SUMMARY (1) Measurements have been made of seasonal variation in the density and composition of the reservoir of germinable seeds present in surface (0-3 cm) soil samples collected at 6-weekly intervals from ten ecologically-contrasted sites in the Sheffield region. (2) The procedure was not designed to provide a complete assessment of the seed flora, and the methods were found to be ineffective in recovering germinable seeds of those species (e.g. Endymion non-scriptus, Viola riviniana, several Umbelliferae) in which there is only a brief interval between fulfilment of a chilling requirement and the onset of germination. (3) The techniques adopted were particularly suitable for the detection of persistent seed banks (i.e. those in which some of the component seeds are at least 1 year old), and also allowed recognition of species in which there is a transient accumulation of detached germinable seeds during the summer. (4) Comparison of the results obtained for populations of the same species in different types of habitat suggests that seasonal variation in seed number is a function of the species rather than of the environment. (5) It is concluded that the major evolutionary force determining the nature of the seed bank is the selective advantage derived from mechanisms of seed dormancy and germination which allow seedlings to evade the potentially-dominating effects of established plants. (6) From the data collected in this study, four types of seed bank (Types I-IV) have been recognized, and an attempt has been made to assess their ecological significance. (7) The transient seed banks (Types I and II) are adapted to exploit the gaps created by seasonally-predictable damage and mortality in the vegetation, whilst the persistent seed bank (Type IV) confers the potential for regeneration in circumstances where disturbance of the established vegetation is temporally and/or spatially unpredictable. A second type of persistent seed bank (Type III) has characteristics intermediate between those of Types I and IV, and contains some seeds which germinate soon after release and others which are more persistent in the soil. (8) A feature of the results was the lack of a general correspondence between the species-composition of the seed flora and that of the associated vegetation. At certain sites, substantial persistent seed banks were detected for species which were either extremely scarce or did not occur at all in the established vegetation. (9) Both transient and persistent types of seed banks were represented at each of the ten sites; this is consistent with the hypothesis that complementary mechanisms of regeneration are involved in the maintenance of floristic diversity.

http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1365-2745/homepage/SeasonalVariation.pdf

Seasonal variation in the seed banks of herbaceous species in ten contrasting habitats

The term phenology is derived from the Greek word phaino meaning to show or to appear. Hence, phenology is defined as the study of the seasonal timing of life cycle events. For plants the seasonal timing of such events can be critical to survival and reproduction. In agriculture the most common failure of introduced crops is the inability to adjust to the seasons imposed by the new, environment (68). In the past few years, interest in the ecology and evolution of timing of life cycle events has grown. Here we review the literature on phenological patterns of germination, flowering, and fruiting (including dispersal).

/pdf/phenological-patterns-of-terrestrial-plants-2339nfk6dp.pdf

Phenological Patterns of Terrestrial Plants

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

Results of a literature survey indicate that weed population density and biomass production may be markedly reduced using crop rotation (temporal diversification) and intercropping (spatial diversification) strategies. Crop rotation resulted in emerged weed densities in test crops that were lower in 21 cases, higher in 1 case, and equivalent in 5 cases in comparison to monoculture systems. In 12 cases where weed seed density was reported, seed density in crop rotation was lower in 9 cases and equivalent in 3 cases when compared to monocultures of the component crops. In intercropping systems where a main crop was intersown with a "smother" crop species, weed biomass in the intercrop was lower in 47 cases and higher in 4 cases than in the main crop grown alone (as a sole crop); a variable response was observed in 3 cases. When intercrops were composed of two or more main crops, weed biomass in the intercrop was lower than in all of the component sole crops in 12 cases, intermediate between component sole crops in 10 cases, and higher than all sole crops in 2 cases. It is unclear why crop rotation studies have focused on weed density, whereas intercropping studies have focused on weed biomass. The success of rotation systems for weed suppression appears to be based on the use of crop sequences that create varying patterns of resource competition, allelopathic interference, soil disturbance, and mechanical damage to provide an unstable and frequently inhospitable environment that prevents the proliferation of a particular weed species. The relative importance and most effective combinations of these weed control tactics have not been adequately assessed. In addition, the weed-suppressive effects of other related factors, such as manipulation of soil fertility dynamics in rotation sequences, need to be examined. Intercrops may demonstrate weed control advantages over sole crops in two ways. First, greater crop yield and less weed growth may be achieved if intercrops are more effective than sole crops in usurping resources from weeds or suppressing weed growth through allelopathy. Alternatively, intercrops may provide yield advantages without suppressing weed growth below levels observed in component sole crops if intercrops use resources that are not exploitable by weeds or convert resources to harvestable material more efficiently than sole crops. Because of the difficulty of monitoring the use of multiple resources by intercrop/weed mixtures throughout the growing season, identification of specific mechanisms of weed suppression and yield enhancement in intercrop systems has so far proven elusive. Significant advances in the design and improvement of weed-suppressive crop rotation and intercropping systems are most likely to occur if three important areas of research are addressed. First, there must be continued attention to the study of weed population dynamics and crop-weed interference in crop rotation and intercropping systems. More information is needed concerning the effects of diversification of cropping systems on weed seed longevity, weed seedling emergence, weed seed production and dormancy, agents of weed mortality, differential resource consumption by crops and weeds, and allelopathic interactions. Second, there needs to be systematic manipulation of specific components of rotation and intercropping systems to isolate and improve those elements (e.g., interrow cultivation, choice of crop genotype) or combinations of elements that may be especially important for weed control. Finally, the weed-related impacts of combining crop rotation and intercropping strategies should be assessed through careful study of extant, complex farming systems and the design and testing of new integrated approaches. Many aspects of crop rotation and intercropping are compatible with current farming practices and could become more accessible to farmers if government policies are restructured to reflect the true environmental costs of agricultural production.

Crop Rotation and Intercropping Strategies for Weed Management

The Demography of Plants

Emergence and longevity of seeds of annual weeds in cultivated and undisturbed soil.

Summary. Freshly-collected seeds of twenty species of annual weeds were uniformly mixed with 2.5, 7.5 or 15 cm of soil confined in cylinders sunk in the ground outdoors. The soil was either cultivated four times a year or left undisturbed, seedling emergence was recorded, and after 5 years the numbers of viable seeds remaining were determined.



Total seedling emergence from seeds incorporated to 2·5, 7·5 and 15 cm amounted to 75, 65 and 54% respectively of those added when the soil was cultivated, and 58, 36 and 21% where it was left undisturbed. The corresponding numbers of viable seeds remaining after 5 years were 2·3, 4·0 and 7·7% respectively for cultivated soil, and 6·8, 16·5 and 31·6% of the seeds initially added and then not disturbed. Individual species varied in their responses to increasing depth of incorporation and to cultivation; these differences are discussed in relation to the dormancy characteritics of the seeds.



Devenir des semences de quelques mauvaises herbes annuelles a differentes profondeur dans un sol cultive et non cultive



Resume. Des semences recoltees recemmeni sur vingt especes de mauvaises herbes an-nuelles furent uniformement melangees avec 2,5; 7,5 ou 15 cm de sol contenu dans des cylindres stockes dans le sol en dehors du laboratoire. Le sol fut travaille quatre fois dans l'annee, ou au contraire laisse sans facon culturale, la levee dts plantules fut enregistree, et, apres 5 ans, les semences viables restantes furent denombrees.



La levee totale des semences incorporees a 2,5; 7,5 et 15 cm fut respectivement de 75, 65 et 54% pour les semences ajoutees au sol travaille et de 58, 36, et 21 % pour les semences en sol non travaille. Les nombres correspondants de semences restees viables apres 5 annees representerent respectivement 2,3; 4,0 et 7,7% du nombre initial dans le sol travaille et 6,8; 16,5 et 31,6% dans le sol non travaille, Les especes, considerees individuellement, presenterent des reactions variees a l'accroissement de la profondeur d'enfouissement et au travail du sol; ces differences sont examinees en relation avec les caracteratiques des dormances des semences.



Verhalten von Samen einiger annueller Unkraauter in versckiedenen Tiefen eines bearbeiteten und nicht bearbeiteten Bodens



Zusammenfassung. Frisrh gesammelte Samen von zwanzig Arten annueller Unkrauter wurden mit Boden gleichiormig gemischt und In Zylindern in 2,5; 7,5 oder 15 cm Tiefe draussen im Boden gclagert. Der Boden wurde entweder viermal im Jahr bearbeitet oder blieb unbcarbeitet. Das Auflaufen der Samen wurde ermittelt, und nach 5 Jahren wurde die Zahl der lebensfahig gebliebenen Samen bestimmt.



Der gesamte Auflauf der Samen, die in 2,5; 7,5 und 15 cm Tiefe eingebracht worden waren, belief sich bei dem bearbeiteten Boden auf 75; 65 und 54% der untergemischten Samen. Im nicht bearbeiteten Boden waren cs 58; 36 und 21%. Die entsprechende Anzahl der nach 5 Jahren noch lebensfaAhig gebliebenen Samen belief sich im bearbeiteten Boden auf 2,3; 4,0 und 7,7% und im nicht bearbeiteten Boden auf 6,8; 16,5 und 31,6% der ausgcbrachten Samen. Vcrsrhiedene Arten zeigten in ihrer Reaktion aufdie zunehmende Tiefe ihrer Aussaat und aufdie Bearbeitung ein abweichendes Verhalten. Diese Unterschiede werden in Bczug zu den Dormanzcharaktc Hstika der Samen diskutiert.

Fate of seeds of some annual weeds in different depths of cultivated and undisturbed soil

Seed banks in soil

Summary. In a 6-year experiment with a naturally occurring population of viable weed seeds, the numbers in the top 9 in. of soil decreased exponentially from year to year in the absence of further seeding. The rates of loss were equivalent to 22% per year in undisturbed soil, 30% per year on plots dug twice a year (March and September) and 36% per year on those dug four times a year (March, June, September, December). Seed numbers of individual species also decreased exponentially, although not all at the same rates.



On the dug plots, the numbers of seedlings that emerged each year decreased exponentially once the regimes had become established. The proportions of the viable seeds which gave rise to seedlings in the course of a year were 7 % on plots dug twice and 9 % on those dug four times a year; although there was some variation, these proportions remained much the same from year to year. On undisturbed soil the number of seedlings that emerged declined rapidly, and in the 4th year represented no more than 03% of the viable seeds still present in the top 9 in. of soil.



Resume. Resume. Au cours d'une experience qui a dure 6 ans, le nombre des semences viables de mauvaises herbes dans les 23 premiers centimetres du sol a decru exponentiellement d'annee en annee, en l'absence de tout nouvel ensemencement. Les taux de diminution furent respectivement de 22% par an dans un sol non travaille, de 30% par an dans les parcelles retournees 2 fois dans l'annee (en mars et septembre) et de 36% par an dans celles retournees qualre fois dans I'annee (mars, juin, septembre et decembre). Les nombres de semences des especes considerees individuellement diminuerent aussi de facon exponentielle mais pas toutes au meme taux.



Dans les parcelles travaillees, le nombre de plantules qui leverent chaque annee decrut exponentiellement a partir du moment ou les regimes de travail du sol furent elablis. Les proportions de semences viables qui donnerent des plantules au cours d'une annee, furent de7 % sur les parcelles retournees 2 fois et de 9 % sur celles retournees quatre fois durant I'annee; malgre quelques variations, ces proportions restarent a peu pres les memes d'une,annae a l'autre. Sur le sol non travaille, le nombre de plantules qui levarent diminua rapidement et, dans la quatriame annec, ne representa pas plus de 0,3% des semences viables encore presentes dans les 23 premiers centimetres du sol.



Zusammenfassung. In 6-jahrigen Versuchen auf einem naturlich verunkrauteten Feld nahm die Zahl der Unkrautsamen im Boden in den obersten 23 em im Laufe der Jahre nach einer Expo ten tialfunk tion ab, sofern Aussamen verhindert wurde. Im unbearbeiteten Boden betrug die Abnahme 22% jahrlich, in zweimal bearbeitetem (Marz und September) 30% und bei viermaliger Bearbeitung (Marz, Juni, September und Dezember) 36 %. Die Samenzahl der einzelnen Arten folgt in der Abnahme derselben Gesetzmassigkeit, obgleich das absolute Ausmass der Abnahme artverschieden war.



Auf den bearbeiteten Flachen nahm die Zahl der auBaufenden Keimpfianzen nach derselben Expotentialfunktion ab und betrug jahrlich 7% der lebensfahigen Samen auf den zweimal bearbeiteten Flachen und 9% auf den viermal bearbeiteten; obgleich diese Bezifhungen in ihrem absoluten Ausmass etwas schwankten, waren sie in jedem Jahr doch weitgehend dieseiben. Auf unbearbeitetem Boden nahm die Zahl der auflaufenden Keimpflanzen sehr rasch ab, im 4. Jahr betrug sie nicht mehr als 0,3% der in den oberaten 23 cm des Bodens noch vorhandenen lebenstahigen Samen.

Effect of cultivation on the numbers of viable weed seeds in soil

Summary: 

Eighty comparisons were made between the numbers of weed seedlings emerging after seedbed preparation and the numbers of apparently viable seeds extracted by sieving and flotation from samples of the top 10 cm of soil. When soil moisture was adequate, the total seedling numbers represented 3–6% of the numbers of seeds; when dry weather followed cultivation, the percentages were lower than this. The percentages were relatively high for Poa annua L. and Stellaria media (L.) Vill. but low for Chenopodium album L. and Papaver spp. Relatively high numbers of seeds of Potygonum aviculare L. gave rise to seedlings in early spring but few appeared on seedbeds prepared after mid-May; the reverse was true for Aphanes arvehsis L. It is suggested that data of this kind could provide a basis for predicting weed floras following cultivation at any time of year from determinations of seed numbers made at the start of the year.

Resume: 

Relations quantitatives, apres travail du sol, entre la fiore des mauvaises herbes et la population des semences dans le sol



Quatre-vingt comparaisons one ete faites entre les nombres de plantules de mauvaises herbes qui ont leve apres la preparation du lit de semences et les nombres de semenees apparemment viables extraites par tamisage et flottaison d'echantillons preleves dans les 10 premiers centimetres du sol. Lorsque l'humidite du sol a ete convenable, les nombres totaux de plantules ont atteint 3 a 6% des nombres de semences; quand un temps sec a suivi le travail du sol, les pourcentages ont ete plus faibles. Ces pourcentages ont ete relativement eleves pour Poa annua L. et Stellaria media (L.) Vill., mais pas pour Chenopodium album L. et Papaver spp. Des nombres relativement importants de semences de Polygonum aviculare L. ont donne naissance a des plantules au debut du printemps, mais peu sont apparues sur les lits de semences prepares apres la mi-mai; le contraire s'est produit pour Aphanes arvensis L., II est suggere que les resultats de cette nature pourraient fournir une bonne base de prevision des flores de mauvaises herbes, apres travail du sol, a tout moment de I'annee, en utilisant les determinations des nombres de semences faites au debut de I'annee.

Zusammenfassung

Quantitative Beziehungen zwisehen der Unkrautfiora nach einer Bodenbearbeitung und der Samenpopulation im Boden



Es wurden 80 Vergleiche zwisehen der Anzahl der naeh der Saatbettbereitung aufgelaufenen Unkrauter und der nach Sieben und Wasehen von Bodenproben der obersten 10 cm-Schicht festgestellten Anzahl augenscheinlich lebensfahiger Samen durchgefuhrt, Bei ausreichender Bodenfeuchtigkeit entsprach die Zahl aufgelaufener Pflanzen 3–6% der im Boden vorhandenen Samen; war es nach der Bodenbearbeitung trocken, war dieser Anteil geringer, Der Prozentsatz an aufgelaufenen Pflanzen war fiir Poa annua L. und Stellaria media (L.) Vill. relativ hoch, fur Chenopodium album L. und Papaver spp. aber gering. Der vergleichsweise groBe Anteil an Samen von Polygonum aviculare L. fuhrte im zeitigen Fruhjahr zu entspre-chend vielen Keimpflanzen, es keimten aber nur wenige, wenn das Saatbett erst nach Mitte Mai bereitet wurde; bei Aphanes arvensis L. war es umgekehrt. Die Kenntnis der Zahl an Un-krautsamen im Boden am Anfang des Jahres konnte die Grundlage fur eine Vorhersage der Verunkrautung (nach einer Bodenbearbeitung) zu einem beliebigen Zeitpunkt wahrend des Jahres sein.

H. A. Roberts

Papers

Emergence and longevity of seeds of annual weeds in cultivated and undisturbed soil.

Fate of seeds of some annual weeds in different depths of cultivated and undisturbed soil

Seed banks in soil

Effect of cultivation on the numbers of viable weed seeds in soil

Quantitative relationships between the weed flora after cultivation and the seed population in the soil