Analyses of spatial patterns and population processes of clonal plants

The nonrandom spatial structure of terrestrial plants is formed by ecological interactions and reproduction with a limited dispersal range, and in turn this may strongly affect population dynamics and population genetics. The traditional method of modelling in population ecology is either to neglect spatial pattern (e.g. in transition matrix models) or to do straightforward computer simulation. We review here three analytical mothods to deal with plant populations in a lattice-structured habitat, which propagate both by seeds that scatter over the whole habitat and by vegetative reproduction (producing runners, rhizomes, etc.) to neighboring vacant sites. [1]Dynamics of global and local densities: Dynamical equations of population density considering nearest-neighbor correlation (spatial clumping) are developed as the joint dynamics of global average density and local density (comparable to mean crowding) based onpair approximation. If there is a linear trade-off between seed production and vegetative reproduction, the equilibrium abundance of the population may be maximized by engaging both means of reproduction. This result is accurately predicted by the pair approximation method, but not by mean-field approximation (neglect of spatial structure). [2]Cluster size distributions: Using global and local densities obtained by pair approximation, we predicted cluster size distribution, i.e. the number of clusters of occupied sites of various sizes. [3]Clonal identity probability decreasing with distance: Multi-locus measurement of allozymes or other neutral molecular markers tells us whether or not a given pair of individuals belong to the same clone. From the pattern of clonal identity probability decreasing with the distance between ramets, we can estimate the relative importance of two modes of reproduction: vegetative propagation and sexual seed production.     

Optimal resource allocation to seeds and vegetative propagules under density-dependent regulation in Syneilesis palmata (Compositae)

Syneilesis palmata reproduces by both seeds and vegetative propagules (short rhizomes). The latter result in the production of new plants that are larger in size and hence have a higher survival probability and a higher growth rate than seeds. A previous study predicted that the optimal reproductive strategy, in terms of maximizing population growth rate (a fitness measure under no density regulations), was pure vegetative reproduction. However, high resource investment to vegetative propagules can cause local crowding resulting in reduced demographic performances of the plants, because the vegetative propagules of Syneilesis are produced close to one another. We examined, in this situation, the impact of allocating a certain proportion of reproductive resource to seeds with relatively greater capacity for dispersal. We simulated dynamics of hypothetical Syneilesis populations with various reproductive resource allocation balances (from pure seed to pure vegetative reproduction), using a density-dependent matrix model. In the model, it was assumed that plants from vegetative propagules experienced density-dependent reduction in their survival probabilities, but this was not the case for plants originating from seeds. Each allocation strategy was evaluated based on an equilibrium population density, a fitness measure under density-dependent regulations. The optimal reproductive strategy predicted was pure vegetative reproduction. Unrealistic conditions were required for seed reproduction to be favoured, such as the production of seeds one hundred times the normal number per unit resource investment. However, the conditions were fairly relaxed compared with those required in the model where no density effects were incorporated. This indicates that escape from local crowding is likely to be one of the roles of seed production in Syneilesis.     

Evaluation of the population dynamics of the forage legume (Lotus corniculatus), using matrix population models

The population dynamics of perennial crop plants are influenced by numerous factors, including management practices. Conditions in the field vary from year to year, and matrix population models are useful for evaluating population behaviour in relation to environmental variability. In Missouri, the stand persistence of birdsfoot trefoil ( Lotus corniculatus ), a perennial legume, is often limited by disease and poor seed production. A stage-based, matrix population model was developed to evaluate the population dynamics of birdsfoot trefoil in relation to clipping treatment. The plant growth stages represented in the model were seeds, seedlings, mature vegetative and reproductive plants. Two phases of population growth were evaluated in clipped and unclipped stands. Establishment-phase populations were characterized by relatively high mortality and low reproduction. Elasticity analysis indicated that growth of these populations was most sensitive to the survival of vegetative plants. Mature vegetative plants and seeds comprised the majority of surviving individuals in clipped and unclipped populations, respectively; however, establishment-phase populations under both management treatments tended toward extinction. Populations in the post-establishment phase of growth were characterized by relatively low mortality and high reproduction. Population growth in this phase of growth was most sensitive to seed production, and most individuals in these populations were at the seed stage.     

The role of sexual and vegetative reproduction in the population maintenance of a monocarpic perennial herb,Cardiocrinum cordatum var. glehnii

Because monocarpic perennial plants have only one reproductive opportunity in their entire life, they need to ensure offspring production. Some plants reproduce both sexually and vegetatively, and vegetative reproduction could possibly compensate for seed production. Therefore, the role and significance of these reproductive modes is likely to differ between monocarps and polycarps, which can reproduce many times. Cardiocrinum cordatum var. glehnii is a monocarpic perennial that reproduces both sexually and vegetatively (bulblet formation). Here, we investigated the characteristics and contribution to population maintenance of sexual and vegetative reproduction to reveal the significance of these two reproductive modes in this species. First, we found that bulblet formation occurred in plants after the three‐leaved rosette stage. Second, resource allocation experiments revealed that although resources were mainly invested in fruit maturation after the flowering season, resource allocation was switched from sexual reproduction to vegetative reproduction if seed production was insufficient. Third, the outcrossing rate in this species varied greatly according to the environment surrounding the population. However, reproductive assurance by selfing kept seed production stable even if flowers did not receive sufficient pollen for full seed set via outcross pollination, and moreover, there was no intensive inbreeding depression. Finally, genotypic identification of ramets suggested that daughter ramets derived from vegetative reproduction received the space that the mother flowering ramet had occupied until the previous year.     

Evolutionarily stable allocation to vegetative and sexual reproduction in plants

I apply the resource allocation approach to construct an ESS model aimed at predicting the rate of vegetative and sexual reproduction in plants. The model provides an integrated explanation of why the mode of reproduction should vary between populations and species, and what factors should affect the trade-off between ramet and seed production. It follows that seed production, usually considered to be less effective and more costly than ramet production, may be maintained in the population. The production of ramets should be favored when their recruitment rate is high or the costs are low. The intensity of vegetative reproduction is also related to conditions which affect seed production directly. The rate of ramet production should be inversely correlated with factors favoring sexual reproduction, such as lower cost of seeds or high opportunity for colonization of new patches, etc. Under such conditions it is beneficial to allocate more resources available for reproduction into seeds, at the expense of ramet production. The model emphasizes the need to consider both modes of plant reproduction whenever one of them is theoretically or empirically studied.     

Effects of resource availability on the trade-off between seed and vegetative reproduction

Aims To explore whether the trade-off between seed and vegetative reproductive modes is flexible in environments with different amounts of available resources to maintain optimal behaviors.Methods A transition matrix model was established to determine the optimal trade-off between seed and vegetative reproduction in resources–variable habitats.Important findings The model predicts that plants allocate more resources to seed reproduction when available resources are scarce. With increasing resources, more vegetative propagules are produced. However, if resources keep increasing to a harmful level, plants would switch to seeds again.     

Warm temperature conditions restrict the sexual reproduction and vegetative growth of the spring ephemeral <Emphasis Type="Italic">Gagea lutea</Emphasis> (Liliaceae)

The responses of reproduction and growth to climate warming are important issues to predict the fate of plant populations at high latitudes. Spring ephemerals inhabiting cool-temperate forests grow better under cool conditions, but how reproductive performance is influenced by warm weather is unclear. The phenological and physiological responses of reproduction and vegetative growth to warm temperature and light conditions were evaluated in the spring ephemeral Gagea lutea. Leaf and bract physiological activities, bulb growth, and seed production were compared among reproductive plants grown in forest, open, and greenhouse (GH; warming manipulation in the open site) plots. In vitro pollen germination ability was tested under various temperatures. In the GH, leaf and bract photosynthetic activities decreased rapidly at the fruiting stage, but dark respiration rates remained high, resulting in higher carbon exhaust in warm conditions. Both leaf and bract sizes and their longevities were reduced in the GH. Annual bulb growth was largest in the forest plot and smallest in the GH plot. Pollen germination was strongly inhibited at high temperature (30 °C). Fruit and seed productions were decreased only in the GH plot. Both vegetative and reproductive activities were negatively affected by warm temperature, resulting in less vegetative growth and lower seed-set, whereas an understory habitat was beneficial for vegetative growth and showed similar seed production to an open habitat over the experimental period. Decreasing population dynamics of spring ephemerals was predicted in response to future warming climate not only by growth inhibition but also by restriction of seed production.     

Pair-edge approximation for heterogeneous lattice population models

To increase the analytical tractability of lattice stochastic spatial population models, several approximations have been developed. The pair-edge approximation is a moment-closure method that is effective in predicting persistence criteria and invasion speeds on a homogeneous lattice. Here we evaluate the effectiveness of the pair-edge approximation on a spatially heterogeneous lattice in which some sites are unoccupiable, or "dead". This model has several possible interpretations, including a spatial SIS epidemic model, in which some sites are occupied by immobile host-species individuals while others are empty. We find that, as in the homogeneous model, the pair-edge approximation is significantly more accurate than the ordinary pair approximation in determining conditions for persistence. However, habitat heterogeneity decreases invasion speed more than is predicted by the pair-edge approximation, and the discrepancy increases with greater clustering of "dead" sites. The accuracy of the approximation validates the underlying heuristic picture of population spread and therefore provides qualitative insight into the dynamics of lattice models. Conversely, the situations where the approximation is less accurate reveals limitations of pair approximation in the presence of spatial heterogeneity.     

Myrmecochorous adaptations ofCorydalis species (Papaveraceae) in southern Japan

Dispersal morphology based on the myrmecochorous adaptations for predator avoidance of sevenCorydalis species including two varieties are investigated in southern Japan. Three types of myrmecochory were distinguished: myrmecochory with autochory (diplochory), the explosive ejection of seeds followed by ant transportation; myrmecochory with vegetative reproduction, seed-transportation by ants and reproduction by tuber; and pure myrmecochory, seed-transporting by ants only. Diplochory occurs in one winter annual plant, which has explosive capsules, a smooth seed surface, a small elaiosome, long pedicels and large bracts. Myrmecochory with vegetative reproduction occurs in two perennials that reproduce by tuber, although they also produce a small number of seeds with a medium-sized elaiosome. the pedicels and bracts are medium in size. Pure myrmecochory occurs in five annuals or biennials that are characterized by a rough seed surface with a large elaiosome, comparatively high seed production, short pedicels and only small bracts. Diplochory is exhibited by only one species distributed widely throughout Japan. Myrmecochory with vegetative reproduction is exhibited by species mainly distributed in cool-temperate northern Japan, while true myrmecochory is exhibited by the majority ofCorydalis species in warm-temperate Japan.     

The seed bank longevity index revisited: limited reliability evident from a burial experiment and database analyses

Background and Aims

Seed survival in the soil contributes to population persistence and community diversity, creating a need for reliable measures of soil seed bank persistence. Several methods estimate soil seed bank persistence, most of which count seedlings emerging from soil samples. Seasonality, depth distribution and presence (or absence) in vegetation are then used to classify a species'' soil seed bank into persistent or transient, often synthesized into a longevity index. This study aims to determine if counts of seedlings from soil samples yield reliable seed bank persistence estimates and if this is correlated to seed production.

Methods

Seeds of 38 annual weeds taken from arable fields were buried in the field and their viability tested by germination and tetrazolium tests at 6 month intervals for 2·5 years. This direct measure of soil seed survival was compared with indirect estimates from the literature, which use seedling emergence from soil samples to determine seed bank persistence. Published databases were used to explore the generality of the influence of reproductive capacity on seed bank persistence estimates from seedling emergence data.

Key Results

There was no relationship between a species'' soil seed survival in the burial experiment and its seed bank persistence estimate from published data using seedling emergence from soil samples. The analysis of complementary data from published databases revealed that while seed bank persistence estimates based on seedling emergence from soil samples are generally correlated with seed production, estimates of seed banks from burial experiments are not.

Conclusions

The results can be explained in terms of the seed size–seed number trade-off, which suggests that the higher number of smaller seeds is compensated after germination. Soil seed bank persistence estimates correlated to seed production are therefore not useful for studies on population persistence or community diversity. Confusion of soil seed survival and seed production can be avoided by separate use of soil seed abundance and experimental soil seed survival.Key words: Arable weeds, Bifora testiculata, Carthamus lanatus, Centaurea solstitialis, longevity index, seed bank persistence, soil seed bank     

Ecology of two cytotypes ofButomus umbellatus II. Reproduction,growth and biomass production

Generative and vegetative reproduction of diploid and triploidButomus umbellatus L., and growth and biomass production of both cytotypes under two different nutrient levels were compared. Seedling survival was studied under controlled conditions in a growth chamber; the response of plants to different nutrient conditions was studied in experimental garden. Both cytotypes do not differ in seed germination and seedling survival. Triploids produce more aboveground and underground biomass, more numerous lateral rhizome buds, and have significantly higher flowering stalks. Low generative reproduction (limited seed production) in triploids is compensated for by more intensive vegetative reproduction. High nutrient level appeared to be stressful for plants of both cytotypes: it limits plant growth and causes plant mortality. Triploids are more viable than diploids in this case, which may be important for their survival under conditions of high trophic level.     

Reproduction of invasive Amur honeysuckle (Lonicera maackii) and the arithmetic of an extermination strategy

Ecological restorations often require removal of invasive species. The abundance of invasives has tended to catalyze research emphasizing removal, not broader understandings, of species mechanisms for persistence in the landscape (e.g. reproductive output and seed dispersal). Asiatic shrub honeysuckles (Lonicera spp.) are pernicious invaders throughout eastern North America. Heavy tree canopy cover apparently reduces growth and reproductive output in Lonicera maackii, which is widespread through the lower Midwestern United States. To help focus control efforts more effectively, we quantified the effect of tree canopy cover on vegetative growth, flowering, and fruit production under three canopy densities. Mean vegetative growth of flowering shoots was not affected by canopy cover. All aspects of sexual reproduction (flower production, fruit set, fruit number, fruit mass, seed number, and seed size) were strongly reduced by moderate shade. Although all individuals modify community and ecosystem properties, a limited number of high light individuals might also provide the greatest proportion of the seeds. Through model simulation of honeysuckle population structure in relation to canopy cover, we argue that it can sometimes be more efficient to initially target reproductive individuals in the high light edge and interior gap environments than to immediately focus on all individuals in the forest interior.     

CLONAL STRUCTURE OF ARCTIC DWARF BIRCH (BETULA GLANDULOSA) AT ITS NORTHERN LIMIT

Betula glandulosa Michx. (Betulaceae) at Tarr Inlet, Baffin Island, Northwest Territories reproduces primarily by vegetative layering with little, if any, viable seed production or seedling recruitment. Information from polymorphic enzyme loci revealed 15 three-locus genotypes with a single genotype usually dominating each of the nine sampled sites. Clonal reproduction combined with self-incompatibility may be one factor contributing to a decreased seed set by reducing the probability of xenogamy. However, multiple genotypes were found at some sites providing an opportunity for cross compatible pollen to effect fertilization. The lack of seed set in these areas appears to be due to additional factors inhibiting sexual reproduction. Based on pollen records, the present limited distribution of Betula glandulosa on Baffin Island represents the remnants of once more widely distributed populations, and the failure of sexual reproduction at the northern edge of its range may be responsible for the decline in abundance. Genotypic diversity observed in the Tarr Inlet area appears to be residual from a once sexually reproducing population. No evidence was found for differential selection of genotypes indicating that random processes are probably eroding the genotypic diversity during the decrease of population size in this species.     

Optimum reproduction and dispersal strategies of a clonal plant in a metapopulation: a simulation study with Hieracium pilosella

Clonal spread is favoured in many plants at the expense of seed production in order to expand rapidly into open habitats or to occupy space by forming dense patches. However, for the dynamics of a population in a patchy landscape seed dispersal remains important even for clonal plants. We used a spatially explicit individual-based metapopulation model to examine the consequences of two trade-offs in Hieracium pilosella L: first, between vegetative and sexual reproduction, and second, between short and far-distance dispersal of seeds. Our main question was, what are the environmental conditions that cause a mixed strategy of vegetative and sexual reproduction to be optimal. The model was parameterised with field data on local population dynamics of H. pilosella. Patch dynamics were given firstly by disturbance events that opened patches in a matrix of a clonal grass that were colonisable for H. pilosella, and secondly by the gradual disappearance of H. pilosella patches due to the expanding grass. Simulations revealed opposing selection pressures on traits determined by the two trade-offs. Vegetative reproduction is favoured by local dynamics, i.e. the need for maintenance and expansion of established populations, whereas seed production is favoured by the necessity to colonise empty habitats. Similar pressures act on the proportion of seeds dispersed over short and far distances. Optimum reproductive and dispersal strategies depended on habitat quality (determined by seedling establishment probability), the fraction of dispersed seeds, and the fraction of seeds lost on unsuitable ground. Under habitat conditions supporting moderate to low seedling establishment, between 20% and 40% of reproductive effort in H. pilosella should be devoted to sexual reproduction with at least 10% of the seeds dispersed over distances suitable to attain empty patches. We conclude that in a spatially heterogeneous landscape sexual seed production in a clonal plant is advantageous even at the expense of local vegetative growth.     

Spread of introgressed insect-resistance genes in wild populations of Brassica juncea: a simulated in-vivo approach

Introgression between transgenic, insect-resistant crops and their wild relatives could lead to a progressive increase of the frequency of resistant plants in wild populations. However, few studies help predict the impact on the population dynamics. To simulate the performance of introgressed insect-resistant plants of wild Brassica juncea, independently from the interspecific hybridization cost, healthy plants were cultivated in pure and mixed stands with damaged plants through cutting leaves in field experiments over two field seasons. As expected, resistant (healthy) plants held a competitive advantage when in competition with susceptible (damaged) plants. Individual biomass and seed production of both types of plants decreased as the percentage of resistant plants increased, so that the relative advantage of resistant plants increased. The combined effects of defoliation and competition on the individual performance of B. juncea were additive. Replacement series experiments confirmed this trend but did not show different seed output in pure stand of susceptible versus resistant plots. The total vegetative and reproductive production of mixed populations was not significantly different of that of pure populations. These results suggest that if a transgene for insect-resistance were to colonize wild populations, high herbivory of susceptible plant and low resource availability would facilitate the spread of resistant individuals. However, at the population level, the shift from an insect-susceptible to a predominantly resistant population would not result in exacerbated habitat colonization.     

State of the reproductive system of populations of species of the genus Glycyrrhiza L. (Fabaceae)

Reproductive features of the valuable medicinal herbs Glycyrrhiza glabra L., G. uralensis Fisch., and G. korshinskyi Grig. were investigated; an assessment was made of the state of the reproductive systems of populations located under introduction conditions in the Central Urals and in the zone contact of the species ranges in the southern cis-Urals. The states of the male and female reproductive systems, seed productivity elements, pollination features, and the viability of seeds produced by different pollination treatments were studied. For the three Glycyrrhiza species, the seed productivity indices were found to decrease considerably starting from the early stages of development of the female reproductive system and continuing to seed maturity. A quantitative analysis of seed productivity elements taking into account the flowering duration of the species showed that the seed production system of the species is very efficient; even in the introduced populations located north of the range, it is able to produce viable seeds in years with favorable growing conditions. Qualitative and quantitative indices of the reproductive system were used to characterize samples from the zone of contact between the ranges. Embryological analysis confirmed the possibility of interspecific hybridization and the prevalence of hybrid forms among the samples. A broad distribution of interspecific hybridization in Glycyrrhiza in various parts of the ranges is assumed. Glycyrrhiza species are characterized by seed and vegetative reproduction. Vegetative reproduction is limited in space but efficiently ensures reproduction of individuals and formation of vegetative clones. Seed reproduction is characterized by indices such as amphimictic seed formation, strict entomophily, and interspecific compatibility. Seed reproduction is a factor that controls the possibility of wide propagation of the genetic material in space and, hence, the formation of mixed and hybrid populations leading to a higher intraspecific and interspecific diversity of the genus.     

Seed quality in hybrid swarm populations of Pinus mugo Turra and P. sylvestris L.

Crop characteristics involving cone size, average number of seeds per cone, and seed germinability have been studied for three hybrid swarm populations of Pinus mugo and P. sylvestris, two pure P. mugo populations and one pure P. sylvestris population, in northern Slovakia. Generally, reproduction capability characteristics were significantly reduced in hybrid swarms in relation to the control populations. Although there were no consistent differences between hybrid swarms and control populations in the average number of seeds per cone or cone size, seed germination rates and full seed production in hybrid swarms were lower than for the control population of P. sylvestris and one of the pure populations of P. mugo. Based on these data a conclusion has been drawn postulating partial rather than full fertility of the hybrid swarm populations of P. mugo and P. sylvestris.     

An analysis of life history evolution in terms of the density-dependent Lefkovitch matrix model.

The evolution of demographic characteristics is considered in terms of the density-dependent Lefkovitch matrix model, which describes a species' population dynamics with a stage-specific pattern of reproduction and mortality. We obtain the invadability condition of a mutant-type into the wild-type population at the equilibrium state. The condition depends on the left and right eigenvectors at the equilibrium state. The condition depends on the left and right eigenvectors at the equilibrium state and the difference, between wild-type and mutant-type populations, of the values of elements in the Lefkovitch matrix at the equilibrium state. It is also shown that if elements of the density-dependent Lefkovitch matrix are decreasing functions of population density, then the equilibrium population density increases in the process of natural selection; that is, K-selection acts even on the stage-structured population. The evolution of life history in perennial plants is discussed through two models as an application of the above results. The evolution of perennial plants with no vegetative reproduction is analyzed in the first example. It is shown that whether monocarpic perennials (which reproduce once and die) or polycarpic perennial plants (which reproduce more than once) are favored depends on the cost of a produced seed. The second example concerns perennial plants that reproduce vegetatively. It is shown that whether monocarpic or polycarpic perennial plants are favored depends on the cost of a seed and that where vegetative reproduction is common, polycarpic perennials with no seed reproduction are favored.     

Spatially structured superinfection and the evolution of disease virulence

When pathogen strains differing in virulence compete for hosts, spatial structuring of disease transmission can govern both evolved levels of virulence and patterns in strain coexistence. We develop a spatially detailed model of superinfection, a form of contest competition between pathogen strains; the probability of superinfection depends explicitly on the difference in levels of virulence. We apply methods of adaptive dynamics to address the interplay of spatial dynamics and evolution. The mean-field approximation predicts evolution to criticality; any small increase in virulence capable of dynamical persistence is favored. Both pair approximation and simulation of the detailed model indicate that spatial structure constrains disease virulence. Increased spatial clustering reduces the maximal virulence capable of single-strain persistence and, more importantly, reduces the convergent-stable virulence level under strain competition. The spatially detailed model predicts that increasing the probability of superinfection, for given difference in virulence, increases the likelihood of between-strain coexistence. When strains differing in virulence can coexist ecologically, our results may suggest policies for managing diseases with localized transmission. Comparing equilibrium densities from the pair approximation, we find that introducing a more virulent strain into a host population infected by a less virulent strain can sometimes reduce total host mortality and increase global host density.