S. pyramidalis is considered a major problem in Australia because it reduces the productivity of native and introduced pastures, and the profitability of industries dependent on grazing animals. It has low palatability and is difficult for stock to graze due to the fibrous nature of its leaf blades. Therefore companion plants are preferentially grazed and S. pyramidalis becomes tall and rank. Over time, if plants are not controlled, they increase in number until almost pure stands of this undesirable grass result. Current control methods are centred on herbicide application and replacement of existing pasture species with more competitive species because little objective ecological information is available. The studies outlined in this thesis were conducted to identify the key ecological characteristics of S. pyramidalis seed that contribute to its success, and those ecological characteristics which may be exploited by land managers to control and reduce the impacts of this grass. Laboratory, glasshouse and field experiments were conducted between 1996 and 1999 to identify these ecological characteristics. Information from this research will be used to improve existing management strategies so that S. pyramidalis will be effectively managed.
Stands of S. pyramidalis are generally characterised by large (780 to 7650 seeds/m2), persistent (3 to 8 years), viable soil seed banks and high seed production (1885 to 85420 seed/m2, 210 to 570 seeds/inflorescence) depending on plant density and rainfall. New seed and seedlings can be produced at any time of the year given adequate soil moisture and suitable temperatures, with the viability of fresh seed generally above 90 %.
Seed falls within a 3 m radius from mature plants indicating the usefulness of buffer strips as a containment measure. Application of glyphosate or desiccation of seeds of S. pyramidalis at four days after anther appearance when they are at early milk stage reduces the viability of seeds produced to less that 5 %.
The seed does not have a burial mechanism, therefore in undisturbed areas more than 50 % and 70 % of the viable seed bank is located in the litter to a depth of 5 mm and in the top centimetre of soil respectively. No more than 8 % of the viable seed bank is located below a depth of 25 mm. The effective maximum emergence depth of 20 to 30 mm is not significantly affected by soil type.
Seed germination of S. pyramidalis is increased by temperature fluctuations in light, green light and darkness, with the greatest response occurring in light. However, as the effects of innate dormancy become negligible (4-6 months) the requirement for both fluctuating temperatures and light is reduced with significant levels of germination occurring in darkness with constant temperatures ranging from 15 to 45 °C. Seed populations which have seed with a range of responses to light and temperature enjoy an ecological advantage, as seeds are able to germinate over a range of environmental conditions. This suggests that, once aged seed is present, a small percentage of the seed bank will be able to germinate in poor light conditions such as in pastures with high levels of vegetation cover, or at depths where temperature and light fluctuations are significantly reduced.
S. pyramidalis seedlings are able to emerge when soil moisture levels are at wilting point, although soil moisture must be at or above wilting point for at least four days for emergence to occur during the warm season. These soil moisture levels are much lower than those required for emergence of seedlings of either Heteropogon contortus or Dichanthium sericeum. These are important native grasses in Queensland in regions where S. pyramidalis is well adapted. Seedling emergence is negligible in undisturbed, burnt and mown plots, while significant emergence occurs only where competition is reduced and light and temperature are increased by herbicide application and cultivation.
The time to 50 % germination for H. contortus, D. sericeum, Chloris gayana cv. Callide, Digitaria milanjiana cv. Jarra and Dichanthium. aristatum cv Floren is at least as fast as that of S. pyramidalis which took four days. S. pyramidalis is relatively tolerant of saline conditions; with time to first germination not affected until electrical conductivity (EC) reaches 20800 µs/cm, while time to 50 % germination increases significantly once EC reaches 7000 µs/cm. No germination occurs when EC reaches 44700 µs/cm while total germination after 15 days only declines significantly when EC is greater than 25300 µs/cm. H. contortus has similar salt tolerance to S. pyramidalis during the germination and emergence growth phases. D. aristatum cv. Floren, D. sericium, and Bothriochloa. insculpta cv. Bissett are all more sensitive to salt during these growth phases than either S. pyramidalis or H. contortus.
Seed viability, soil seed bank viability and seedling survival are significantly affected by the dry heat of fire. Seed viability is reduced to zero when seeds are subjected to dry heat at/or greater than 125 ˚C for as little as 15 s duration, and viable seed banks are reduced by up to 50 % when subject to the heat generated by a relatively hot fire.
Seedlings of S. pyramidalis are relatively fire tolerant although significant mortality can occur when seedlings are burnt when severely wilted. Fires with large fuel loads (7140 kg/ha) can kill all but a few of the oldest (15 weeks) wilted plants, while wilted plants as young as five weeks (15 cm high, base diameter 0.5 cm) can survive when burnt with 1790 kg/ha of fuel. A similar pattern of plant survival occurs with plants that are not wilted 0resh) at the time of burning, only with more plants surviving at higher fuel loads and younger ages. Fresh plants as young as seven weeks (30 cm high, base diameter 1 cm) can survive burning with 7140 kg/ha of fuel while one week old plants (first leaf) can survive burning with 3570 kg/ha of fuel. All 15 week old fresh plants (105 cm high, base diameter 2 cm) can survive burning regardless of fuel load.
S. pyramidalis seedlings (aged 4-26 days after sowing) are more tolerant of severe moisture stress than similarly aged seedlings of D. sericeum and H. contortus. No seedlings of D. sericeum survived in soil at wilting point for more than seven days while seedlings of H. contortus did not survive more than 14 days in soil at wilting point. Seedlings of S. pyramidalis can survive in soil at wilting point for 112 days. Seedlings appeared dead prior to re-watering except at seven days without water when surviving plants were extremely wilted, but continued to grow as normal apart from some necrosis of leaf margins. With surviving S. pyramidalis seedlings, the exposed part of the plant was necrotic. New growth occurred by the continuation of leaf extension so that leaves had a necrotic section at the end, with new leaves emerging from the crown of the plant.
The research reported in the thesis identifies the production of large amounts of viable seed; long lived soil seed banks; seed germination in a wide range of temperature, light and moisture regimes; an high degree of tolerance of moisture stress during the seedling stage; tolerance of seedlings to fire; salt tolerance during seed germination; and seeds becoming viable within a few days after pollination as key attributes of S. pyramidalis contributing to its success in Australia. Several attributes which may be perceived as weaknesses include relatively slow germination and emergence, seed susceptible to death caused by dry heat such as that caused by fire, and natural seed fall close to the parent plant.
The main effects of management are to change the amount of plant cover/competition (fire, slashing, grazing, chemicals, cultivation, species replacement), disturb soil at various intensities (cultivation, grazing), and directly kill plants or seed (fire, cultivation, chemicals). In ecological terms, each of these impacts alters the light, temperature or moisture conditions which enhance or inhibit seed germination and emergence, prevent or promote seed production and change the balance of plant competition. Management must use these options to reduce the viable seed bank to negligible levels and prevent its replenishment if effective control is to be achieved.
Development of grazing management strategies for S. pyramidalis infested areas to maintain productivity is an area that needs considerable research. Herbicide use is constrained by a lack of cost effective selective herbicides. The integration of grazing management, strategic herbicide use and perhaps strategic burning currently offers the greatest potential for improving the management of S. pyramidalis. The continual improvement of current management strategies is critical to the success of any weed control activities. Perhaps the greatest challenge is the transfer of developed technologies to land managers, so that it becomes part of everyday management of S. pyramidalis.