School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemPopulation biology and ecophysiology of Phalaris aquatica L. in grazed pastures( 2002)Perennial grasses are an important component of sustainable grazing systems in southern Australia. Phalaris aquatica L. (phalaris) is considered to be well adapted to the summer dry areas of southern Australia, regions with an average annual rainfall of 450- 650 mm. The species survives summer drought via dormant regenerative buds formed during spring at the nodes on the base of reproductive tillers. Despite the adaptive strengths of the species, producers find it difficult to achieve persistence of phalaris pastures for the long periods (> 1 0 years) required to achieve both the economic and environmental outcomes that ensure sustainability. Phalaris plant and tiller population characteristics were monitored under set stocked and rotational grazing treatments for three years in south-western Victoria to determine the effects of management on productivity and persistence. Rotational grazing treatments increased the phalaris contribution to total pasture dry matter by increasing tiller size, but did not arrest the overall decline in tiller density observed. On all treatments the tiller density declined by over 50 percent in three years. Summer drought, grazing and low soil nutrient availability were identified as three stresses that may limit phalaris plant survival. On a per tiller basis, the number of regenerative buds formed and the bud outgrowth characteristics in the following growing season, were not affected by grazing management. The regenerative capacity of phalaris (defined as the number of regenerative buds/m2) reflected the reproductive tiller density, which declined in a similar pattern to the total tiller density. Phalaris breeding programs, and pasture management research, should focus on increasing reproductive tillering to enhance phalaris persistence. When exposed to the combined stresses of frequent, severe defoliation and low soil nutrient availability under controlled conditions, phalaris responded more strongly to the below ground stress, favouring root more than leaf growth, compared to Lolium perenne L. (perennial ryegrass). This indicates that phalaris has more of a stress-tolerator growth strategy (after Grime 1981), compared to the competitor strategy of perennial ryegrass. This result provides a physiological explanation for the poor survival of perennial ryegrass when set stocked in summer-dry environments, and for the improved persistence of phalaris under these conditions. In a longer-term defoliation experiment conducted under controlled conditions, phalaris was found to have a similar level of grazing tolerance to Festuca arundinaceae Schreb. (tall fescue). These species were less tolerant of frequent, severe defoliation than Dactylis glomerata L. (cocksfoot), but more tolerant than perennial ryegrass. The grazing tolerance of phalaris was mediated by a change in C allocation patterns to favour leaf regrowth following defoliation, reduced specific leaf weight, and a longer leaf appearance interval. In the field, C resources were shared between growing tillers and the reproductive tiller from which they originated. Older plant structures received minimal C resources from actively growing leaves, which may lead to plant fragmentation, and could be one reason for the loss of phalaris clumps observed in grazed pastures. Phalaris appears to be well adapted to cope with the individual stresses of summer drought, low nutrient availability, defoliation and grazing, so the constraints to persistence must be due to interactions between these stresses. The response of phalaris to defoliation and low soil nutrient availability was to favour root development, a strategy that makes it well adapted to the summer dry environments. The interaction between grazing management and the summer survival mechanism showed that regenerative bud characteristics could not be manipulated on a per tiller basis. However, there may be an opportunity to increase reproductive tiller density through grazing management or plant improvement programs, and it is predicted that this will enhance phalaris persistence.