Extract from Rangelands 2008 — Taking the Pulse 4. Focus Bioregions - Mitchell Grass Downs bioregion (QLD and the NT) © Commonwealth of Australia 2008 Suggested citation This work is copyright. It may be reproduced for Bastin G and the ACRIS Management Committee, study, research or training purposes subject to the Rangelands 2008 — Taking the Pulse, published on inclusion of an acknowledgment of the source and behalf of the ACRIS Management Committee by the no commercial usage or sale. Reproduction for National Land & Water Resources Audit, Canberra. purposes other than those above requires written Acknowledgments permission from the Commonwealth. Requests should be addressed to: Rangelands 2008 — Taking the Pulse is the work of many people who have provided data and information that Assistant Secretary has been incorporated into this report. It has been Biodiversity Conservation Branch possible due to signicant in-kind contributions from the Department of the Environment,Water, Heritage State andTerritory governments and funding from the and the Arts Australian Government through the Natural Heritage GPO Box 787 Trust. Particular thanks are due to sta of the Desert Canberra ACT 2601 Knowledge Cooperative Research Centre, including Australia Melissa Schliebs,Ange Vincent and Craig James. Disclaimer Cover photograph This report was prepared by the ACRIS Management West MacDonnell Ranges (photo Department of Unit and the ACRIS Management Committee.The the Environment,Water, Heritage and the Arts) views it contains are not necessarily those of the Australian Government or of state or territory Principal author: Gary Bastin, CSIRO and Desert governments.The Commonwealth does not accept Knowledge CRC responsibility in respect of any information or advice Technical editor: Dr John Ludwig given in relation to or as a consequence of anything Editors: Biotext Pty Ltd contained herein. Design: Design ONE Printed in Australia by Lamb Print ISBN 978 0 642 37146 1 ISBN 978 0 642 37147 8 (CD-ROM of the report) Printed with vegetable-based inks on stock that ISBN 978 0 642 37148 5 (PDF) comprises 80% recycled bre from postconsumer Product number: PN21387 waste and 20% totally chlorine-free pulp, sourced from sustainable forests. August 2008 Mitchell Grass Downs bioregion (Queensland and the NT) The Mitchell Grass Downs bioregion extends across central Queensland into the NT (Figure 4.15). Reporting here is conned to the Queensland portion except where otherwise stated. The bioregion encompasses rolling, largely treeless, plains with cracking clay (‘black’) soils. The predominant vegetation is Mitchell grass tussock grassland with some low-tree overstorey of gidgee (Acacia cambagei) and other species (Figure 4.16). Most of the bioregion is under either leasehold or freehold tenure and is grazed by cattle and sheep. There has been a gradual movement out of woolgrowing in recent years. Major populatio n centres are Longreach, Blackall and Hughenden. Figure 4.15 Mitchell Grass Downs bioregion Area: QLD = 242,245 km 2 NT = 93,075 km 2 QLD Mitchell Grass Downs NT Mitchell Grass Downs 4 Focus bioregions 151 Figure 4.16 Landscapes of the Mitchell Grass Downs bioregion, Queensland. Central western Queensland Mitchell Grass Downs in a And when it rains! good season.The shrub is mimosa bush (Acacia farnesiana), which provides winter protein and shade. Photos: Queensland Department of Primary Industries and Fisheries Regional issues reporting period as a whole had a significantly positive cumulative rainfall deviation from the long‑term Regional issues in the Mitchell Grass Downs median, and seasonal quality for the whole period was bioregion include: considerably better than for some other blocks of n high levels of pasture utilisation (in two 14‑year periods in the past (ie 1918 to 1940 and sub‑IBRAs and in individual years), which have 1955 to 1973 were much drier periods than in the implications for persistence and recovery of recent past). palatable and productive perennial grasses Spatial averaging of rainfall conceals spatial variation n Mitchell grass death, with areas of non‑recovery in seasonal quality for the Queensland portion of to date the bioregion.The variability shown for 1997 (as an example) was based on simulated pasture biomass n pasture composition changes to more Aristida species produced by AussieGRASS and ‘season quality’ derived n increasing woody Weeds of National Significance from the NDVI44 (Figure 4.17, right panels). (WoNS), particularly prickly acacia (Acacia nilotica) n increasing cover of trees and shrubs in former Change in landscape function grassland areas (eg mimosa and gundabluey). Landscape function declined across most of the bioregion between 1994 and 2005 (Figure 4.18), Seasonal quality — 1992–2005 and significantly so in the Georgina Limestone Annual rainfall, as an indicator of seasonal quality, sub‑IBRA.The underlying data to support this was quite variable throughout the reporting period assessment were extracted from the Rapid Mobile (Figure 4.17, left).The period from April 1998 to Data Collection database (Hassett et al 2006).These March 2001 was wetter (deciles 9 and 10 in terms data were collected along repeat road traverses. of the 1890–2005 record). Rainfall was quite variable at other times. Several years in the 1990s and at the end of the reporting period had below‑median rainfall. Despite these drier years, the 1992–2005 44 See http://www.environment.gov.au/erin/ndvi.html. 152 Rangelands 2008 — Taking the pulse Figure 4.17 Indicators of seasonal quality, Mitchell Grass Downs bioregion 700 mean median mean 700 rainfall 600 median 500600 rainfall 400500 300400 200300 Rainfall (mm) 100200 Rainfall (mm) Rainfall (mm) 1000 0 1991–1992 1992–1993 1993–1994 1994–1995 1995–1996 1996–1997 1997–1998 1998–1999 1999–2000 2000–2001 2001–2002 2002–2003 2003–2004 2004–2005 1991–1992 1992–1993 1993–1994 1994–1995 1995–1996 1996–1997 1997–1998 1998–1999 1999–2000 2000–2001 2001–2002 2002–2003 2003–2004 2004–2005 1991–1992 1992–1993 1993–1994 1994–1995 1995–1996 1996–1997 1997–1998 1998–1999 1999–2000 2000–2001 2001–2002 2002–2003 2003–2004 2004–2005 >3000 kg/ha Annual rainfall. Long-term (1890–2005) mean and median 10 109 98 87 76 median <100 kg/ha 65 6 median 54 43 Aussie-GRASS simulated total 2 Rainfall decile 3 standing dry matter - 1997 21 Rainfall decile 2 Rainfall decile 10 0 1991–1992 1992–1993 1993–1994 1994–1995 1995–1996 1996–1997 1997–1998 1998–1999 1999–2000 2000–2001 2001–2002 2002–2003 2003–2004 2004–2005 1991–1992 1992–1993 1993–1994 1994–1995 1995–1996 1996–1997 1997–1998 1998–1999 1999–2000 2000–2001 2001–2002 2002–2003 2003–2004 2004–2005 1991–1992 1992–1993 1993–1994 1994–1995 1995–1996 1996–1997 1997–1998 1998–1999 1999–2000 2000–2001 2001–2002 2002–2003 2003–2004 2004–2005 Annual rainfall as deciles of the long-term (1890–2005) rainfall 500 400500 400300 300200 200100 1000 -1000 zero deviation -100-200 zero deviation highest -200-300 quality (100) -300 Index of cumulative rainfall deviation Index of cumulative rainfall deviation Index of cumulative rainfall deviation rainfall cumulative of Index 1890–1903 1900–1913 1910–1923 1920–1933 1930–1943 1940–1953 1950–1963 1960–1973 1970–1983 1980–1993 1990–2003 lowest 1890–1903 1900–1913 1910–1923 1920–1933 1930–1943 1940–1953 1950–1963 1960–1973 1970–1983 1980–1993 1990–2003 1890–1903 1900–1913 1910–1923 1920–1933 1930–1943 1940–1953 1950–1963 1960–1973 1970–1983 1980–1993 1990–2003 quality (0) Cumulative percentage deviations of annual (April–March) NDVI-based image of ‘season quality’ for rainfall from the long-term (1890–2005) median for all 14-year 1997. Each pixel has a relative value according to the greeness of vegetation periods between 1890–1903 and 1992–2005 (ie photosynthetic activity) Left: Rainfall Right: Simulated pasture biomass and vegetation greenness (NDVI) Note: Indicators are based on spatially averaged annual rainfall (April–March) between 1991–92 and 2004–05. For cumulative percentage deviations, periods below the dashed zero line indicate 14‑year sequences with generally less rainfall (poorer seasonal quality) and periods above the line indicate sequences of increased rainfall (better seasonal quality). All data are for the Queensland part of the bioregion. 4 Focus bioregions 153 Figure 4.18 Change in landscape Sustainable management function, Queensland sub-IBRAs of the Mitchell Change in critical stock forage Grass Downs bioregion Three sub‑IBRAs (Georgina Limestone, Northern Downs and Barkly Tableland; Figure 4.19a, brightest not assessed green) had levels of AussieGRASS simulated annual some loss of function pasture utilisation between 1991 and 2005 that significant loss of function were less than the specified safe threshold.The utilisation level for the Barkly Tableland sub‑IBRA was considerably less than the threshold, implying relatively conservative (and sustainable) grazing management.The Kynuna Plateau and Southern Wooded Downs sub‑IBRAs had the highest utilisation levels during the same period (27% and 208 26%, respectively, darkest green, Figure 4.19a).Those 212 utilisations are at a level that causes loss of palatable 209 211 perennial grasses and are of considerable concern. 210 213 Average utilisation elsewhere (Southwestern Downs and Central Downs) was close to the safe threshold. 214 Time‑averaged utilisation levels declined between 1976–90 and 1991–2005 in the Barkly Tableland, Northern Downs, Central Downs and Southern Wooded Downs sub‑IBRAs (bright red in Figure 4.19b). The Georgina Limestone, Southwestern