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

Seed mass is a trait that occupies a pivotal position in the ecology of a species. It links the ecology of reproduction and seedling establishment with the ecology of vegetative growth, strategy sectors that are otherwise largely disconnected (Grime et al., 1988; Shipley et al., 1989; Leishman and Westoby, 1992). There is a startling diversity of shapes and sizes of seeds among the plant species of the world. Seeds range from the dust seeds of the Orchidaceae and some saprophytic and parasitic species (around 10 6 g), across ten orders of magnitude to the double coconut Lodoicea seychellarum (104 g) (Harper et al., 1970). Within species, seed size typically spans less than half an order of magnitude (about fourfold: Michaels et al., 1988). Most within-species variation occurs within plant rather than among plants or populations (Michaels et al., 1988; Obeso, 1993; Vaughton and Ramsey, 1998), indicating environmental effects during development rather than genetic differences between mothers. This chapter is concerned with the differences in seed size among species, and the consequences for vegetation dynamics and community composition. During the last 10–15 years, there has been considerable progress in the ecology of seed mass. Unlike many other areas of comparative plant ecology, we have substantial published information from several different scales and research styles. As well as field experiments and demographic studies with a few species at a time, we have simple experiments with larger numbers of species (ten to 50), quantification of seed mass and its correlates in whole-vegetation types (hundreds of species) and tests of consistency across different continents. The wide-scale quantification began as early as Salisbury (1942) and Baker (1972), but has been much added to and consolidated over the past 10 years (e.g. Mazer, 1989, 1990; Leishman and Westoby, 1994a; Leishman et al., 1995; Eriksson and Jakobsson, 1998). The work spanning large numbers of species is complementary to detailed experiments involving only a few species, giving a stronger sense of how widely the results from particular experiments can be generalized. Much of the literature examines how natural selection on seed size might be influenced by various environmental factors. In this context, it is at first glance surprising that seed size varies within communities across a remarkable five to six orders of magnitude (Leishman et al., 1995; Fig. 2.1). Further, there is strong overlap of seed-size distributions between quite different habitats. Within the temperate zone,

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The Evolutionary ecology of seed size

The hypothesis that large seed size provides an advantage for seedlings establishing in low soil moisture was tested under both field and glasshouse conditions, using a range of sympatric species of different seed sizes and growth forms from the semiarid woodland flora of western New South Wales. The hypothesis predicted that the relative advantage of large seeds should be greater under dry-soil than better-watered conditions. In the field, seedlings from large seeds had higher percentage emergence and survival than small seeded seedlings, but there was no evidence of a relatively greater advantage under drier soil conditions. Daytime temperatures were very high during the field experiment and even the most favourable watering treatment showed quite low rates of emergence and survival (...)

The role of seed size in seedling establishment in dry soil conditions--experimental evidence from semi-arid species.

Quantifying plant drought resistance is important for understanding plant species' association to microhabitats with different soil moisture availability and their distribution along rainfall gradients, as well as for understanding the role of underlying morphological and physiological mechanisms. The effect of dry season drought on survival and leaf-area change of first year seedlings of 28 species of co-occurring woody tropical plants was experimentally quantified in the understory of a tropical moist forest. The seedlings were subjected to a drought or an irrigation treatment in the forest for 22 weeks during the dry season. Drought decreased survival and growth (assessed as leaf-area change) in almost all of the species. Both survival and leaf-area change in the dry treatment ranged fairly evenly from 0% to about 100% of that in the irrigated treatment. In 43% of the species the difference between treatments in survival was not significant even after 22 weeks. In contrast, only three species showed no significant effect of drought on leaf-area change. The effects of drought on species' survival and growth were not correlated with each other, reflecting different strategies in response to drought. Seedling size at the onset of the dry season had no significant effect on species' drought response. Our study is the first to comparatively assess seedling drought resistance in the habitat for a large number of tropical species, and underlines the importance of drought for plant population dynamics in tropical forests.

Comparative drought-resistance of seedlings of 28 species of co-occurring tropical woody plants.

The Ecology of Seeds: Seedling establishment

SUMMARY
‘Seeds’ of 15 species collected from a range of habitats contrasting in soil water status were germinated in soils of known matric potentials ranging from near field capacity to the permanent wilting potential (– 0.05, –0.5, –1.0 and –1.5 MPa). Germination was very sensitive to soil water potential and species responded in various ways. Some showed germination responses which correlated with the soil water status of their native habitat: none of the wetland species studied could germinate to any great extent at low soil water potentials; in contrast some species associated with drier habitats achieved high levels of germination in soils as dry as –1.0 MPa (and –1.5 MPa for one ruderal species). However, other species from drier habitats failed to germinate at low soil water potentials, and it is suggested that this may be a mechanism to avoid exposing the seedling to an unfavourable environment.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1990.tb00482.x

The effect of soil water potential on seed germination of some British plants

summary

Seedlings of 15 species from a range of habitats contrasting in soil water status were grown at known soil water matric potentials ranging from near field capacity to the permanent wilting potential (−0.05. −0.5, −1.0 and − 1.5 MPa). Root and shoot growth were differentially very sensitive to soil water matric: potential: root growth continued at lower soil water matric potentials than shoot growth. All Species from drier habitats established better in drier soils than wetland species, with increased root growth in drier soils in contrast to decreased root growth of wetland species in drier soils. Seed size was thought to be an important factor in determining establishment in dry soils; the only wetland species to maintain substantial root growth in dry soils had comparatively large seeds. The seedling growth responses were compared with the same species' germination responses in relation to the above range of soil water matric potentials (established in a previous study). Sensitivity to soil moisture may occur at the germination or seedling stage of growth.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1469-8137.1991.tb00998.x

John R. Etherington

Papers

The effect of soil water potential on seed germination of some British plants

The effect of soil water potential on seedling growth of some British plants