Rajesh K. Jalota

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.

Impact of tree clearing on soil pH and nutrient availability in grazing systems of central Queensland, Australia

In Queensland, land is cleared at high rates to develop pastures for enhanced production and the associated monetary gains. However, pasture production declines over time in cleared pastures until a new equilibrium is reached. The present study focussed on elucidating the reasons for decline in pasture production and finding the key soil properties that are affected due to clearing. Paired sites for cleared and uncleared pastures were selected to represent 3 dominant tree communities of the semi-arid tropics in central Queensland, i.e. Eucalyptus populnea, E. melanophloia, and Acacia harpophylla. The cleared pastures were chosen to represent 3 different durations of time since clearing (5, 11–13, and 33 years) to evaluate the temporal impact of clearing on soil properties. Various soil parameters were studied: macronutrients—available N (NH4+and NO3–), total N, and available P; micronutrients—Cu, Fe, Zn, and Mn; exchangeable cations—Ca, Mg, Na, and K (also macronutrients); pHw; and electrical conductivity. Of these, pHw showed a significant response to time of clearing for all 3 tree communities. Soil pHw increased significantly at cleared sites relative to uncleared (native woodland) pastures, and the increase was highly correlated with concentrations of exchangeable Ca, Mg, and Na. The change in soil pHw and exchangeable cations was more evident at >0.30 m soil depth. The increase in soil pHw in cleared pastures decreased the availability of soil nutrients for plant growth and, hence, pasture productivity. The interactions of different soil properties down the profile as a result of changes caused by clearing are important when interpreting the effects of clearing on soil properties.

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.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.

Impact of tree clearing on soil attributes for a pastoral property in central Queensland, Australia

Impacts of tree clearing on soil properties in grazing systems of Queensland largely remain undetermined. In the past, most of the land was cleared to develop pastures to enhance pasture production. The present research highlights how tree clearing affects soil attributes over time on a pastoral property in the semiarid zone of central Queensland, Australia. Paired cleared and uncleared pasture systems were selected to represent three different times since clearing (5, 11-13, and 33 years). Three woodland plant communities, which are common in the region, were studied: poplar box (Eucalyptus populnea), ironbark (Eucalyptus melanophloia), and brigalow (Acacia harpophyllla). Soil macronutrients and micronutrients and exchangeable cations were studied down the profile for 0-5, 5-10, 10-20, 20-30, and 30-60 cm depth. The results suggest that tree clearing initially increased concentration of some nutrients (such as total N in the topsoil) that consequently augmented pasture production. However, this scenario changes with increased time since clearing: concentrations of some soil nutrients increased, whereas some decreased, thus disturbing the equilibrium in soil functions that further impacts upon pasture yield. The impacts of clearing on soil properties were more apparent at deeper layers of soils. Increase in concentrations of Na at 30-60 cm in cleared pastures of E. populnea and E. melanophloia raises concern for sodicity. Results from this study support other findings in the region that changing native woodlands to introduced grass pastures is likely to have consequences on soil nutrients, which can affect cattle production gains and the sustainability of pasture systems.

Soil respiration rate and its sensitivity to temperature in pasture systems of dry-tropics.

Abstract Tree clearing is a topical issue the world over. In Queensland, the high rates of clearing in the past were mainly to increase pasture production. The present research evaluates the impact of clearing on some soil biological properties, i.e. total soil respiration, root respiration, microbial respiration, and microbial biomass (C and N), and the response of soil respiration to change in temperature. In-field and laboratory (polyhouse) experiments were undertaken. For in-field studies, paired cleared and uncleared pasture plots were selected to represent three major tree communities of the region, i.e. Eucalyptus populnea, E. melanophloia, and Acacia harpophylla. The cleared sites were chosen to represent three different time-since-clearing durations (5, 11–13, and 33 years; n=18 for cleared and uncleared plots) to determine the temporal impact of clearing on soil biological properties. Experiments were conducted in the polyhouse to study in detail the response of soil respiration to changes in soil temperature and soil moisture, and to complement in-field studies for estimating root respiration. The average rate of CO2 emission was 964 g CO2/m2/yr, with no significant difference (P<0.05) among cleared and uncleared sites. Microbial respiration and microbial biomass were greater at uncleared compared with those at cleared sites. The Q 10-value of 1.42 (measured for different seasons in a year) for in-field measurements suggested a small response of soil respiration to soil temperature, possibly due to the limited availability of soil moisture and/or organic matter. However, results from the polyhouse experiment suggested greater sensitivity of root respiration to temperature change than for total soil respiration. Since root biomass (herbaceous roots) was greater at the cleared than at uncleared sites, and root respiration increased with an increase in temperature, we speculate that with rising ambient temperature and consequently soil temperature, total soil respiration in cleared pastures will increase at a faster rate than that in uncleared pastures.