individuals resided within varieties. This variation may be mostly environmental.In the high nutrient treatment, where competition for light was most intense, variation in individual plant size was lower in mixed culture as compared to monocultures. Genetic variation for competitive ability may actually lead to greater size uniformity among survivors by promoting an early initiation of thinning, thereby reducing the density stress on survivors.     

Effects of copper and mineral nutrition on growth,copper accumulation and mineral element uptake in two Rumex japonicus populations from a copper mine and an uncontaminated field sites

Two Rumex japonicus populations, one from a copper mine and the other from an uncontaminated site, were studied in hydroponic experiments for the plant growth, copper accumulation and mineral nutrient content under excess copper and nutrient deficiency conditions. The tolerance indices of the contaminated population were significantly higher than that of the uncontaminated population, indicating the evolution of Cu resistance in the former. At control and low Cu treatment, there was no difference in Cu accumulation in roots between the two populations. At high Cu (100 μM) treatment, however, the contaminated population accumulated less Cu in roots than the uncontaminated one, suggesting the root exclusion mechanism existing in the former. The contaminated population was also more tolerant to general nutrient deficiency than the uncontaminated one. The results indicated that the contaminated population had evolved not only Cu-tolerance but also tolerance to low nutrient supply. Under Cu stress, the contaminated population had less change in nutrient composition than the uncontaminated one. The similar result was observed in general nutrient deficiency experiment. The results indicated that the mineral composition homeostasis under the stresses was important in metal tolerance and colonizing the Cu-enriched soils for the Cu-tolerant population. At high Cu (100 μM) treatment and general nutrient deficiency treatment, the contaminated population accumulated significantly lower copper and higher phosphorus in both roots and shoots than the uncontaminated one. This was not the case for other mineral nutrients Ca, Mg and Fe except for root Mg and root Fe at Cu treatment. The result suggested that the high Ca-metabolism in R. japonicus was uncorrelated with high Cu-tolerance and that P might play an important role in governing Cu bioaccumulation.     

Ektodesmenstudien

Summary Lambertz (1954) did not mention Gramineae as plants containing ectodesmata, although he stated that ectodesmata are not bound to special groups of angiosperms but should occur in all families of angiosperms. Schnepe (1959) expressly pointed out that he never succeeded in demonstrating such structures in leaves of Gramineae with his method. In our studies, however, ectodesmata could be shown in leaves of wheat and maize if the fixation mixture contained nitric acid. As in other objects, including species of gymnosperms and pteridophytes, the same distribution of ectodesmata could be observed which was characterized by the formation of rows along the anticlinal walls, crowding in guard-cells and seattering in periclincal walls.     

Competition between plant populations with different rooting depths II. Pot experiments

Summary In a previous paper in this series a model was proposed lor the competition between plant populations with different rooting depths. This model predicts that in mixtures of plant populations with different rooting depths the Relative Yield Total will exceed unity. Secondly it predicts that in these mixtures the relative crowding coefficient of the deep rooting population with respect to the shallow rooting one will decrease with increasing plant frequency. Two competition experiments in deep pots are carried out to test these predictions. In the first experiment a comparison is made between a series in which nutrients limited plant growth severely and a series in which nutrients were applied in a quantity that allowed high productivity. In the nutrient poor series the Relative Yield Total exceeded unity by about twenty per cent, while in the nutrient rich series only a small deviation from one was observed. The second experiment was divided into a series of deep pots in which different rooting depths were possible and a series of shallow pots in which the two species were forced to share the same space. In the series of shallow pots the relative crowding coefficient of the two species was found to be independent of plant frequency. In the series of deep pots the relative corowding coefficient of the two species with respect to each other did decrease significantly with increasing frequency.The observed frequency-dependence for the shallow rooting species could be explained by an extension of the theory presented previously. In the spacing series used in the same experiment a decrease of the shoot to root ratio with increasing plant density was observed. Furthermore, in the replacement series the shoot to root ratio in mixture was found to be lower than in monoculture. The consequences of these phenomena for the estimation of the relative crowding coefficient and the Relative Yield Total on the basis of shoot weights are discussed. However, it is argued that the fact that shoot to root ratios change with plant frequency does not provide an alternative explanation for the frequency-dependence observed.Grassland Research Group Publication No. 33     

The nature value of the ditch vegetation in peat areas in relation to farm management

Farmers can play an important role in enhancing the diversity of plant species in ditches, since they carry out most of the management. Little is known about the measures farmers can take to enhance aquatic biodiversity in ditches. Therefore, a field study was set up to determine how dredging, ditch cleaning, and nutrient supply in the adjacent fields affect the floristic values of the ditch vegetation in peat areas. A total of 240 ditches on 84 dairy farms were selected, to establish the effect of ditch cleaning, dredging, and nutrient supply. Generalised linear modelling was used to determine the most relevant factors, and to obtain quantitative relations between those factors and two indices of floristic value. Ditch management, field management, and water management were found to affect the floristic values of the ditch vegetation. Increasing the water depth proved to be the most cost-effective measure. Further improvements can be obtained by choosing certain machines or specific management periods, or reducing the N supply. The effect of the P supply is not yet sufficiently clear. The water boards can enhance floristic values by raising the ditch water table and by lowering the Cl^– concentration in the ditches.     

Spatio-temporal modeling of the crowding conditions and metabolic variability in microbial communities

The metabolic capabilities of the species and the local environment shape the microbial interactions in a community either through the exchange of metabolic products or the competition for the resources. Cells are often arranged in close proximity to each other, creating a crowded environment that unevenly reduce the diffusion of nutrients. Herein, we investigated how the crowding conditions and metabolic variability among cells shape the dynamics of microbial communities. For this, we developed CROMICS, a spatio-temporal framework that combines techniques such as individual-based modeling, scaled particle theory, and thermodynamic flux analysis to explicitly incorporate the cell metabolism and the impact of the presence of macromolecular components on the nutrients diffusion. This framework was used to study two archetypical microbial communities (i) Escherichia coli and Salmonella enterica that cooperate with each other by exchanging metabolites, and (ii) two E. coli with different production level of extracellular polymeric substances (EPS) that compete for the same nutrients. In the mutualistic community, our results demonstrate that crowding enhanced the fitness of cooperative mutants by reducing the leakage of metabolites from the region where they are produced, avoiding the resource competition with non-cooperative cells. Moreover, we also show that E. coli EPS-secreting mutants won the competition against the non-secreting cells by creating less dense structures (i.e. increasing the spacing among the cells) that allow mutants to expand and reach regions closer to the nutrient supply point. A modest enhancement of the relative fitness of EPS-secreting cells over the non-secreting ones were found when the crowding effect was taken into account in the simulations. The emergence of cell-cell interactions and the intracellular conflicts arising from the trade-off between growth and the secretion of metabolites or EPS could provide a local competitive advantage to one species, either by supplying more cross-feeding metabolites or by creating a less dense neighborhood.