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Seedling Emergence and Longevity of Senecio Madagascariensis Poir

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Seedling Emergence and Longevity of Senecio Madagascariensis Poir 14 Plant Protection Quarterly Vol.11(1) 1996 when seedlings emerged, how long plants Seedling emergence and longevity of Senecio lived and when they flowered as a basis for developing a management strategy. madagascariensis Poir. (fireweed) in coastal south- eastern Australia Materials and methods Study sites B.M. SindelA and P.W. MichaelB, School of Crop Sciences, University of Three permanent quadrats, each 8 m2 (2 × Sydney, New South Wales 2006, Australia. 4 m), were marked out in January and February 1986 at three pasture sites in the Present addresses: County of Cumberland, New South A Department of Agronomy and Soil Science, University of New England, Wales. Quadrats 1 and 2 were located on Armidale, New South Wales, 2351, Australia. the University of Sydney Agronomy Unit, Camden (34°01’S; 150°41’E), and a third B 5 George Street, Epping, New South Wales, 2121, Australia. on a farm 20 km north-east at Hoxton Park (33°56’S; 150°51’E). Quadrat 1 was Summary pyrrolizidine alkaloid content (Culvenor located in a lightly wooded area on low Monitoring of the population dynamics unpublished data, cited by Bull et al. 1968, fertility, unimproved pasture with mod- of Senecio madagascariensis Poir. McBarron 1976) and is a vigorous com- erate cover (approximately 50%) of (fireweed), an introduced African weed petitor (Sindel 1987). Whilst its potential Paspalum dilatatum Poir. (paspalum) and of pastures in south-eastern Australia, distribution appears to be restricted to Themeda triandra Forssk. (kangaroo showed that population size varied eastern Australia (Sindel and Michael grass). Quadrat 2 was in a comparatively greatly between sites and between 1992a), it is one of the most troublesome bare cultivation paddock (unworked for years; peaks were reached in autumn weeds in that region (Sindel and Michael 18 months), had compacted soil and scat- and spring, coinciding with flushes of 1988). tered plants of Cynodon dactylon (L.) Pers. germination. Small numbers of seed- The life cycle and population dynamics (couch), P. dilatatum and Plantago lings emerged throughout most of the of S. madagascariensis are complex. Al- lanceolata L. (lamb’s tongue) (approxi- year. Survival curves typically showed though the plant is most commonly a mately 20% cover). Both Camden quadrats rapid mortality following germination winter annual, under some conditions it were ungrazed. Quadrat 3 was a grazed and again nearing senescence in late acts as a biennial or a short-lived peren- C. dactylon dairy pasture, had compara- spring and summer. Mean life expect- nial (Sindel 1986). Individual plants can tively dense vegetation cover (approxi- ancy was 1.5 months at a site where the flower at most times of the year, and den- mately 90%), and was continuously pasture was grazed and relatively vigor- sities fluctuate dramatically, similar to stocked at approximately 1.6 cows ha-1. It ous, compared with 3.7 months at two species such as Senecio jacobaea L. (rag- also contained many Soliva pterosperma less-productive, ungrazed sites. The bi- wort) (Harper and Wood 1957, Forbes (Juss.) Less. (bindii) plants and was heav- ennial behaviour of plants depended 1974) and Carduus nutans L. (nodding ily infested with Senecio madagascariensis more on site characteristics, e.g. soil con- thistle) (Popay et al. 1987). As with many in 1985 after a summer fire promoted es- dition, than climatic factors. Comple- other weeds (Roberts 1964), flushes of tablishment. Quadrat 1 was the most shel- mentary data on the demise of S. germination and patterns of seedling re- tered of the three, being surrounded by madagascariensis were obtained from a cruitment of S. madagascariensis are tall grass and shaded by a nearby tree; high density population (>5000 plants closely related to seasonal climatic condi- Quadrats 2 and 3 were more exposed. m-2) which was stimulated by cultiva- tions, particularly temperature and rain- The major grass species present at each tion. Fertilizing of pastures in autumn fall. Changes in pasture management site were warm-season perennials, grow- increased the rate of seedling emer- practices and soil conditions are also ing most vigorously from spring through gence, growth and development, while likely to interact with environmental fac- to autumn. P. dilatatum and T. triandra slashing once during early vegetative tors to affect population dynamics. both form tussocks while C. dactylon is a growth reduced S. madagascariensis sur- Moreover, because S. madagascariensis prostrate, stoloniferous and rhizomatous vival by up to 38.2%. Fast growing seeds prolifically, its pattern of flowering species. P. lanceolata is a perennial plants flowered within six weeks of may be an important determinant of po- broadleaf weed which germinates in the emergence. Vigorous pasture growth re- tential infestations. autumn to winter period while S. ptero- duced emergence of S. madagascariensis Weed management involves maxim- sperma is a prostrate stoloniferous, annual despite favourable climatic conditions izing mortality as well as lowering repro- weed which grows vigorously through for germination. Early autumn cohorts duction (Mortimer 1983). An understand- spring and summer. and large over-summering plants would ing of the factors that regulate the popula- appear to have the greatest potential to tion dynamics of S. madagascariensis will Experiment 1. Seedling emergence, increase the soil seed population. assist in the proper evaluation and timing flowering and survival of control methods. For example, can pas- All over-summering S. madagascariensis Introduction ture competition be used at particular plants were removed and counted and Senecio madagascariensis Poir. (fireweed), times of the year to suppress germination other weed and pasture growth cut to a native to southern Africa and Madagas- and seedling emergence? Population height of 10 cm. Then each month for two car, is an opportunistic weed (Fernández studies also provide base line data against years, newly-emerged S. madagascariensis and Verona 1984) which has invaded and which to assess the effect of biological seedlings were tagged at their bases with colonized a variety of habitats in coastal control. If sensitive stages in the life cycle coloured plastic rings and their life cycles areas of south-eastern Australia. It occurs of S. madagascariensis can be identified monitored through to flowering (defined predominantly on cultivated land and in then these may be targeted to make con- here by the appearance of unopened poorly grassed, neglected and heavily trol measures more effective. The aim of capitula) and senescence. Owing to an ex- grazed pastures (Green 1953, Martin and this study, in which three naturally- tremely high germination of S. madagas- Colman 1977, Launders 1986). The weed occurring field populations of S. madagas- cariensis seeds at Quadrat 2 in May 1986, is poisonous to livestock because of its cariensis were examined, was to find out two subquadrats, each 0.5 m2, were Plant Protection Quarterly Vol.11(1) 1996 15 subsampled within the large quadrat for were chosen according to expected S. In late May 1987, two of the four addi- further assessment. madagascariensis densities. On 5 March tional quadrats at Hoxton Park which The effect of fertilizer application on 1987 half the area (randomly allocated) of were used to compare fertilizer treat- survival and flowering of S. madagascar- each of the four quadrats at the two sites ments were slashed at a height of 5–10 iensis was measured over a period of 12 received a surface dressing of ‘Starter 18’ cm, thus providing the opportunity to de- months in additional quadrats estab- fertilizer equivalent to 66 kg N and 30 kg termine the effect of cutting on survival lished on 6 February 1987. Four 0.5 m2 P ha-1; the other half remained unferti- and the pattern of flowering. Data were quadrats (0.5 × 1 m) were located at the lized. Established S. madagascariensis combined over the two fertilizer treat- Agronomy Unit, Camden (pastures of plants were removed and new seedlings ments and subjected to analysis of vari- variable botanical composition), and four tagged and monitored on a monthly ba- ance. quadrats, each 4 m2 (2 × 2 m), at the sis, as in the year-old quadrats. Emer- Hoxton Park site (pasture type similar to gence data were subjected to analysis of Experiment 2. Survival and seed Quadrat 3). The different quadrat sizes variance. production of uprooted plants The ability of S. madagascariensis plants to set viable seed after being uprooted was assessed. On 22 April 1985, five plants with immature capitula (ray florets had not yet opened) were uprooted and dropped in an open paddock at Camden. On 10 May, an additional five plants at a similar stage of development were up- rooted at Goulburn (34°45’S; 149°43’E) and placed in water in a lighted room. The fate of all plants was closely fol- lowed, and any seed they produced was collected and germinated on moist filter Temperature (°C) Precipitation (mm) paper at room temperature. Experiment 3. Dynamics at high densities Extremely large numbers of S. madagascariensis seeds germinated con- Months 1986–1987 currently with the emergence of a culti- vated crop of forage sorghum during the Figure 1. Monthly precipitation and mean daily maximum and minimum summer of 1985/86 at Albion Park temperatures for Liverpool, New South Wales, from 1986 to 1987. (34°34’S; 150°46’E). In March 1986, four quadrats, each 0.25 m2 (0.5 × 0.5 m), were positioned in an area of the paddock with a high uniform density of seedlings. Four smaller 0.01 m2 quadrats (10 × 10 cm) were placed within these quadrats for regular counting between March and September 1986 and on each visit destruc- 2 ) tive harvests (each 0.01 m ) were taken -2 within two of the larger quadrats. Height, dry weight, and number of leaves and capitula were determined for harvested plants.
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