What determines the number and size of the seeds produced by a plant? How often should it reproduce them? How often should a plant produce them? Why and how are seeds dispersed, and what are the implications for the diversity and composition of vegetation? These are just some of the questions tackled in this wide-ranging review of the role of seeds in the ecology of plants. The authors bring together information on the ecological aspects of seed biology, starting with a consideration of reproductive strategies in seed plants and progressing through the life cycle, covering seed maturation, dispersal, storage in the soil, dormancy, germination, seedling establishment, and regeneration in the field. The text encompasses a wide range of concepts of general relevance to plant ecology, reflecting the central role that the study of seed ecology has played in elucidating many fundamental aspects of plant community function.

The ecology of seeds

Contents

Summary321

I.   Introduction321
II.     Theory on costs of reproduction322
III. Methodological aspects324
IV. Empirical evidence328
V. Plant size and costs of reproduction330
VI.  Costs of reproduction in sexually dimorphic plants331
VII. Compensation of the costs333
VIII. Concluding comments and future perspectives336

Acknowledgements337


References337



Summary
This review reports on the processes associated with costs of reproduction, including some theoretical considerations, definitions and methodological aspects, followed by a list of the situations where costs are difficult to find. Despite some exceptions, case studies, examined by trade-offs between reproduction and other life-history traits, generally support the predictions of the cost of reproduction hypothesis. The cost of reproduction as an evolutionary determinant of sexual dimorphism in life history traits in dioecious species was specifically tested, considering that the higher cost of reproduction in females has driven the life history traits related to sexual dimorphism. Females of woody dioecious species were consistently smaller than males supporting the costs of reproduction hypothesis. By contrast, females of herbaceous perennials were generally the larger sex, which did not fit the expectations of the hypothesis. Finally, the mechanisms that enable the compensation of the reproductive costs are detailed, including the plastic responses of photosynthesis and growth, the effects of the timing of investment, plant architecture and plant physiological integration.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.2002.00477.x

The costs of reproduction in plants

Germination behavior is one of the earliest phenotypes expressed by plants. This fact has several consequences for the evolution of postgermination traits, ecological niches, and geographic ranges. By determining the conditions that plants experience after they germinate, germination influences phenotypic expression of postgermination traits, natural selection on them, and their genetic basis. The breadth of germination niches may influence the ecological breadth and geographic ranges of species. Because germination is expressed early, it is frequently subjected to natural selection before other traits are expressed. We review evidence for natural selection on and adaptation of germination and discuss how the breadth of the germination niche is associated with the ecological niche and range of plant species. We review evidence for the coevolution of germination and postgermination traits and compare germination to postgermination niches. Finally, we discuss how germination responses to altered environments can influence species distribution and the evolution of postgermination traits after environmental change.

/pdf/germination-postgermination-adaptation-and-species-2g92osmo5g.pdf

Germination, Postgermination Adaptation, and Species Ecological Ranges

more frequently (Fragaria), and removal of floral biomass causes a reduction in the frequency of insect visits proportional to the fraction of biomass removed (Impatiens). (ii) Removal of attractive structures may cause a decline in the probability that a fruit will be formed but has no effect on the number of seeds set per fruit; thus, mutilation of essentially solitary flowers has no effect on seed-set per fruit (Impatiens), while removal of flowers from inflorescences in a species that forms several many-seeded fruits per inflorescence reduces fruit-set per inflorescence but has no effect on seed-set per fruit (Asclepias), and removal of sterile flowers from an inflorescence in which the fertile flowers yield one-seeded fruits is effective in reducing seed-set per inflorescence (Viburnum). (iii) Larger flowers may disperse a greater fraction of their pollen in unit time (Impatiens) and the removal of flowers from inflorescences causes a steep reduction in total pollen exported and a weak decline in the quantity of pollen exported per flower (Asclepias). These results are consistent with the hypothesis that a single insect visit (or a very few visits) suffices to fertilize almost all available ovules and is procured by a very small allocation to attractive structures, while much greater allocation is necessary to procure the numerous visits required to disperse a large fraction of the pollen. This inference is supported by a comparative survey of sexually dimorphic plants, in which male flowers are generally larger than female flowers, male inflorescences bear more flowers, and male plants bear more inflorescences. It is concluded that the flower is primarily a male organ, in the sense that the bulk of allocation to secondary floral structures is designed to procure the export of pollen rather than the fertilization of ovules. This conclusion may be sensitive to whether it is the flower or the inflorescence as a whole that represents the primary unit of attraction to insects. It was found that the performance of a given flower was substantially affected by other flowers in the same compact inflorescence (Asclepias), though not by other inflorescences on

http://biology.mcgill.ca/faculty/bell/articles/23.Bell_1985_ProcRoySoc223.pdf

On the Function of Flowers

Plant compensation for arthropod damage is a general occurrence of considerable importance in both natural and agricultural systems. In natural systems, plant species that can tolerate or compensate (e.g. recover equivalent yield or fitness) for herbivore feeding have obvious selective advantages that lead to genotype maintenance. Scientists publishing in this area often cite an optimal strategy for enhancing fitness (90, 108). In agricultural crops, reports of plant compensation mostly are concerned with yields rather than fitness (164). However, variation in compensatory response also affects sampling strategies and economic threshold levels [sensu Stern (166)] and provides viable tactic for breeding insect resistance to key arthropod pests into plants. Not surprisingly, the relative importance of the various forms of compensation in agricultural and natural systems is still relatively unknown. Therefore, a primary purpose for this review is to integrate the forms of compensation reported in the literature in the context of natural and agricultural habitats. Several previous reviews and articles have had a significant impact on the development of an understanding of plant compensatory responses. Some provide extensive lists of examples of plant compensation and the pest-yield relationship (4, 164), while others stress the complexity and interrelatedness

/pdf/plant-compensation-for-arthropod-herbivory-1kdda8vqew.pdf

Plant compensation for arthropod herbivory

Dioecy has evolved independently, many times, among unrelated taxa. It also appears to have evolved along two contrasting pathways: (1) from hermaphroditism via monoecy to dioecy and (2) from hermaphroditism via gynodioecy to dioecy. Most dioecious plants have close cosexual relatives with some means of promoting outcrossing (e.g., herkogamy, dichogamy, self-incompatibility, or monoecy). To the extent that these devices prevent inbreeding, the evolution of dioecy in these species cannot logically be attributed to selection for outcrossing. In these cases, the evolution of dioecy is, we believe, due to selection for sexual specialization. However, in other species, that lack outbreeding close relatives, dioecy may have evolved from gynodioecy (males and hermaphrodites) as an outbreeding device. Subsequent disruptive selection and selection for sexual specialization may have also shaped the evolution of dioecy from gynodioecy in these species, resulting in two genetically determined, constant sex morphs. Both pathways for the evolution of dioecy require the operation of disruptive selection, though the gynodioecy route involves more restrictive disruptive selection and a genetic designation of gender. In contrast, the monoecy route is not dependent on the genetic designation of two sex morphs, but, rather, allows the possibility of sexual intermediates and sexual lability. Both pathways produce one morph in which maleness is suppressed and another in which the female function is negligible or nonexistent—the reproductive mode recognized as dioecy. Evidence is presented here to support the thesis that instances of sexual lability, the presence of an array of sexual intermediates, sex-switching, and sexual niche segregation can be explained in terms of the pathway that was taken in the evolution of a particular dioecious species. In addition, the degree of sexual dimorphism seen in dioecious species is correlated with mode of pollination (insector wind-pollinated) and other ecological factors.

Sexual specialization and inbreeding avoidance in the evolution of dioecy

A major theme in plant evolutionary ecology concerns the allocation of resources to fitness-enhancing activities. Patterns of allocation have been regarded as reflecting evolved reproductive strategies. Here I review recent studies that focus on problems of resource allocation. These studies concern resource currencies, plant ontogeny and bud dynamics, reproductive costs, and clonal growth. While there is a certain amount of tension between the descriptive utility and the evolutionary validity of the reproductive strategy concept, the idea of strategy remains a robust one.

Plant reproductive strategies and resource allocation

Eleven populations of Arisaema triphyllum (L.) Schott (jack-in-the-pulpit) were stud- ied in 1978-1979. There was a range of population gender ratios, from strongly female biased to strongly male biased; gender dimorphism with respect to body size and phenology was found. The proportion of a plant's biomass found in reproductive and ancillary reproductive organs was determined for plants of each gender. The mean total dry mass of carpellate individuals was 3.5 times greater than that of staminate plants. Reproductive effort was 12% of dry mass for flowering carpellate plants, 17% for staminate plants, and 44% for mature fruiting carpellate plants. Approximately 10% of the reproductive plants in all populations were monoecious. In 1978 and 1979 several environmental factors were measured at four of the sites, and these were related to population gender ratios at each site. Carpellate individuals were predominant in less- shaded sites where both pH and soil nutrient levels were higher. Staminate individuals were concen- trated in darker microsites and in areas of lower pH. A reciprocal replant/transplant experiment was initiated in 1978, using 267 ramets of Arisaema triphyllum, from two sites with contrasting population gender ratios. Staminate, carpellate, and mon- oecious plants, reciprocally, were either replanted at the "home" site, or transplanted to the "alien" site. The gender of ramets was monitored in 1979 and 1980. The sequence of gender change in plants was nondirectional. Plants transplanted to an alien site did not maintain the gender ratio characteristic of the home site. Highly significant effects on gender change could be attributed to the environment in which plants were grown. We support the conclusion that sex change is adaptive where female reproductive costs are greater than male costs. A simple model of gender change for reproductive plants at each site was constructed, based on the results of the transplant experiment. The differential survivorship and patterns of gender flux provide possible explanations of contrasting gender ratios.

https://www.jstor.org/stable/pdfplus/1936800.pdf

Sex and Gender Dynamics in Jack-in-the-Pulpit, Arisaema Triphyllum (Araceae)

In a number of disciplines including ecology, ecotoxicology, water quality management, water resource management, fishery biology etc., there is significant interest in the testing of new materials, environmental samples (of water or sediments) and specific sites, in terms of their effects on biota. In the first instance, we consider various sources of aquatic pollution, sources typically associated with developed areas of the world. Historically, much water quality assessment has been performed by researchers with a background in chemistry or engineering, thus chemical analysis was a dominant form of assessment. However, chemical analyses, particularly of such materials as organochlorines and polyaromatic hydrocarbons can be expensive, and local environmental factors may cause the actual exposure of an organism to be little correlated with chemical concentrations in the surrounding water or sediments. To a large extent toxicity testing has proceeded independently of environmental quality assessment in situ, and the work has been done by different, and differently-trained researchers. Here we attempt to bring together the various forms of biological assessment of aquatic pollution, because in our opinion it is worth developing a coherent framework for the application of this powerful tool. Biotic assessment in its most primitive form involves the simple tracking of mortality in exposed organisms. However, in most natural environments it is extended, chronic exposure to contaminants that has the most wide-ranging and irreversible repercussions--thus measures of sub-lethal impairment are favoured. From an ecological standpoint, it is most valuable to assess ecological effects by direct study of in situ contaminant body burdens and impairment of growth and reproduction compared with 'clean' sites. A distinction is made here between bioindication and biomonitoring, and a case is made for including aquatic macrophytes (angiosperms) in studies of contaminant levels and effects in the biota. It is apparent that there is a concurrent need for laboratory-based testing of new industrial by-products before any are released in the environment, and such studies should aid the investigation of mechanisms and modes of toxicity, but environmental assessment, and tracking of improvements in environmental quality are most effectively achieved by active biomonitoring experiments.

Jon Lovett Doust

Papers

Sexual specialization and inbreeding avoidance in the evolution of dioecy

Plant reproductive strategies and resource allocation

Sex and Gender Dynamics in Jack-in-the-Pulpit, Arisaema Triphyllum (Araceae)

Biological assessment of aquatic pollution: a review, with emphasis on plants as biomonitors*

Plant Reproductive Ecology: Patterns and Strategies