Run-off, bedload and sediment concentration data were collected over a five-year period from unbounded catchments in grazed and exclosed pastures in woodlands. Cover varied from 4% during drought conditions to almost 100% in exclosed areas after above-average rainfall. High bedload soil loss, sediment concentration and run-off percentages were associated with low cover (<30%). Run-off as a percentage of rainfall increased linearly with rainfall intensity; decreased linearly with cover; decreased slightly as soil moisture status declined; and reached a maximum at intermediate rainfall events. Interactions between these factors were observed. Run-off was up to 30% of rainfall in moderate rainfall events (30-40 mm) where maximum rainfall intensity over any 15 minute period (I15) exceeded 70 mm/h. When soil moisture status was high, mean run-off exceeded 30% for 40-80 mm rainfall events. For all rainfall event sizes, run-off exceeded 20% where I15 exceeded 60 mm/h. Cover had very little effect on run-off when rainfall intensity was low (I15<20 mm/h), soil water deficit was low (<10 mm) or when rainfall events were >75 mm or <10 mm. Bedload plus suspended sediment loads ranged from negligible to 1000 kg/ha/a, depending principally on cover. Soil movement from areas with >40-50% cover was very low. Pastures dominated by Bothriochloa pertusa (a stoloniferous, naturalised grass) had lower run-off and lower rates of soil movement than pastures dominated by Heteropogon contortus (a native tussocky perennial grass) when compared at the same level of cover. Differences between grazed and exclosed areas could be attributed solely to differences in cover.

Native pasture yield and species composition within naturally occurring Eucalyptus communities of central Queensland were studied. Within a site, herbaceous yield decreased as tree basal area increased with the greatest relative decrease at sites with low pasture production potential. Mitscherlich regressions between herbaceous yield and tree basal area Y = A + B*e−kx) accounted for >80% of the observed variation at all sites. The absolute value of ‘k’ was greatest for those sites that had the lowest yield in the absence of trees.Pasture composition in treeless plots varied widely among sites. However, the direction of species compositional change was similar for all sites except one subject to short‐term inundation on a heavy clay soil. Grass tribes differed in their occurrence in areas of high tree basal area. Andropogoneae had lower actual yield and percentage composition (by dry weight) at high tree basal area, while Paniceae showed the reverse trend. Non‐grass herbaceous plants made a greater contribution to pasture composition at higher tree basal area although actual dry matter yield remained relatively constant over a wide range of basal areas.

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

Abstract. A transition matrix model was used to explore the dynamics, rate and potential extent of changes in landscape vegetation patterns on a southern Texas Prosopis savanna. Transitions between seven vegetation classes were determined for the periods 1941–1960 and 1960–1983 on aerial photographs of three sites. During these periods, the sites were heavily grazed by cattle and were fire‐free. Vegetation states assessed in grids of 20 m x 20 m cells superimposed on photographs ranged from grass‐dominated to woody plant‐dominated. The 1941–1960 period (denoted DRY) was characterized by prolonged drought, whereas annual rainfall during the 1960–1983 period (denoted WET) was typically normal to above‐normal. The 1941 landscape consisted of herbaceous zones (6% of cells), woodland (50% of cells) and savanna parkland (44% of cells with grass/woody plant mixtures). The woodland state was the most stable, with probabilities of no change being 0.970 and 0.873 in WET and DRY periods, respectively. The herbaceous state was least stable, with corresponding values of 0.074 and 0.353. Past and future landscape structure was modelled by randomly selecting DRY or WET transitions at 20 year time steps. The model was run under a series of rainfall scenarios where the probability of selecting the WET transition matrix (P[WET]) ranged from 0 (DRY always chosen) to 1 (WET always chosen). Historical records indicate P[WET] has approximated 0.3 to 0.4 in the region. The rate of succession to states of greater woody cover increased as P[WET] increased. Forward simulations based on P[WET] > 0.2 suggest the present landscape is unstable and will develop into a closed‐canopy woodland within the next 180 years, assuming the processes operating between 1941 and 1983 continue (e.g. grazing by cattle and lack of fire). Reverse simulations concur with historical observations and projections derived from woody plant growth rates in other studies and suggest that 200 to 300 yr BP these landscapes contained a substantially greater proportion of cells dominated by grassland or grassland with scattered woody plants (43 to 74%) than was present in 1983 (19%). Based upon elapsed time between predicted past and future steady states, succession from open savanna to closed‐canopy woodland may occur in ca. 400 to 500 yr for P(WET) ≥ 0.33. Arresting or reversing the projected trend may require changes in climate and/or changes in livestock grazing and land management practices. The approaches employed in this study illustrate how time series maps, aerial photographs and satellite imagery can be analyzed and used to interpret, project and reconstruct local and regional changes in ecosystem structure. Difficulties and limitations associated with the use of Markov chains to model succession are identified and discussed.