INTRAPOPULATIONAL VARIATION IN POLLEN-MEDIATED GENE FLOW IN PLANTAGO LANCEOLATA L.

The extent of genetic variation in pollen-mediated gene flow distance was measured for a population of the wind-pollinated herb, Plantago lanceolata. Twenty-nine genotypes were collected from a natural population, clonally replicated, and grown to reproductive maturity in a greenhouse. Relative gene flow distances were measured for each replicate and genotype in a wind tunnel. Approximately five percent of the total variation in gene flow distance was attributable to variation among genotypes. Most of the remaining variation was attributable to differences between the early part of the growing season, when most of the flowering occurs, and the remainder of the season, when flowering is sparse. The rankings of the genotypes' gene flow distances showed significant concordance between the early-season measures and measures from later in the summer. There was no correlation between the average inflorescence height for a plant and the average gene flow distance for that plant. For analysis of pollen characteristics suspected as causal factors of variation in gene flow distance, the variation in gene flow distance was divided into two components: that due to environmental (seasonal) differences, and that due to the differences among genotypes within runs. Variation in both the buoyant properties of the pollen grains and their adhesiveness was significantly partially correlated with environmental variation in gene flow distance, while only the buoyant properties of the pollen grains were significantly partially correlated with among-genotype variation in gene flow distances.     

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.     

松嫩平原异质生境羊草种群营养繁殖特征

多年生无性系禾草主要依靠营养繁殖实现种群的持续更新,而各类芽和由芽形成的苗为其种群进行营养繁殖的潜在种群。通过单位面积挖掘取样,利用营养繁殖世代数划分分株、根茎和各类型芽和苗的龄级,对松嫩平原封育草甸、长期割草草甸、封育积沙草甸和林间草地的羊草潜在种群组成和大小进行了比较研究。结果表明: 在生长季末期,羊草的潜在种群均由根茎芽和各龄级分蘖节向上生长的芽以及根茎苗和分蘖节苗组成。在4种生境条件下,羊草分株潜在种群由3～4个龄级组成,1～4龄级根茎均可形成潜在种群,其中,根茎潜在种群占整个羊草潜在种群数量比例为68.3%,占绝对优势。1龄分株和根茎产生的潜在种群占总体的60.2%,是潜在种群组成的主要部分,是种群更新的主要来源。分株潜在种群数量以长期割草草甸最高,根茎潜在种群数量以封育积沙草甸最高。营养繁殖力均以年轻龄级分株和根茎最高,根茎的营养繁殖力是分株的10.0倍,在羊草种群营养繁殖过程中始终占据优势。羊草的潜在种群构成在不同生境条件下变异较大,但体现出对生境差异的趋异适应。     

Patterns of vegetative growth and reproduction in relation to branch orders: the plant as a spatially structured population

The patterns of vegetative growth and reproduction in relation to orders of terminal branches were examined in the evergreen woody plant, Eurya japonica. The branch order number was determined centrifugally. The trunk was given order number 1; branches issuing directly from the trunk were order 2; branches growing on order 2 branches were order 3, and so on. The results of this study show the differential patterns of vegetative growth and reproduction in relation to the branch orders. Lower-order shoots of terminal branches grew more, but produced few flowers. On the other hand, for the higher-order terminal branches, shoot growth was very limited but flowering was more intense. The results show that a tree can be interpreted not as a mere population of equivalent modules but as a spatially structured population. Thus, it is essential to consider the position of modules within the branch system when patterns of vegetative growth and reproduction are examined. It is hypothesized that the difference in the opportunity cost in relation to the branch orders is the main cause of the spatial structure for patterns of vegetative growth and reproduction. Furthermore, for same-order terminal branches, current-year shoot elongation was independent of flowering intensity. These results suggest that this species only invests resources in reproduction that are surplus to its requirements for the functions associated with survival, such as growth and/or storage. Received: 22 July 1999 / Accepted: 12 January 2000     

Production dependence of vegetative propagation inDisporum smilacinum A. Gray

The dependence of vegetative propagation on the production of individual plants was examined inDisporum smilacinum A. Gray on the basis of shading experiments and field surveys. This species typically showed four types of reproductive behavior: sterile plants producing one plantlet (no propagation), sterile plants producing more than two plantlets (vegetative propagation), fertile plants producing one plantlet (sexual reproduction) and fertile plants producing more than two plantlets (both sexual reproduction and vegetative propagation). The propagation ofD. smilacinum was clearly related to the annual net production of each individual plant. The probability of a mother plant producing more than two vegetative propagules (plantlets) increased with net production of the plant in the current year. The number of propagules per plant and runner length increased with net production. It was possible to explain the types of reproductive behavior of this species on the basis of both the initial plant size before sprouting and its net production during the growing season. There was a critical initial plant size for sexual reproduction and a critical level of production for vegetative propagation.     

SEXUAL DIMORPHISM IN BIOMASS ALLOCATION AND CLONAL GROWTH OF XANTHOXYLUM AMERICANUM

Reproduction can have a high resource cost. It has been suggested that greater investments in sexual reproduction by female dioecious plants leads to a lower rate of vegetative growth in females than in males. In this study, we investigated sexual dimorphism in biomass allocation and genet growth of the dioecious clonal shrub, northern prickly ash (Xanthoxylum americanum). The allocation of biomass over the course of one growing season to reproductive tissue, leaves, and growth of aboveground first-year wood, was compared in 18 clones growing in fields and six clones in woods in southeastern Wisconsin during 1985 and 1986. In addition, the number of shoots per clone, and weight of nonfirst-year wood (accumulated biomass) above- and below-ground were estimated. In open field sites, male clones allocated more biomass to new wood and less to reproduction than females, although males allocated more to flowers alone. Accordingly, male clones had significantly more shoots and more accumulated biomass both above- and below-ground than female clones. In the woods, where fruit set was near zero, there were few significant differences between male and female clones in either biomass allocation or accumulated biomass. These results support the hypothesis that the high resource investment in fruit production by females reduces their vegetative growth relative to males.     

不同生境自交植物黄花大苞姜生殖分配的研究

为了探讨自交植物黄花大苞姜(Caulokaempferia coenobialis)对石壁附生这一特殊生境的生态适应,对其不同物候期和不同生境的生殖分配进行了对比研究。结果表明,在生殖生长过程中,黄花大苞姜种群用于营养生长的生物量分配占有绝对优势,而用于生殖的生物量分配比例较小(<13%)。在黄花大苞姜各构件的生物量分配中,根茎和叶的比重较大(24.22%~43.25%)。在光线较弱生境中的种群,为了提高资源获取能力,黄花大苞姜分配到叶的比重明显高于光线较强的种群,而分配给根茎的比例却明显低于光线较强的种群。随着物候期的推移,黄花大苞姜生殖分配的比例不断增加,到果期达到最大值。不同种群间和年度间黄花大苞姜分配给生殖构件的比例没有显著差异,推测其生殖分配可能受遗传因素控制。个体大小与根茎生物量呈极显著线性函数同速生长,而与生殖分配在云天海种群没有表现出相关性,在上坪和天堂顶种群表现为同速生长关系,但决定系数小于40%。因此,黄花大苞姜能有效调节其在不同生境的生物量分配以适应石壁附生的特殊生境,在光线较弱的种群提高叶的生物量分配并降低根茎的生物量分配以提高资源的获取能力。整体上投资到营养构件的生物量占比高达87%以上,生殖构件在居群间和年度间均保持稳定。这种繁殖策略,一方面较高的营养构件投资可以获得更多的资源,另一方面稳定的生殖投资可以保证种群的延续,各构件相互协调以更好适应石壁这一资源匮乏的生境。     

Sexual reproduction in the phyllodioicous bryophyte Homalothecium lutescens (Hedw.) H.Rob. in relation to habitat age,growth conditions and genetic variation

Sporophyte production and female fertility were investigated in seventeen calcareous grassland demes of the moss Homalothecium lutescens (Hedw.) H.Rob. on the Baltic island of Öland, with the aim of understanding the relationships between sexual reproduction, habitat age, genetic variation and factors related to growth conditions. The overall proportion of fertile female shoots (with perichaetia) was 35%. Fertility status at the level of individual shoots was positively associated with shoot length and density, while within deme fertility was positively associated with bush cover. There was no association between female fertility and habitat age, genetic diversity (H_S) or allelic richness. Out of 1344 investigated shoots, only two were normal-sized fertile males. Dwarf males were also extremely rare, and found almost exclusively on shoots with sporophytes. Few sporophytes were observed (in the two demes with highest fertility and bush cover). No relationship between genetic variation and the frequency of sporophytes and males was found. The lack of a relationship between sexual reproduction and genetic variation suggests that sexual reproduction may not occur in the same grassland fragments as the recruitment of new clones (from spores or vegetative fragments). The majority of the dry, open grassland habitats, where H. lutescens is typically found in the study area, appear to be suboptimal for both dwarf males and fertilization. Sexual reproduction is more likely to occur in shaded (although grazed) grassland patches, where moisture levels are likely to be higher and the moss colonies are generally more vigorous.     

Establishment and reproduction of Aeschynomene virginica (L.) (Fabaceae) a rare,annual, wetland species in relation to vegetation removal and water level

Aeschynomene virginica is a rare annual plant found in freshwater tidal wetlands of the eastern United States. We hypothesized that standing vegetation and water inundation were two important environmental factors in its population dynamics. To test these hypotheses, we sowed seeds into plots with undisturbed vegetation or plots with all aboveground vegetation removed in 1998 and 1999. Presence/absence of seedlings was noted and seedling survival to reproduction, final size, and seed set were measured throughout both growing seasons. Seedling establishment from germination to the first true leaf stage increased with decreasing water depth. Vegetation removal plots had greater seedling establishment, higher seedling survival, and higher seed set per plant than non-removal plots. In a greenhouse study designed to test the effects of water level on seed germination and seedling establishment, no seedlings established in submerged soils, and seed germination and seedling establishment were lower in waterlogged soil than in wet soil. Physical stress associated with deeper water likely limits the distribution of A. virginica to higher elevations, where seeds that colonize patches with low vegetative cover are more likely to produce reproductive adults that produce more seeds relative to patches with established vegetation. A. virginica appears to be a fugitive species specializing on open habitat patches in tidal wetlands. This species may be dependent on disturbances for population establishment and maintenance.     

Resource allocation to sexual and vegetative reproduction in a forest herbSyneilesis palmata (Compositae)

The balance between sexual and vegetative reproduction inSyneilesis palmata was examined in relation to environmental conditions and the amount of reproductive resources, which is defined here as the total quantity of dry matter invested in both modes of reproduction. The allocation balance was measured for individual plants of two populations, with different densities in an open habitat, for 2 years, and those of two other populations, under different light intensities in a plantation forest (forest floor and edge), for 3 years. Relative allocation to sexual reproduction decreased with increasing reproductive resources in all populations except for the forest edge, which showed a constant allocation balance. The high density population showed lower relative allocation to sexual reproduction than the low density population, irrespective of the amount of reproductive resources. However the between-year comparison of the high density population suggested that under extremely high density, plants with a small amount of reproductive resources enhanced sexual reproduction, while plants with a large amount of reproductive resources reproduced vegetatively. On the forest floor, plants with small amounts of reproductive resources had higher relative allocation to sexual reproduction than the forest edge population, while plants with large amounts of reproductive resources had a somewhat lower one. The adaptive significance of such allocation patterns are discussed, based on qualitative data on the characteristics of both types of offspring.     

扎龙湿地保护区异质生境芦苇种群分株构件的数量特征

采用单位面积取样,计数和测量的调查与统计方法,对扎龙湿地保护区4个生境单优群落芦苇种群分株构件数量特征进行比较分析。结果表明,4个生境芦苇种群从5月10日左右返青后进入营养生长期,分株高度和分株密度均持续增加到生殖生长初期的8月份,其中6-8月份差异均达到显著水平(P0.05),8-10月份差异均未达到显著水平(P0.05),芦苇进入生殖生长期后,分株便停止高度生长,地下芽的输出也不再形成分株补充现实种群;分株生物量和种群生物量均持续增加到生殖生长旺盛期的9月份,至休眠期的10月份均有所降低,各月份间的差异均达到显著水平,芦苇种群在生长季末期,具有将生产的物质分配转移到地下储藏与营养繁殖器官的形成与生长上的特性。芦苇种群分株数量特征与返青后实际生长时间之间均较好地符合对数函数关系,R~2在0.818-0.994之间,拟合方程均达到了P0.05的显著水平。4个生境芦苇种群分株密度以湿生生境最大,依次为水生生境、旱生生境,盐碱生境最小,分株高度、分株生物量和种群生物量均以水生生境最大,依次为湿生生境、旱生生境,盐碱生境最小。因此,4个生境芦苇种群分株构件数量特征均表现出基本一致的生育期节律性,同时,芦苇种群的个体生长和种群动态存在明显的环境效应,土壤含水量、pH是影响该地区芦苇分株数量特征的主要因子。     

Vegetative reproduction and clonal diversity in Rhytidium rugosum (Rhytidiaceae, Bryopsida) inferred by morpho-anatomical and molecular analyses

Many bryophytes exhibit specific life forms that result in the formation of distinct patches. This is primarily achieved by consequent vegetative multiplication through indeterminate growth and vegetative reproduction. Little, however, is known about the underlying mechanisms and genetic patterns. In this study on vegetative multiplication in bryophytes, we apply morpho-anatomical and molecular (AFLP fingerprinting) techniques to analyze the vegetative reproduction system, clonal diversity, and habitat colonization of a pleurocarpous moss, Rhytidium rugosum (Rhytidiaceae). Morpho-anatomically, three types of vegetative diaspores are identified and illustrated: ramets, separated after decay and disintegration of older shoot parts (clonal reproduction); brood branches/branchlets; and caducous shoot apices (both vegetative reproduction s.str.). The AFLP fingerprinting of 35 samples (from two plots in Thuringia, further German populations, and from France, Russia, and Canada) resulted in the identification of three clones from the two plots, each comprising two to 15 samples with identical fingerprints. Samples from one clone occurred in both plots, thus proving dispersal of vegetative diaspores. The AFLP analysis further revealed close relationships of the plot samples, which suggest a clonal rather than generative (sexual) origin of the population.     

RELATIVE FITNESS OF SELFED AND OUTCROSSED PROGENY IN A SELF-COMPATIBLE,PROTANDROUS SPECIES,SABATIA ANGULARIS L. (GENTIANACEAE): A COMPARISON IN THREE ENVIRONMENTS

The consequences of selfing were examined for a population of self-compatible, protandrous, Sabatia angularis L. (Gentianaceae). Field-collected plants were hand-pollinated in the greenhouse to produce selfed progeny and outcrossed progeny from parents separated by a maximum of 5 m (near-outcross) and 85 m (far-outcross) in the field. Self, near-outcross, and far-outcross half sib progeny were grown in the greenhouse, a garden plot, and their native habitat. Progeny were compared by multiplicative fitness functions based on seed production per hand-pollination, seed germination, rosette formation, survival to reproduction, and reproduction in each environment. Variation in reproduction among progeny groups was influenced by the environment in which they were grown. Significant inbreeding depression was detected between the self and far-outcross progeny in each environment. Only the natural environment demonstrated a greater than 50% reduction in relative fitness of self compared to near-outcross progeny. This is of biological relevance since near-outcross hand-pollinations occurred within the range of pollen and seed dispersal suggesting that inbreeding depression in S. angularis is strong enough to maintain outcrossing in the study population. In the field, the far-outcross progeny outperformed the near-outcross progeny suggesting local population substructure. The magnitude of the inbreeding depression expressed among the self progeny was the greatest in the field, intermediate in the garden plot, and the least in the greenhouse.     

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.     

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.     

Plasticity in life-history traits of Plantago major L. ssp. pleiosperma Pilger

Summary Plasticity in life-history characteristics was investigated in three populations of Plantago major L. ssp. pleiosperma (Pilger), a self-compatible, wind pollinated species with a high self-fertilization rate. The populations studied were selected for their marked differences in biomass accumulation and habitat characteristics such as nutrient availability and interspecific interaction. Plants, raised from seeds collected at three sites, were grown in a greenhouse at three nutrient levels. In addition, a reciprocal transplant experiment was carried out. In both experiments variances in variables of growth and reproduction were largely due to environmental factors. Besides this overall result, population and population x environment interaction effects existed for most of the variables. Differences in plasticity between populations were further analysed. In the greenhouse experiment plants from a river-side population showed a high degree of plasticity in reproductive effort, whereas plants from two other populations, one series from a beach plain and the other from a salt meadow, showed a high degree of plasticity in shoot-root ratio. Plasticity in biomass allocation to either vegetative or generative parts is suggested to be an important response to selective forces related to either interspecific competition or temporal variability.     

Variation and covariation among life-history traits in Rumex acetosella from a successional old-field gradient

In this study morphological variation and the potential for competition to affect biomass and seedling selection of the families of five populations of Rumex acetosella L. sampled along a successional old-field gradient have been investigated. Seeds from 25 families were submitted to four competitive regimes: no competition (one plant per pot), medium competition (two plants/ pot taking plants from the same population), high within-population competition (four individuals from the same population in a pot) and high between-population competition (four individuals from two different populations in a pot). Eight traits were analysed after 3 months of growth for variation among families within populations. A significant difference among families within the two older populations was recorded for sexual biomass and related components. High sensitivity of these traits to density was observed in all populations except the youngest, suggesting specialization to particular environmental conditions in late successional populations, and a good adaptive capacity to buffer environmental variation in the pioneer population. Little significant interaction between competitive regimes and families within populations was found, i.e. genotypes within each population showed little variation in their response to environmental variation. Genotypic variance decreased with increasing competitive conditions for the majority of the traits. However, the percentage of variance in sexual reproduction explained by family was stable among treatments. Tradeoffs between vegetative reproduction and sexual reproduction were recorded at the population level along the successional gradient, with increasing competitive conditions. As succession proceeds, we observed a decrease in sexual reproduction and an increase in vegetative reproduction. At the family level, correlation among traits were similar when plants were grown in the absence of competition and at high density, with a significant negative correlation between sexual reproduction and vegetative reproduction. For both sprout number and sexual biomass, the performance of families grown under all the treatments was positively correlated. Together these results indicate allocational constraints on the reproductive biology of R. acetosella that may be favoured by natural selection and have influenced population differentiation along the successional gradient. However, they also revealed that the potential exists for evolutionary specialization through plasticity, in response to variation in environmental conditions.     

东北草地异质生境芦苇种群根茎芽年龄结构及输出规律

为明确异质生境条件下芦苇种群根茎芽年龄结构及输出规律,揭示芦苇种群的营养繁殖特性,采用单位土体挖掘取样,分别计数各龄级根茎芽的调查与统计方法,对东北草甸草原草甸土和盐碱土两个生境单优群落芦苇种群根茎芽动态进行比较分析。结果表明,两个生境芦苇种群根茎芽库主要均由6个龄级组成;草甸土生境在6—10月均为增长型年龄结构;盐碱土生境6—7月份为衰退型年龄结构,8月份为稳定型年龄结构,9—10月份为增长型年龄结构。根茎芽数量1—4a普遍以草甸土生境高于盐碱土生境,5—6a普遍以盐碱土生境高于草甸土生境,各龄级根茎芽数量与月份之间均符合y=a+bx直线关系(P0.05)。随着龄级的增加,休眠芽比率呈逐渐下降趋势,而萌发芽比率则呈逐渐上升趋势,5个生育期的休眠芽比率和萌发芽比率与龄级之间均符合y=a+bx直线关系(P0.01)。各龄级根茎的休眠芽具有一个相对稳定的萌发输出过程,草甸土生境根茎休眠芽按每年11%的比率萌发输出,而盐碱土生境根茎休眠芽按每年7%的比率萌发输出。虽然芦苇种群根茎芽年龄结构及年龄谱在异质生境中存在显著差异,但却有着相同的季节变化规律,均以不断形成新根茎的芽来维持着种群的营养繁殖更新。     

Variation in reproductive modes and population genetic structures of a monocarpic perennial herb,Cardiocrinum cordatum,in relation to habitat fragmentation

Reproductive systems are recognized as having a profound influence on the extent and structure of genetic variation in plant populations. To investigate the spatiotemporal variation in the reproductive modes (sexual and vegetative reproduction) and population genetic structures of a monocarpic perennial herb, Cardiocrinum cordatum (liliaceae), we selected a variety of habitats (e.g. large forested area including primeval forest, small fragmented secondary forest, and so on) around Sapporo City, Japan. We conducted breeding experiments, monitored the fate and growth of marked individuals for 3 years, and also analyzed the spatiotemporal genetic variation of flowering plants within the populations using allozyme variation. Plants emasculated prior to anthesis produced mature fruits in all populations examined. However, seed production was significantly lower in the small fragmented populations, possibly because of the low availability of pollinators and subsequent pollen limitation. In these fragmented populations, the mature flowering plants tended to be more dependent on vegetative reproduction for their recruitment, because they can only produce flowers once in their lifetime. Genetic diversity using samples from mature flowering plants in each population was lower in the small fragmented populations than in the large populations. In addition, although genotypic compositions in the fragmented populations were more or less similar during the 3 years of the study, the dominant genotypes changed temporally and spatially every year in the large populations. The present study demonstrated that the reproductive features of C. cordatum can be altered in various environmental conditions, such as habitat fragmentation, and these changes considerably affected the population genetic structures and vice versa.     

Differential costs of sexual and vegetative reproduction in wild strawberry populations

Summary The CO₂ costs of producing sexual and vegetative reproductive propagules were calculated for two species of wild strawberry, Fragaria virginiana and F. vesca. Five populations on sites representing a gradient of successional regrowth near Ithaca, New York, USA, were studied for two or three years each. Field studies of phenology, biomass, demography, and environment and laboratory studies of CO₂ exchange were integrated using a computerbased model of CO₂ dynamics to estimate costs of propagules.The percentage of plants flowering and the number of flower buds produced were highest in an open, recently disturbed habitat and lowest in a forest habitat. The openhabitat plants had the greatest success in converting flower buds into ripe fruits and also produced the highest numbers of runners and runner plantlets. On the basis of total investments in structure and respiration minus any photosynthetic gain of all reproductive structures, the cost per seed was lowest in the most open habitats and highest and increasingly variable in the more closed habitats. The cost of plantlets also was lowest in the most open habitat. The differences among habitats in cost of plantlets alive after one or two growing seasons increased due to differential survivorship of plantlets, with the open habitat continuing to have the lowest cost per plantlet. Theoretical treatments of life history characteristics such as reproductive effort should recognize that costs of equivalent type and size of propagule may vary among environments.