Litter production, decomposition and nutrient release in cleared and uncleared pasture systems of central Queensland, Australia

The temporal impact of clearing trees on litter production, litter decomposition and on C, N and P release through decomposition of litter was examined in the pasture systems of a semi-arid zone of central Queensland. Paired sites for cleared pastures (developed from clearing woodlands) and uncleared (intact woodland) pastures were selected to represent three dominant tree communities of the region i.e. Eucalyptus populnea, E. melanophloia and Acacia harpophylla, with three different time-since-clearing (5, 11–13 and 33 y) treatments. Yearly litter production was greater at uncleared sites (1732–1948 kg ha−1 y−1 for eucalypt and 2596 kg ha−1 y−1 for acacia communities) compared with cleared sites (1038–1282 kg ha−1 y−1 for eucalypt and 1100 kg ha−1 y−1 for acacia communities averaged over three time-since-clearing treatments). Rates of litter decomposition and of release of C, N and P from decomposing litter were higher at cleared than uncleared sites for all three tree communities. The cleared and uncleared sites did not differ significantly in total amount of C and N released per year since the concentrations of C and N were greater in litter from uncleared sites but the rate of release was less than that at cleared sites. Slow but continuous release of nutrients in eucalypt and acacia woodlands may be an adaptation of these communities to maintain the nutrient cycle and to avoid leaching of nutrients in the nutrient-poor soils of the region.

Pasture composition in cleared and uncleared woodlands

Land clearing in Queensland is often practised to enhance pasture production, and hence, increase financial returns from beef production. The benefits of clearing have been quantified in terms of short-term gains in pasture yield but have not adequately accounted for possible medium- or longer-term impediments that may be attributed to clearing. Therefore, impacts of clearing and the subsequent sowing of exotic grasses such as Cenchrus ciliaris L. on pasture composition and production were studied. To achieve this, paired sites were selected representing cleared and uncleared pastures across three different times since clearing (i.e. 5, 11–13 and 33 years since clearing) for the three dominant tree communities of central Queensland (i.e. Eucalyptus populnea F.Muell. (poplar box), E. melanophloia F.Muell. (silver-leaved ironbark) and Acacia harpophylla F.Muell. ex. Benth. (brigalow)). The results demonstrated that species diversity declined with clearing and sowing of exotic pastures. Species diversity and pasture production were negatively related. Although pasture yield was 2–3 times greater 13 years after clearing of E. populnea and A. harpohylla, the gains in pasture yield were not consistent over time, yields being only 1.5 times greater after 33 years of clearing. In E. melanophloia, an increase in the yield of only 1.5–1.8 times occurred 5 years after clearing compared with uncleared pastures, whereas 33 years after clearing, yield was 3/4 of that in uncleared pastures. The initial gains in pasture yield were accompanied by a loss of plant diversity that may affect ecosystem functions such as nutrient cycling or soil mineralisation, and the longer-term production gains.

Pasture production in cleared and uncleared grazing systems of central Queensland, Australia

Clearing land of trees and introducing exotic pastures to enhance pasture and cattle production and hence enterprise financial performance are widely practised in Queensland. The results from many previous studies on tree clearing have emphasised the gains in pasture production, but over periods of less than 10–15 years after clearing. The present study questioned the sustainability of pasture production in cleared systems over a longer time-frame (>10 years of clearing). For this, three different age groups of clearing i.e. 5 year, 11–13 year and 33 year were selected in each of 3 major types of tree communities i.e. Eucalyptus populnea, E. melanophloia and Acacia harpophylla in central Queensland. Paired comparisons of cleared and uncleared (intact) pasture systems were selected for each age group of clearing. The results suggest that the initial gains in pasture production upon clearing were compatible with published studies. However, for longer periods of time since clearing, the gains in pasture production were not sustained and were accompanied by risks of land degradation and loss of pasture plant diversity. For E. populnea and A. harpophylla, the maximum benefits from clearing were achieved at 13–15 years whereas for E. melanophloia, any benefits existed only over a short period of 5–6 years. The study emphasises that each tree community exhibits a specific response with regard to the duration of increased pasture production following clearing. To estimate the total benefits from tree clearing in pasture development, it is important to consider both monetary benefits and non-monetary losses from clearing for different types of tree communities.

The Relationship Between Distance From Water and the Soil Seed Bank in a Grazed Semi-Arid Subtropical Rangeland

The germinable soil seed bank of a grassy Eucalyptus populnea F.Muell, open woodland was investigated in relationship to distance from water (away from a bore-drain) on North Yancho Station, near Bollon in southern semi-arid Queensland. The germinable soil seed bank was both diverse and abundant, 69 taxa of seedlings being identified to species and 2 more to genus. A mean density of 13 207 ± 4160 seeds m-2 was recorded, with over 80% of the seed being from the annual forbs Crassula sieberana (Schultes & J.H.Shultes) Druce, Wahlenbergia tumidifructa P.J.Smith and Dysphania gomulifera (Nees) Paul G.Wilson. Most common species showed spatial variation in their germinable seed density with relation to distance from water, resembling the patterns previously reported by other authors for standing vegetation under stocking pressure gradients.

Herbivore diet in a semi-arid Eucalyptus populnea woodland. 2. Feral goats

The diet of feral goats was recorded by direct observation in a shrub-dominated Eucalyptus populnea woodland between Nyngan and Bourke, N.S.W., on 7 sampling occasions over 2 years. Different nutritional opportunities were provided by the effects of rainfall on 4 contrasting grazing treatments. The goats foraged evenly over the paddocks and encountered most foods with the same frequency as they occurred. With unlimited quantities of forage available the goats spent 63% of their time grazing and only 28% browsing. Most browsing took place under dry conditions when pasture quality was low. When forage was in short supply the goats accepted some shrubs not normally eaten, and these plants died; other woody plants were consumed erratically or not at all. Acacia aneura was the only woody plant which was both palatable and resilient to goat browsing. Goats mostly ate the same plant species as sheep and with the same preference, but tended to eat more browse and less forbs than sheep in dry times. The browse consumed was principally the same as consumed by sheep, demonstrating sequential competition for dry-season fodder. Goats demonstrated a potential for overgrazing in dry times, indicating that stocking rates must be adjusted at such times to avoid pastures becoming extinct.