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

Using a simple two-patch model, we examine how patterns of spatial and temporal variation in carrying capacities affect natural selection on dispersal. The size of the population in each patch is regulated separately, according to a discrete-generation logistic equation, and individuals disperse from each patch at propensities determined by their genotype. We consider genotypes that express the same dispersal propensities in both patches and genotypes that express patch-specific disperal propensities. Contrary to previous analyses, our results show that some level of dispersal is favored by selection under almost all regimes of habitat variability, including a spatially varying and temporally constant environment. Furthermore, two very different polymorphisms are favored under different conditions. When carrying capacities vary spatially but not temporally, any number of genotypes with patch-specific dispersal propensities in ratios inversely proportional to the ratio of the carrying capacities can coexis...

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The Evolution of Dispersal in Spatially and Temporally Varying Environments

This review assesses the state of our knowledge about comparative seed shadows. Using data presently available in the literature, I compare the slopes (on a log-linear scale) of seed shadows for plants with different morphologically characterized modes of dispersal. The seeds of many species have no evident morphological adaptation for dispersal and seem to achieve only short-distance dispersal. Seed shadows for herbaceous species with devices for wind have flatter slopes and more distant modes and maxima than those of ballists, which in turn exceed those with no special devices. Seed shadows for wind-dispersed trees and shrubs had similar or steeper slopes than those for vertebrate-dispersed species in this sample. Species with poor mechanisms for dispersal in space only sometimes had the capacity for better dispersal in time (dormancy). Although some species exhibited seed shadows sufficiently steep to be predicted to colonize new-habitat in a ‘front’ or phalanx pattern, actual colonization patterns must reflect many other factors.

Dispersal mode, seed shadows, and colonization patterns

Maternal effects on seed traits such as germination are important components of the life histories of plants because they represent the pathway from adult to offspring: the pathway that completes the life cycle. Maternal environmental effects on germination influence basic life-history expression, natural selection on germination, the expression of genetic variation for germination and even the genes involved in germination. Maternal effects on seed traits can even influence generation time and projected population growth rates. Whether these maternal environmental effects are imposed by the maternal genotype, the endosperm genotype or the embryonic genotype, however, is as yet unknown. Patterns of gene expression and protein synthesis in seeds indicate that the maternal genotype has the opportunity to influence its progeny's germination behaviour. Investigation of the phenotypic consequences of maternal environmental effects, regardless of its genetic determination, is relevant for understanding the variation in plant life cycles. Distinguishing the genotype(s) that control them is relevant for predicting the evolutionary trajectories and patterns of selection on progeny phenotypes and the genes underlying them.

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Completing the cycle: maternal effects as the missing link in plant life histories

The time at which a seedling emerges can determine its future success as a plant. Despite the large number of studies that have examined the effect of emergence time on different components of plant fitness (survival, growth, and/or fecundity), the potential evolutionary response to selection on seedling emergence date is still poorly known. In this study, we review 55 of those studies by a random-effects meta-analysis, considering the phylogenetic relatedness among taxa. We test the following hypotheses: (1) early emergence increases seedling survival, growth, and fecundity, (2) early emergence is more advantageous to large-seeded species than to small-seeded ones, as the former can compensate for the lower number of seeds by increasing seedling survival, (3) perennial plants benefit more than annuals from early emergence, as the iteroparity of the former allows them to risk seedling emergence to the best conditions each year, whereas the semelparity of the latter forces them to spread the risk of emergence over time, and (4) the effect of emergence time may depend upon the experimental conditions (field vs. controlled experiments in a greenhouse or laboratory). Our results show that early emergence differentially affects components of plant fitness, with no effect on seedling survival but large benefits to seedling growth and fecundity. Such effects vary depending upon intrinsic factors like seed size or life-form, and also upon methodology (census time and experimental conditions). Large-seeded species gain from emerging early by growing more during their first growing seasons, although they survive and reproduce similarly to small-seeded species. The survival benefit of early emergence is greater in perennial than in annual species, thus supporting hypothesis 3. The relationship between emergence time and seedling growth appears to be stronger under controlled conditions than in the field, probably as a result of the unlimited nutrient and water resources of the former. In field conditions, in contrast, limited resources probably decelerate the growth of early seedlings, precluding the detection of differences between these and late seedlings.

Early emergence enhances plant fitness: a phylogenetically controlled meta‐analysis

Heterosperman pinnatum is an annual composite that produces heteromorphic achenes, with the proportion of achene morphs varying geographically. Field, greenhouse, and laboratory experiments were used to examine the ecological consequences of the heteromorphism. Achenes were classified into three types based on morphology. Central, intermediate, and peripheral morphs, named for the positions occupied within the fruiting head, range in shape from long, thin—beaked achene with barbed awns (central) to short wide achenes without beaks or awns (peripheral). Considerable within— and between—population phenotypic variation in achene and head characteristics was documented. Field experiments using artificial dispersal agents showed that central achenes are the most likely to adhere to animals and that the proportion of a population's achenes presented for dispersal that adhere depends on the proportion of central achenes produced in the population and on the proportion of achenes with adhesive awns. Once achenes had reached the ground, mean and median secondary dispersal distances were short (@<20 cm). Central achenes lose innate dormancy earlier than peripheral achenes during the period between autumn achene production and the onset of the summer rainy season. Germination is inhibited by darkness, though darkness becomes less inhibitive at the onset of the germination season (earlier for central achenes). In the laboratory, peripheral achenes germinated over a slightly narrower range of temperatures than central achenes. Percent germination was greater for central than intermediate achenes, and for intermediate than peripheral achenes, in two natural germination events in the field. Seed bank samples indicated that few if any achenes remain dormant between years. No significant differences in the growth rates of young seedlings were found in the greenhouse. A demographic experiment documented trade—offs resulting from the dormancy differences: central achenes tended to germinate earlier than intermediate and peripheral ones, and early germination resulted in greater mortality, but survivors that had germinated earlier attained greater size and produced more seeds. Intermediate achenes, while intermediate in all components of fitness, had the highest total fitness. This was because they resembled peripheral achenes in germinating late and having high survival, but they resembled central achenes in having higher percent germination. The results of these experiments suggest that a achene behavior ranges from "low—risk" (peripheral achenes) to "high—risk" (central achenes) within the progeny of a single individual, while individuals and populations vary in the proportion of offspring exhibiting each type of behavior.

The Ecology of Seed Heteromorphism in Heterosperma Pinnatum in Central Mexico

Heterosperma pinnatum is an annual composite that produces heteromor- phic achenes, with the proportion of achene morphs varying geographically. Field, green- house, and laboratory experiments were used to examine the ecological consequences of the heteromorphism. Achenes were classified into three types based on morphology. Cen- tral, intermediate, and peripheral morphs, named for the positions occupied within the fruiting head, range in shape from long, thin-beaked achenes with barbed awns (central) to short wide achenes without beaks or awns (peripheral). Considerable within- and be- tween-population phenotypic variation in achene and head characteristics was documented. Field experiments using artificial dispersal agents showed that central achenes are the most likely to adhere to animals and that the proportion of a population's achenes presented for dispersal that adhere depends on the proportion of central achenes produced in the population and on the proportion of achenes with adhesive awns. Once achenes had reached the ground, mean and median secondary dispersal distances were short (<20 cm). Central achenes lose innate dormancy earlier than peripheral achenes during the period between autumn achene production and the onset of the summer rainy season. Germination is inhibited by darkness, though darkness becomes less inhibitive at the onset of the ger? mination season (earlier for central achenes). In the laboratory, peripheral achenes ger- minated over a slightly narrower range of temperatures than central achenes. Percent germination was greater for central than intermediate achenes, and for intermediate than peripheral achenes, in two natural germination events in the field. Seed bank samples indicated that few if any achenes remain dormant between years. No significant differences in the growth rates of young seedlings were found in the greenhouse. A demographic experiment documented trade-offs resulting from the dormancy differences: central achenes tended to germinate earlier than intermediate and peripheral ones, and early germination resulted in greater mortality, but survivors that had germinated earlier attained greater size and produced more seeds. Intermediate achenes, while intermediate in all components of fitness, had the highest total fitness. This was because they resembled peripheral achenes in germinating late and having high survival, but they resembled central achenes in having higher percent germination. The results of these experiments suggest that achene behavior ranges from "low-risk" (peripheral achenes) to "high-risk" (central achenes) within the progeny of a single individual, while individuals and populations vary in the proportion of offspring exhibiting each type of behavior.

Gabriela Corral

Papers

The Ecology of Seed Heteromorphism in Heterosperma Pinnatum in Central Mexico

Heterosperma pinnatumin central mexico1