Bibliothek

Notizen: Herbicides were applied annually for 3 years to a pasture population of Emex australis Steinh. Herbicide performance was highly variable, with significant effects on weed survival only in single seasons. In most cases, seed bank densities declined monotonically from an initial level of 1900 m−2. Seed banks decreased after seasons when E. australis contributed less than 25% to pasture dry matter. The annual decay rate of the seed bank was estimated to be 70%. Owing to the unreliability of the herbicides when applied at rates low enough to protect pasture legumes, the potential for tactical chemical control of E. australis is limited. Furthermore, it is argued that the potential for strategic chemical control is low, since the species can tolerate periods of population decline. E. australis could be managed more effectively by maintaining high densities of annual grasses and legumes, against which it is a poor competitor. Efficacité herbicide et lutte contre Emex australis dans une pâture annuelleDes herbicides ont été appliqués chaque année pendant trois ans sur une population d' Emex australis Steinh. dans une pâture. L'efficacité herbi- cide était très variable, et les effets significatifs sur la survie des mauvaises herbes ne furent que des évènements uniques, observés lors de saisons isolées. Dans la plupart des cas, les stocks semenciers ont décru de façon monotone à partir d'un niveau initial 1900 m−2. Us décrurent après les saisons oùE. australis contribuait pour moins de 25%à la matière sèche de la pâture. Le taux de décroissance annuel du stock semencier était estiméâ 70%. En raison du manque de fiabilité des herbicides lorsqu'ils sont appliqués â des doses suffisamment basses pour respecter les légumineuses de la pâture, leur intérêt pour la lutte chimique à court terme contre E. australis est limité. Leur intérêt à moyen terme est égale-ment faible car cette espece peut supporter des périodes de déclin démographique. On pourrait lutter plus efficacement contre E. australis en maintenant de fortes densités de graminées annuelles et de légumineuses, à l'égard desquelles cette mauvaise herbe est un com-pétiteur médiocre. Herbizidwirkung und die Bekämpfung von Emex australis auf einjährigem WeidelandBei jährlicher Anwendung von Herbiziden über 3 Jahre hin zur Bekämpfung von Emex australis Steinh. war die Wirkung sehr variabel; eine sig-nifikante Wirkung auf das Überleben des Unkrauts gab es nur in einzelnen Vegetations-perioden. Die Bodensamenbank nahm meistens gleichförmig von einem Ausgangsbestand von 1900 m−2 ab. Die Samendichte sank in den Jahren, wo Emex australis weniger als 25% der Trockenmasse der Weidepflanzen ausmachte. Der jährliche Rückgang der Lebensfähigkeit der Samen im Boden wurde auf 70% geschätzt. Wegen der Unzuverlässigkeit von Herbiziden, wenn sie zum Schutz der Weide-pflanzen niedrig dosiert werden, sind die Aussichten für eine chemische Bekämpfung dieser Art begrenzt. Außerdem sind langfristige Erfolge auf diese Weise kaum zu erreichen, da die Art zeitweiligen Populationsrückgang übersteht. Die Verbreitung von Emex australis sollte durch hohe Dichten einjähriger Gräser und Leguminosen wirkungsvoller zurückgedrängt werden können, weil sie gegenüber diesen Arten ein schlechter Konkurrent ist.

Notizen: Very few seeds of M. pulegium germinated in the absence of light, but relatively high levels of germination were attained over a wide range of temperatures when seeds were exposed to light. This response to temperature was reflected in patterns of seedling appearance in several field sites, where new plants were recorded between autumn and early summer. Initial mortality rates differed between cohorts which occurred within and outside a pasture community, and were higher in the presence of pasture species. Seedlings survived prolonged periods of inundation and new plants were detected under water to a depth of 10 cm. The capacity for growth under water is considered to be an important aspect of the ability of M. pulegium to invade and persist in sites which are flooded seasonally.

Notizen: Soil samples were taken along transects cut through thickets of Rubus polyanthemos at three sites, the seeds were extracted by flotation, and the viability of filled seeds of R. polyanthemos determined by staining with tetrazolium chloride. Exotic annual species were the dominant component of the soil flora at two sites formerly undercultivation and a change in seed bank composition, both in number of individuals and numberof species, was evident from the exterior to the interior of the thickets. This was not observed in a relatively undisturbed native community. The apparent absence of suitable replacement species in the seed bank at all three sites means that they must be introduced if the sites are to be rehabilitated after the thickets are removed. Viable seeds of R. polyanthemos were present in relatively low numbers (mean 179 m−2:), so that loss of viability appears to be fairly rapid.

Notizen: Populations of the winter annual Emex australis were established by initiating irrigation at monthly intervals, beginning in mid-summer. Natural seedling densities varied from 100 m−2 when irrigation was begun during late January to almost 700 m−2 when it began at the end of March. Densities were increased by sowing non-dormant seeds, but most sown seeds became dormant when introduced before autumn. Plants which emerged after the earliest irrigation grew most rapidly. Those originating from the latest irrigation grew most slowly, in association with vigorous growth of other annual species. Because it minimizes interspecific competition, early commencement of irrigation, coupled with sowings of nondormant seeds is the best method for establishing E. australis during summer. Maintenance of such populations may enhance the persistence of non-aestivating biological control agents and/or boost their numbers prior to the onset of mass germination. This approach could also prove useful for other winter annuals which are targeted for biological control.

Notizen: An investigation of seed banks found under populations of M. pulegium located within and outside pasture systems revealed that seed densities were sixteen times greater when this species occurred in pastures. The sizes of seed banks were not positively related to current seed production, but larger seed banks appeared to be associated with higher levels of soil disturbance arising from trampling by grazing stock. Densities of buried seed were very high under pasture populations (range 55 000–176 000 m−2), which illustrates the impracticality of attempts to eliminate M. pulegium from this type of community.

Notizen: Factors influencing germination and seed survival of Celtis sinensis Pers. and Cinnamomum camphora (L.) Presl. were investigated in a series of experiments conducted under natural rainfall regimes in the field and under irrigation in a growth tunnel. Germination from surface-sown seeds of C. camphora was virtually nil under natural rainfall, in contrast to almost 100% germination under irrigation. This suggests that surface-lying seeds of this species will germinate only in highly protected microsites in the field. For C. sinensis, however, in one of the two field experiments germination from surface-sown seeds was similar (≈40%) to that achieved under irrigation. Germination from intact fruits of both species was generally considerably less than from seeds. Seeds of both species were relatively short-lived under field conditions. A maximum of 1% of seeds of C. camphora remained viable at 12 months post-sowing under field conditions, whereas a maximum of 8% viable seeds of C. sinensis was detected at 24 months post-sowing in one of the field experiments.

Notizen: Homoclime analysis of three weed species of potential significance to agriculture in New Zealand was undertaken. Using the Bio-climate Prediction System computer program (BIOCLIM), climatically suitable regions in New Zealand were estimated for Homeria flaccida Sweet, Chondrilla juncea L. and Emex australis Steinh., on the basis of their respective distributions in Australia. These estimates indicated that the current eradication campaign for Homeria is warranted, owing to the existence of extensive areas with a suitable climate on both of New Zealand's main islands. For Homeria, most of the known occurrences fell within regions predicted to be climatically suitable. For Chondrilla, only very small areas were predicted to be climatically suitable, due to the generally low temperatures that prevail during its reproductive period. This suggests that the designation of Chondrilla as a quarantine pest may be unwarranted. Much of the land in the interior of the North Island was estimated to be climatically suitable for Emex. However, invasion has not occurred, despite the presence of this weed in New Zealand for over 100 years. The failure of Emex to invade perennial pastures is probably a result of its poor competitiveness, since climatic and edaphic characteristics appear to be suitable over large areas. It is concluded that homoclime analysis can be used to identify regions at risk of invasion by a species which has been weedy elsewhere, when potentially modifying factors such as soils, land use/management practices and competitive interactions are taken into account.

Notizen: Isozymic variation was investigated in five Australian populations and six reference collections of mesquite (Prosopis section Algarobia). Within nine enzyme systems, a total of 18 loci were examined in detail. 17 of which were polymorphic. Classification and ordination techniques revealed that two of the Australian populations were referable to P. pallida (syn. P. limensis) and another to a polyploid form of P. juliflora. Although the remaining two Australian populations were closely related, only one showed direct evidence of introgression of genes from P. pallida, and the existence of an additional, undetermined parent species was suggested. Owing to the apparent widespread distribution of P. pallida in northern Australia, it may be expedient to seek potential biological control agents from extra-Australian populations of either this or other species within series Pallidae. However, little is known regarding the general level of host specificity exhibited by the fauna which attacks species in section Algarobia. The overall degree of similarity of the taxa examined in this survey suggests that useful agents for Australian Prosopis might be obtained from the fauna associated with species from both series Pallidae and series Chilenses of section Algarobia.