No abstract

No abstract

The dormancy and germination requirements of five native grass species (Themeda australis, Chrysopogon fallax, C. latifolius, Sorghum plumosum, S. stipoideum) from the savannah woodlands of the Northern Territory were studied under controlled conditions. Results were related to the ecology of these species in the native grasslands of the region. All species were dormant at seed fall, and dormancy was found to be broken by exposure to dry heat. Gibberellic acid was also found to have a stimulatory effect on germination of dormant seed but there was no effect on stratification. In the field, temperatures existent at the soil surface during the dry season ensure that dormancy was broken before the heavy rainfall of the following wet season. Seeds of all species studied germinated over a similar range of temperatures with optimum germination at high temperature (c. 30°C). Although seeds of all species were not adapted to germina- tion under moisture stress, the existence of sharply pointed calluses and hygroscopically active awns on the seeds ensured that they would be buried below the moisture extremes of the immediate soil surface.

The tropical tallgrass rangelands of Australia are declining in condition in response to increased grazing pressure. However, large areas are still in good condition and many of the deteriorated areas are not yet irreversibly damaged. Increases in grazing pressure have been associated with the loss of perennial grasses, woody weed invasion, and increased run-off and soil loss in some areas. The population dynamics, diet selection patterns, defoliation responses of the perennial grasses and impacts of fire are outlined and ways this understanding can be incorporated into management are presented. The perennial grasses are sensitive to defoliation and can only be lightly utilised. Annual utilisation rates should not exceed 25% in areas of moderate and high fertility and this threshold 'safe' level decreases to 15% on infertile soils in the monsoon zone of the north and north-west. Spelling pastures during the wet season, when they are particularly sensitive to defoliation, may enable utilisation during the rest of the year to be increased. Such a grazing regime allows fuel accumulation, increasing the opportunity for use of fire in managing exotic woody weeds and the treetshrub layer. Fire can also be used to improve animal distribution and reduce the formation of patches which are prone to soil degradation. Sown pastures and tree clearing can be used to increase carrying capacity and improve flexibility in the management of native pastures but careful consideration needs to be given to these improvements to prevent problems such as salinisation and unwanted spread of exotic pasture plants. One of the difficulties in developing recommendations relevant to management is that most of the ecological understanding is at the plant and plant community scale but most problems occur at the paddocWlandscape scale where our knowledge base is limited. Future work should focus at this large spatial scale so that ecological principles derived from a range of scales can be better integrated into guidelines more appropriate to extensive management of tropical tallgrass rangelands. Key words: grazing, population dynamics, defoliation, diet selection, fire, grass decline

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Ecology.

Two perennial tussock grasses of savannas were compared in a glasshouse study to determine why they differed in their ability to withstand frequent, heavy grazing; Cenchrus ciliaris is tolerant and Themeda triandra is intolerant of heavy grazing. Frequent defoliation at weekly intervals for six weeks reduced shoot biomass production over a subsequent 42 day regrowth period compared with previously undefoliated plants (infrequent) in T. triandra, but not in C. ciliaris. Leaf area of T. triandra expanded rapidly following defoliation but high initial relative growth rates of shoots were not sustained after 14 days of regrowth because of reducing light utilising efficiency of leaves. Frequently defoliated plants were slower in rate of leaf area expansion and this was associated with reduced photosynthetic capacity of newly formed leaves, lower allocation of photosynthate to leaves but not lower tiller numbers. T. triandra appears well adapted to a regime where defoliation is sufficiently infrequent to allow carbon to be fixed to replace that used in initial leaf area expansion. In contrast, C. ciliaris is better adapted to frequent defoliation than is T. triandra, because horizontally orientated nodal tillers are produced below the defoliation level. This morphological adaptation resulted in a 10-fold higher leaf area remaining after defoliation compared with similarly defoliated T. triandra, which together with the maintenance of moderate levels of light utilising efficiency, contributed to the higher leaf area and shoot weight throughout the regrowth period.

A methodology is presented to estimate the safe carrying capacity of properties in extensive cattle- grazing regions within tropical, semi-arid woodlands of north-eastern Australia. Carrying capacities for 45 properties were calculated from resource information collected from the properties. These calculated carrying capacities were then compared with graziers' estimates and with Queensland Department of Lands' ratings. The rated carrying capacities were not correlated with either the calculated values or the graziers' estimates, and in general were much lower than both other values. The graziers' estimates and the calculated values were highly correlated with a slope not significantly different from 1 (p>0.1). This methodology could form the basis of a review of rated carrying capacities on an objective basis. Refinements would be necessary to improve the determination of individual cases with particular emphasis on spatial variability of resource use and fine scale variability in soil fertility and tree and shrub density.

No abstract