摘要:

AbstractProgress in plant demography will depend upon being able to synthesize a large body of data and this requires a means of comparing populations between sites and species. We have employed a comparative technique using elasticity analysis of stage‐projection matrices to partition the contributions of fecundity (F), survival (L) and growth (G) to the finite rate of increase λ. Ordination of populations of 77 perennial herbs and trees in G‐L‐F space has shown that species segregate in this triangular space according to their life history and habitat. In the present paper we use the correspondence between demographic parameters and habitat revealed by this method to predict how succession and a variety of environmental factors such as grazing and fire are likely to alter the demography of populations and ultimately to change the composition of commu

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1993.tb00058.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractCompetition between individual plants is defined as a mathematical function for the mean growth rate of individuals in a vertically mono‐layered crowded stand as compared with growth of an isolated individual of the same size. The degree of competitive asymmetry can be described mathematically by using this definition. A canopy photosynthesis model for individual growth in a mono‐layered stand shows that physiological and environmental variations have little effects on the size‐structure dynamics of plant populations undergoing strongly asymmetric competition. These variations affect greatly the size‐structure dynamics under symmetric competition. It has also been shown that one‐sided competition (the most asymmetric competition) is never realised in a mono‐layered stand. A spatial competition model shows that variation in the spatial distribution pattern of individuals greatly affects the size‐structure dynamics of populations undergoing symmetric competition, whilst it has little effect on populations undergoing strongly asymmetric competition. Therefore, it can be concluded that a population undergoing strongly asymmetric competition is a stable system little affected by variations in various factors and that a population undergoing symmetric competition is an unstable system greatly affected by these variations.Based on these theoretical results, the relationships between the mode of competition, species diversity and community stability are discussed and a hypothesis on these processes and mechanisms is proposed for grassland and forest plant communities. One‐sided competition is associated with the stability of community structure, and this type of competition works as a structuring force for plant communities. Symmetric competition is associated with species diversity, and this type of competition cannot be a structuring force of plant communities, i.e. symmetric competition is irrelevant to the deterministic community dynamics. Distinctly multi‐layered forests show “apparent” one‐sided competition between layers (i.e. little direct interactions between canopies in different layers, but higher‐layer trees suppress lower‐layer trees one‐sidedly), bringing about structural stability, but trees in each layer undergo symmetric competition, bringing about species diversity. Mono‐layered grassland shows symmetric competition, bringing about small‐scale spatio‐temporal changes in species' abundance and hence species diversity. The real world lies between the two extreme types of competition, one‐sided and completely symmetric, and each plant community has its own position in this contin

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1993.tb00059.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractData on shoot turnover, shoot size distributions and individual shoot growth in stands of clonal plants are reviewed and compared with shoot density regulation in non‐clonal plants. Shoots are the smallest units capable of independent existence in clonal as well as unitary plants and this comparison allows an investigation of the impact of integration and other factors on clonal shoot dynamics. Physiological integration, the capacity of clonal plants to exchange resources and growth substances between interconnected shoots, has long been held responsible for the ability of clonal plants to prevent shoot overproduction and for the general lack of density‐dependent mortality and self‐thinning. A review of published data leads to the conclusion that the effects of integration on shoot dynamics in clonal plants have been overestimated. Other characteristics such as the density‐dependent control of meristem activity, the synchronization of shoot growth in spring and the maximum shoot size, seem more important for the deviating behaviour of shoot populations of clonal

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1993.tb00060.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe effects of physiological integration on clone behaviour are examined at various structural scales, using data from the stoloniferous herbGlechoma hederacea. The consequences for clone expansion of traumatic fragmentation of the connections between clonal ramets are also illustrated. These results, together with information from other species, are used to refute the commonly‐quoted view that physiological integration between the ramets of clonal herbs evens out the effects of variation in environmental quality, and promotes equitable ramet performance. Instead, clonal species are responsive, at a variety of structural scales, to environmental quality. Therefore, in a heterogeneous habitat, within‐clone variation in the performance of ramets and clonal sub‐structures is to be expected. The seminal study which purports to demonstrate environmental averaging in clonal herbs is shown to be both inadequately designed and inappropriately analysed to accomplish its aim.Physiological and architectural reasons for the local responses to environmental quality seen in the majority of clonal herbs are disc

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1993.tb00061.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractTree gaps may be important habitats for the establishment and spread of understory herbs in eastern (US) deciduous forests. Data from past studies, however, have not shown any clear patterns for either species diversity or increased percent cover. The lack of clear patterns of response to gaps by herbs may be due to evolutionary differences among species or, in part, due to the fact that most studies have been short‐term and long‐term patterns have, therefore, not been observed. In addition, few studies have considered the responses of individual genets and/or ramets. We have been conducting a long‐term study ofCynoglossum virginianum, now in its 16th year, to determine subpopulation and individuals' responses in gap and non‐gap habitats. A larger percentage of individuals flowered in the gap subpopulation shortly after it was created. Seed production rates were also higher in the gap subpopulation. The long‐term benefits gained by the short‐term flowering response have, however, been few as all but two of the seedlings produced within a few years of gap formation have died. In addition, all of individuals that flowered and produced seeds decreased in size and most have remained in the smaller size classes. In addition, only one of the individuals in the gap subpopulation has flowered since the initial flowering response while plants in non‐gap subpopulations have flowered and produced seeds throughout the study period. Gaps may only be beneficial toC. virginianumif seeds are dispersed into non‐gap areas or if seeds can remain dormant in the soil until conditions are more suitable for se

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1993.tb00062.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractWe use a combined model of forest canopy dynamics and patch‐specific demography to examine the response of a forest understory herb,Cynoglossum virginianum, to changes in rates of forest disturbance, rates of long distance seed dispersal, and patterns of seed dormancy. The matrix modelling approach followed that of Horvitz and Schemske (1986), in which empirically‐determined demographic parameters of a population experiencing open canopy conditions are modified depending upon demographic decay functions describing plant demography in a series of patch‐types undergoing succession toward fully closed canopy. In our analysis, demographic transition elements were estimated from 14 years of long‐term census data on three subpopulations at the Smithsonian Environmental Research Center, Anne Arundel County, Maryland. One of the three subpopulations was subjected to full canopy exposure in the second year of data collection, while the other two subpopulations remained in full understory conditions for the entire census period. Our results suggest small negative effects of a doubling of the rate of canopy disturbance (defined as the rate at which new treefall gaps appear in closed canopy) and only small positive effects of long distance dispersal on population growth rate, which predicts tittle selection for long distance dispersal. However, our analyses predict positive benefits of enhanced seed dormancy in this species, particularly if dormancy is higher in new gaps relative to understory conditions. These results are primarily a product of enhanced reproductive output in the years immediately following gap opening which is counterbalanced by increased seedling mortality in those years and a near complete suppression of reproductive output in later years (closing gaps). The suppression of reproduction in closing gaps may be a manifestation of a delayed effect of high costs of reproduction imposed in prior years. We discuss our results in terms of observed patterns of seed dispersal and in terms of the overall potential influence of forest gap dynamics on the demography of understory herbs similar toC. virginianum, in which demographic responses to changes in canopy conditions differ in strength and direction among life history

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1993.tb00063.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractCompetition among relatives can modify the genetic structure of plant populations; in turn, competitive outcomes can depend on the genetic relatedness of the individuals competing. The offspring from individual parents exhibit a continuum of genetic relatedness, depending on parental reproductive systems. Competition among relatives may have evolutionary significance as a selection pressure; sibling competition, for example, has been invoked to explain the evolution of sexual systems, seed packaging within fruits, seed dispersal dimorphisms, and germination behavior.Density‐dependent fitness consequences of sibling competition have been documented in a population of the annual grassSporobolus vaginiflorus. This species produces seeds matured in cleistogamous spikelets within leaf sheaths along tillers in autumn: when seeds along the tillers of a maternal parent (i.e. a sibship) germinatein situthe following spring in close proximity to one another, sibling competition results in a high‐density zone centered around the original senescent parent. Both intra‐ and intersibship interactions can occur within a population. Although fitness is much reduced for siblings inside the zone of competition, potential seed rain and net primary productivity per unit area are significantly higher relative to outside the zone. This annual is functionally analogous to a perennial ramet producer with a phalanx growth strategy and the unit of selection may be the sibling group. It is not yet known whether sibling competition is a significant selection pressure in other species, but indirect evidence suggests it may be relatively wides

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1993.tb00064.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractSurveys of mating‐system parameters in populations of the annual, self‐compatible, tristylous, emergent aquatic, Eichhornia paniculata (Pontederiaceae) from N.E. Brazil and Jamaica have indicated that the species exhibits a wide range of outcrossing rates. To investigate whether temporal variation in outcrossing rate was also a feature of populations, open‐pollinated families were sampled from five populations of contrasting style morph structure from N.E. Brazil over three consecutive years (1987–1989). Multilocus estimates of outcrossing rate (t) were obtained from assays of isozyme polymorphisms using starch gel electrophoresis. There was significant variation both among populations and between years in the frequency of outcrossing. Outcrossing in three tristylous populations was high (t>0.80), with relatively small fluctuations occurring over the three‐year sampling period. In contrast, in a dimorphic and monomorphic population considerable self‐fertilization occurred and the frequency of outcrossing declined significantly from 1987 to 1989 in both populations. In the dimorphic population, increased selfing was associated with a marked reduction in population size and an increase in the frequency of selfing variants of the mid‐styled morph. The significance of temporal variation in outcrossing frequency in plant populations is discussed in relation to its effect on population genetic structure and recent models of mating‐s

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1993.tb00065.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe flux of individuals within populations is dependent upon six controlling processes: the intrinsic rate of increase of the plant, intraspecific competition for resources, interspecific competition, natural enemies, mutualisms and refuge effects such as the immigration of seeds from other populations. Although population interactions are generally believed to play a major role in determining the flux of individuals within populations, little attempt has been made to quantify the strength of these interactions and their role in the dynamics of populations. In this paper we examine the role of competition, herbivory and mutualistic interactions in determining the dynamics of a range of annual plant species. Firstly, it is shown that the dynamics of three weed species (Bromus sterilis, Galium aparine, Papaver rhoeas) in an experimental community in an arable cropping system of winter wheat are determined primarily by the rapid population growth ofB. sterilis. Interactions between the species play a minor part in the dynamics of the system. Secondly, it is shown that current levels of grazing by overwintering populations of brent geese have a minor impact on the abundance ofSalicornia europaea, but that increased grazing has the potential to reduce abundance and increase the instability ofS. europaeapopulations. This is a consequence of the aggregative response of the geese, which results in an increasing proportion of the seeds ofS. europaeabeing eaten as plant density increases. Thirdly, it is shown that there is a complex interaction between root pathogenic and arbuscular mycorrhizal fungi in natural field populations ofVulpia ciliataand that the benefit of mycorrhizal fungi to the plant is in providing protection against pathogens.

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1993.tb00066.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractWe report preliminary results of a series of experiments designed to explore the importance of interspecific competition within arable weed communities at different scales. Competition hierarchies were apparent from a pot experiment with different levels of nutrients and water. Two field experiments looked atBromus sterilis, Galium aparineandPapaver rhoeasin winter wheat in the field, in a range of combinations and management treatments, and a fourth field experiment included a wider variety of species. There was little effect of fertilizer on population behaviour in the the field.Bromusincreased around ten fold per year on minimum‐tilled plots, regardless of other treatments.Galiumincreased on organically‐fertilized and minimum‐tilled plots, but only in the absence ofBromus. Papaverdensities remained low, but again were depressed in the presence of high densities ofBromus. Taken together, the experiments demonstrate the existence of competition between weed species. However, as the design of the experiment increased to include greater levels of environmental variation, so competition became more difficult to detect, and less useful for interpreting the results than knowledge of the biology of the individual species. At the scale of interest to the farmer, the level of competition is not a good predictor for weed population dyn

ISSN:0913-557X    

DOI:10.1111/j.1442-1984.1993.tb00067.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY