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Summary In the Tai region of the Ivory Coast, mature forest is felled and burned and a crop of rice is grown; forest is then allowed to regenerate for 20 or more years before being cropped again. The top 10 cm of soil beneath a 21-year-old forest prior to felling contained 2000 viable seeds m−2 of mainly woody species of previous successional stages no longer represented in the vegetation. Seedling emergence was greater in full sunlight than under leaf canopy, especially for herbaceous species of open habitats. Burning destroyed almost half of the seeds, but overall germination of those that survived was advanced by about 2 weeks. Germination of some common pioneer species was enhanced by the fire. Provided that burning took place and the rice was planted immediately, vegetation arising from the seed bank did not present a problem. The density never exceeded 70 seedlings m−2 and little or no weeding was required. Species present at densities greater than 1 seedling 10 m−2 were all recorded in the seed bank. Resume Graines dans un sol forestier humide et leur relation avec le changement des methodes culturales en Cole d'Ivoire Dans la region Tai de Cote d'Ivoire, quand une foret atteint sa maturite, elle est abattue et brulee, puis une culture de riz est implantee; puis une foret est regeneree pour 20 ans ou plus avant d'etre de nouveau coupee. Les 10 premiers cm du sol sous une foret de 21 ans avant l'abattage contenaient 2000 graines viables m−2 des principale especes forestiers, des stades successifs anterieurs et plus representees dans la vegetation. La levee des plantules est plus elevee en pleine lumiere que sous un couvert de feuilles, specialement pour les varietes herbacees des habitats ouverts. Le brulage detruit environ la moitie des semences, mais en outre la germination de celles qui restent est avancee d'environ 2 semaines. La germination de certaines especes pionnieres communes est accrue par le feu. Pourvu que l'on brule et que le riz soit plante immediatement, la vegetation issue du stock grainier ne represente pas un probleme. La densite n'est jamais superieure a 70 plantules m−2 et il est soit inutile de desherber, soit tres facile de desherber. Les especes presentes a des densites superieures a une plantule pour 10 m2 sont toutes enregistrees dans le stock grainier. Zusammenfassung Samenvorrat im Boden bei wandernder Rodungskultur im tropischen Regenwald der Elfenbeinkuste Im Tai-Gebiet der Elfenbeinkuste wird nach dem Fallen und Abbrennen des Regenwalds Reis angebaut, bis sich der Wald fur 20 oder mehr Jahre bis zur nachsten Nutzung regenerieren kann. Die oberste 10-cm-Bodenschicht eines 21 Jahre alten Waldes enthielt vor der Rodung 2000 Samen m−2, uberwiegend Samen von Baumarten fruherer Sukzessionsstadien, die im akluellen Bestand nicht mehr vorhanden waren. Die Keimung war im vollen Licht besser als im Bestand, besonders bei krautigen Arten der Lichtungen. Durch das Abbrennen wurde etwa die Halfte der Samen vernichtet, aber die uberlebenden Samen keimten rund 2 Wochen fruher. Bei einigen Pionierarten war die Keimungsforderung durch das Abbrennen besonders deutlich. Wenn der Reis unmittelbar nach der Brandrodung gesetzt wurde, stellten die wildwachsenden Pflanzen kein Problem dar. Ihre Dichte uberschritt 70 Pflanzen m−2 nie, so dass sie uberhaupt nicht oder kaum gejatet werden mussten. Von allen Arten, die mit mehr als 0,1 Pflanzen m−2 auftraten, wurden im Boden Samen gefunden.

Seeds of Chenopodium album L. were buried under field and controlled conditions. The germination capacity of these seeds was tested over a range of conditions at regular intervals. Seed buried in the field only showed small seasonal changes in germination capacity when tested at constant temperatures in incubators. However, when germination was tested at field temperatures, seasonal changes in germination were more obvious. Nitrate and light always promoted germination. There was a strong positive interaction between the effects of the two factors. When nitrate and light were combined, exhumed seeds germinated over a much longer period of the year than in water with or without light. Desiccation only stimulated under particular conditions, for example, when germination was tested in nitrate in darkness. A regression model was developed with the data from the germination tests in incubators. The model describes the changes in dormancy and germination and estimates germination at field temperatures accurately throughout the year. Despite the absence of clear seasonal changes in the temperatures suitable for germination (computed with the model), germination in the field showed seasonal periodicity, because the field temperature and the germination-temperature range only overlapped from spring to late summer.

SUMMARY The effect of environmental conditions on changes in dormancy and germination of seeds of Sisymbrium officinale (L.) Scop, was studied. Seeds were buried in soil in the field or in incubators and at regular intervals, germination of exhumed seeds was tested over a range of conditions. Seeds that were buried in the field showed clear seasonal changes in dormancy. Fluctuations in soil moisture and nitrate content were not required for the changes in dormancy. Temperature seemed to be the only factor regulating these changes. Dormancy was relieved in periods of low temperatures and induced in periods of high temperatures. Fresh seeds and seeds buried for only a few months germinated best at elevated temperatures whereas seeds buried for a longer time germinated best at low-temperatures. Light, nitrate and desiccation stimulated germination of exhumed seeds. As a consequence, these treatments extended germination over a much longer period of the year. Germination of exhumed seeds in Petri dishes at field temperature was reasonably described by a model based on the dual role of temperature, in the regulation of both dormancy and subsequent germination.

1. It is hypothesized that the recent intensification of agricultural practice in the coastal fringe of West Connemara, Ireland, has affected the vegetation of vulnerable ecosystems like lakes and wetlands. 2. Lakes and wetlands, arranged in three transects running inland from the coast, were studied to detect an assumed decrease in the effects of eutrophication as a result of a decrease in the intensity in land use. 3. The data obtained from the vegetation survey in 1988 were compared with those collected at the same sites in 1975. The data were subjected to various multivariate statistics to detect changes in species composition and relationships between these changes and environmental variables such as pH and orthophosphate concentration in the surface waters. 4. Changes in species composition are accompanied by changes in environmental factors; notably a marked increase in orthophosphate level indicating an anthropogenic origin for most of the observed changes in vegetation. 5. Changes in species composition have also been used to construct a transition matrix for sites between 1975 and 1988. This has served as a basis for a prognosis on the state of the vegetation at these sites circa 2000. Data have also been grouped to indicate lake-specific patterns in the changes. 6. The dynamics of all plant species have been established based on their frequency of disappearance from sites since 1975 and their rate of establishment in new sites in 1988. Special attention has been given to species from the red data list. It is concluded that over the past 13 years species diversity decreased in the whole area, and especially in coastal lakes. Red data list species appeared to be the more vulnerable ones.

Competition between weeds and groundnuts was studied in two field experiments in the inland Zanderij area of Suriname. The crop was kept weed-free or without weed control for increasing periods of time after planting. Observations made at the end of each period with or without weed control provided an analysis of the growth of a crop with weed control and of a crop without weed control. Uncontrolled weed growth was concentrated between the rows. Competition with weeds reduced ground cover and LAI of the crop. DM accumulation was affected, resulting in lower yields. Plant density and number of branches per plant were not influenced. Weed competition increased stem length and reduced the number of leaves and pods per plant. Timing of flowering was not influenced. Competition for nutrients was absent, but there was competition for light and water. About 15 weed-free days after planting were sufficient to prevent yield losses. Yield losses were due to a reduction in number of pods per plant. Competition should especially be prevented in the period when the number of pods per plant is established, i.e. around 35-60 days after planting. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Two field experiments were carried out in the inland Zanderij area of Suriname to study competition between soyabeans and weeds. The crop was kept weed-free or without weed control for increasing periods of time after sowing. An analysis of the growth and development of a crop with weed control and of a crop without weed control was obtained by making observations at the end of each period of time with or without weed control. Uncontrolled weed growth concentrated mainly between the rows. Competition with weeds reduced ground cover and LAI of the crop, leading to a lower weight and yield. Plant density was not affected, but weed competition reduced the number of branches, inflorescences, and pods per plant. Timing of flowering was not influenced. Competition for water was inferred and competition for light seemed likely. Competition for nutrients was weak. Weeding in the period of pod initiation, i.e. around 45-70 days after sowing, appeared to be essential to avoid competition with weeds. A period of around 4 weed-free weeks after sowing was necessary to avoid yield reduction or too much weed growth at harvest. (Abstract retrieved from CAB Abstracts by CABI’s permission)

Summary: Resume: Zusammenfassung The effect of broadcast or band-placed application of fertilizer on the growth of weeds was studied at two locations on low-fertility acid soils in Suriname. Band placement of fertilizers considerably reduced weed growth compared to fertilizer-applied broadcast. With band-placed fertilizer the increased weed growth due to fertilizer application was concentrated on the fertilized bands. Weed growth was also stimulated in planter wheel-tracks. The spatial distribution of the weed growth is discussed in relation to the crops groundnut, sorghum and soybean, grown at the same location. Reponse des adventices au mode d'application des engrais sur des sols acides de faible fertilite au Surinam L'effet d'application d'engrais sur le rang ou en plein sur la croissance des mauvaises herbes a eteetudiee en deux lieux du Surinam sur sols acides pauvres. Le traitement sur le rang des engrais a considerablement reduit la croissance des adventices comparea l'application en plein. Avec l'application de l'engrais en ligne l'augmentation de croissance due a la fertilisation etait concentree sur les bandes fertilisees. La croissance des adventices etaient egalement stimulee dans les traces de roues de tracteurs. La distribution spatiale de la croissance des adventices est discutee en relations avec la distribution spatiale de la croissance des adventices trouve chez le sorgho, le soja, l'arachide cultives dans les memes lieux. Wirkung der Dungungsmethode auf die Verunkrautung ertragarmer, saurer Boden in Surinam An 2 Ortem mit ertragarmen, sauren Boden in Surinam wurde die Wirkung der Dungungs-methode auf das Wachstum von Unkrautern untersucht. Banddungung reduzierte das Unkrautwachstum im Vergleich zur breitwurfigen Dungung erheblich. Ein gefordertes Unkrautwachstum zeigte sich nur auf dem Dongungsband und auch auf den Fahrspuren. Die unterschiedliche Verunkrautung in Erdnus, Sorghum und Sojabohne wird diskutiert.

Summary: To assess the potential contribution of growing highly competitive crops to control Cyperus esculentus in the Netherlands, the influence of silage maize, hemp, winter barley and silage winter rye on emergence, growth and reproduction of this weed was studied in two field experiments. Growth and reproduction of C. esculentus (determined both as tuber production per plant and density of the weed in the subseuent crop maize) was reduced by each crop, compared to plots in which no crop was grown. In hemp, hardly any tuber production was observed. By growing a green manure crop after harvest of the winter barley the tuber production of the weed was reduced to about 40% of that in winter barley followed by fallow. Decreasing the light regime in a greenhouse experiment in the same order as was observed in the crops winter barley, maize and hemp, caused a similar reduction of tuber production to that found in the field. Therefore, it is suggested that competition for light is the main factor explaining the observed crop effects on the population dynamics of the weed. It is concluded that growing competitive crops as hemp in rotation may effectively complement chemical control of C. esculentus. Resume: Pour mesurer la contribution potentielle de cultures fortement competitives pour lutter centre Cyperus esculentus aux Pays-Bas, l'influence du mais ensilage, du chanvre, de l'orge d'hiver, du seigle d'hiver sur la levee, la croissance et la reproduction de cette adventice a eteetudiee dans deux experimentations au champ. La croissance et la reproduction de C. esculentus (determinees a la fois par la production de tubercules par plante et la densite d'adventice dans la culture suivante de mais) ont ete reduites par chaque culture comparativement aux parcclles laissees sans culture. Dans le chauvre, une assez forte production de tubercules a ete observed. En semant un engrais vert apres la moisson de l'orge de l'hiver, on a reduit la production de tubercules de l'adventice de 40% par rapport a celle observee en orge d'hiver suivi par une jachere. Dans une experimentation de serre en regime de lumiere decroissante de la meme nature que celle qui est observee sous les couverts d'orge d'hiver, de mais et de chanvre a entraine une reduction de la production de tubercules similaire de celle observee en plein champ. De cela, on deduit que la competition pour la lumiere est le principal facteur expliquant les effets observes des cultures sur la dynamique des populations de l'adventice. On en conclut que l'introduction d'une culture competitive comme le chanvre dans la rotation peut completer efficacement la lutte chimique contre C. esculentus. Zusammenfassung: Um den moglichen Beitrag konkurrenzstarker Kulturen zur Bekampfung von Cyperus esculentus L. in den Niederlanden zu erfassen, wurde der Einflus von Mais, Hanf, Wintergerste und Roggen auf den Austrieb, den Wuchs und die Reproduktion dieses Unkrauts in 2 Feldver-suchen untersucht. Wuchs und Reproduktion, beide in Mais als Folgekultur mit der KnoUen-zahl pro Pflanze und der Wuchsdichte bestimmt, wurden— im Vergleich zu Parzellen ohne Einsaaten— durch alle Kulturen herabgesetzt; in Hanf wurde kaum eine Bildung neuer Knollen beobachtet. Bei Anbau von Grundungungspflanzen nach der Ernte der Wintergerste wurden nur etwa 40% so viele Knollen gebildet wie bei folgender Brache. In einem Gewachshausversuch mit zunehmender Beschattung bis zu der in Wintergerste, Mais und Hanf beobachteten ergab sich eine ahnliche Abnahme der Knollenbildung wie im Freiland. Deshalb wird angenommen, das die Konkur-renz um Licht der wichtigste Faktor fur die festgestellten Einflusse der Kulturen auf die Populationsdynamik dieser Unkrautart ist. Der Anbau konkurrenzstarker Kulturen wie z. B. Hanf in einer Fruchtfolge kann die chemische Bekampfung von Cyperus esculentus wirkungs-voll erganzen.

The effects of incorporating mustard as a non-crop plant within a broccoli planting was tested for pest regulation. By varying sowing dates of the mustard, a critical period for crop-weed competition was determined, after which time, crop competition was insignificant. Mustard sowed one week after broccoli transplanting showed no reduction of broccoli yield and tended to reduce numbers of the cabbage aphid, Brevicoryne brassicae L., while increasing effective predation by syrphid larvae. Mechanisms explaining this effect are discussed in relation to diversity. The data also suggest that mustard serves as a trap crop for pest insects that normally feed on brassica crops.

The vegetation of the embankments of the Zak van Zuid-Beveland were surveyed phytosociologically, the vegetation being assigned to the Lolio-Cynosuretum plantaginetosum mediae, Arrhenatheretum elatioris picridetosum, Arrhenatheretum elatioris alopecuretosum and Ulmo-Rubetum ulmifolii . In total, including the subordinate fragmentary communities and variants, 12 communities are described, together with an indication of their synecology. The composition of the vegetation in 1986 is compared to that in 1972 and the influence of different management practices, i.e. grazing, mowing, burning and no management, on the vegetation changes is illustrated. The changes in vegetation composition and structure are clearly related to management practices. This even applies to the low level of syntaxonomic hierarchy, i.e. subassociations and variants. In general the highest intensities of grazing by the flock, i.e. between 8 and more than 15 h a month by 200 sheep per 500 m of embankment, were best suited for the improvement or maintenance of the conservation value. Under the same conditions, light grazing (less than 8 hours) proved to be insufficient.

The population biology of Galium aparine L. needs to be better understood, in order to be able to rationalize decisions about the short- and long-term control of this weed species for different cropping practices.A population dynamics model was developed to simulate the basic processes of the life cycle of G. aparine as influenced by environmental conditions. The effect of temperature, light, moisture, nitrate supply and soil structure on germination and emergence and on growth and seed production were investigated. Data were integrated in a model simulating field emergence and an ecophysiological model on crop-weed competition. The information was used to evaluate the effects of crop rotations, control levels, cultural measures and timing and choice of soil cultivation practices on the population dynamics of G.aparine. The ecophysiological model of competition between G. aparine and sugar beet was extended to predict reductions in crop yield reduction and increases or decreases in the weed population on the basis of the actual situation in the field. The variation caused by weather conditions appeared to be small.The results of simulations demonstrate the possibilities of reducing herbicide use to control G.aparine in both the short and long term, by adapting crop rotation and cropping practice, increasing mechanical control in cereals and not controlling lateemerging G. aparine plants in sugar beet.