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

At the core of plant regeneration, temperature and water supply are critical drivers for seed dormancy (initiation, break) and germination. Hence, global climate change is altering these environmental cues and will preclude, delay, or enhance regeneration from seeds, as already documented in some cases. Along with compromised seedling emergence and vigour, shifts in germination phenology will influence population dynamics, and thus, species composition and diversity of communities. Altered seed maturation (including consequences for dispersal) and seed mass will have ramifications on life history traits of plants. Predicted changes in temperature and precipitation, and thus in soil moisture, will affect many components of seed persistence in soil, e.g. seed longevity, dormancy release and germination, and soil pathogen activity. More/less equitable climate will alter geographic distribution for species, but restricted migratory capacity in some will greatly limit their response. Seed traits for weedy species could evolve relatively quickly to keep pace with climate change enhancing their negative environmental and economic impact. Thus, increased research in understudied ecosystems, on key issues related to seed ecology, and on evolution of seed traits in nonweedy species is needed to more fully comprehend and plan for plant responses to global warming.

Climate change and plant regeneration from seed

Survival Analysis: Techniques for Censored and Truncated Data

We compiled information from the international literature to quantify the relationships between seed mass and survival through each of the hazards plants face between seed production and maturity. We found that small-seeded species were more abundant in the seed rain than large-seeded species. However, this numerical advantage was lost by seedling emergence. The disadvantage of small-seeded species probably results from size-selective post-dispersal seed predation, or the longer time small-seeded species spend in the soil before germination. Seedlings from large-seeded species have higher survival through a given amount of time as seedlings. However, this advantage seems to be countered by the greater time taken for large-seeded species to reach reproductive maturity: our data suggested no relationship, or perhaps a weak negative relationship, between seed size and survival from seedling emergence through to adulthood. A previous compilation showed that the inverse relationship between seed mass and the number of seeds produced per unit canopy area per year is countered by positive relationships between seed mass, plant size and plant longevity. Taken together, these data show that our old understanding of a species' seed mass as the result of a trade-off between producing a few large offspring, each with high survival probability, versus producing many small offspring each with a lower chance of successfully establishing was incomplete. It seems more likely that seed size evolves as part of a spectrum of life history traits, including plant size, plant longevity, juvenile survival rate and time to reproduction.

Seed size and plant strategy across the whole life cycle

Germination in light and darkness was compared after cold stratification of seeds of 54 species known or suspected to accumulate persistent seed banks. Germination became less dependent on light with increasing seed mass. This pattern was clear in a direct correlation of individual species data (P <0.0001) as well as when considering phylogenetically independent contrasts (P <0.001). The latter analysis suggests that light response and seed mass coevolved.

Large-seeded species are less dependent on light for germination than small-seeded ones

Variation in dormancy level was tested in seeds of four species, each collected from three populations in 1994 and 1995 (experiment 1). Germination was tested in light and darkness on recently-harvested seeds and on those after-ripened in dry storage for one year. In addition, seeds from each of eight individual plants within each of eight populations were tested for germination when recently harvested and after warm stratification or cold stratification followed by a drying period (experiment 2). Seeds from the two years differed in dormancy level in Silene noctiflora, Sinapis arvensis and Spergula arvensis. Germination percentage differed significantly among populations in Sinapis arvensis and Spergula arvensis in both experiments and in Thlaspi arvense in experiment 2. Furthermore, dormancy level in seeds from different mother plants also varied in the three species tested in experiment 2. Variations at the three levels tested (year, population and mother plant) indicate that these species have a random pattern of variation in dormancy level. It is concluded that variation in seed dormancy among mother plants, populations and years must be taken into account when testing the germination characteristics of a species and also when attempting to model weed seed bank dynamics.

Variation in seed dormancy among mother plants, populations and years of seed collection

We recorded germination in three different light environments (light, dark, and after a short light exposure) in eight annual weed species. Seeds were buried outdoors at the end of November 1994 and exhumed monthly from March 1995 to April 1996. All species exhibited substantial seasonal changes in dormancy level, and the patterns suggest that seeds of Papaver rhoeas germinate strictly in the autumn; Capsella bursa-pastoris, Descurainia sophia, Spergula arvensis, and Urtica urens mainly in the autumn; Chenopodium suecicum strictly in the spring; and Matricaria perforata mainly in the spring. Lapsana communis showed inconsistent dormancy changes over the year. All species had acquired a light requirement for germination after being in the soil, and in many cases the short light exposure (1050 μmol∙m−2) was enough to fulfil this requirement. The demonstrated seasonal changes in light sensitivity in several of the species will have to be taken into account in attempts to photocontrol weeds. By using the shor...

Seasonal variation in dormancy and light sensitivity in buried seeds of eight annual weed species.

A soil disturbance treatment often results in more annual weed seedlings emerging when conducted in light than when conducted in darkness, suggesting that seed germination in some species can be induced by short-duration light exposure (SDLE). We tested 70 species, most of them agricultural weeds, for their response to SDLE. Seeds were wet and cold stratified in darkness for 18 weeks before being subjected to light (12 hours per day), darkness, or 5 s light exposure (PAR: 210 mol m -2 s -1 ) followed by darkness. Of the 44 species for which meaningful comparison was possible, germination was stimulated by SDLE in 24, while the other 20 showed no or inconsistent responses. Eight of 11 plant families contained species that responded to SDLE. Extreme light sensitivity was shown equally by summer annual, winter annual and perennial species. There was no tendency for small-seeded species to be more light sensitive than largeseeded ones. Most species were represented by seeds from three populations and in many cases, large differences in percentage germination were found. Statements about a species' germination biology based on studies of a single population must therefore be interpreted with caution. The results suggest that a large number of weed species could potentially be 'photocontrolled'. However, the large variations in dormancy level between populations and the relatively small reduction in germination percentage in many cases imply that the effects of a strategy to photo-control, e.g. harrowing in darkness, can be unpredictable and sometimes small.

Seed germination after short-duration light exposure : implications for the photo-control of weeds

Populations of seeds can vary greatly in their dormancy-breaking and germination characteristics. The purpose of this study was to determine if such dormancy differences are levelled out by cold stratification. Seeds of 33 annual weed species, each represented by three populations, were tested in light and darkness 7 weeks after harvest and after two stratification treatments: 18 weeks at 3 °C in the laboratory and 19 weeks outdoors in soil during winter. Cold stratification removed population differences in some species, but in several species such differences became apparent only after stratification. This happened either because dormancy became stronger in weakly dormant seeds (winter annuals) or weaker in strongly dormant seeds (summer annuals). In several species, the light requirement for germination increased after stratification. These results clearly indicate that germination tests performed on fresh seeds from a single population may not adequately predict germination percentages in the field.

Lars Andersson

Papers

Large-seeded species are less dependent on light for germination than small-seeded ones

Variation in seed dormancy among mother plants, populations and years of seed collection

Seasonal variation in dormancy and light sensitivity in buried seeds of eight annual weed species.

Seed germination after short-duration light exposure : implications for the photo-control of weeds

Does cold stratification level out differences in seed germinability between populations