Tropical Grasslands (2004) Volume 38, 17–30 17 Dynamics of plant populations in Heteropogon contortus (black speargrass) pastures on a granite landscape in southern Queensland. 1. Dynamics of H. contortus populations D.M. ORR1, C.J. PATON2 AND D.J. REID1 Introduction 1 Department of Primary Industries, Rockhampton 2 Department of Primary Industries, Heteropogon contortus (black speargrass) Brian Pastures Research Station, Gayndah, pastures are an important forage resource for the Queensland, Australia breeding and finishing of 3–4 million beef cattle in Queensland and are, therefore, of considerable economic importance (Burrows et al. 1988). Abstract These pastures occupy 25 M ha and occur on a The dynamics of Heteropogon contortus (black wide variety of soil types which receive between speargrass) populations were measured in a 700 and 1200 mm of annual rainfall (Weston et subset of treatments contained within an exten- al. 1981). However, recent evidence (Tothill and sive grazing study conducted between 1990 and Gillies 1992) indicates that these pastures have 1996 in H. contortus pasture in southern Queens- undergone deleterious changes in pasture com- land. This subset included 2 landscape positions position under some current grazing management and 3 stocking rates in both native pasture and practices. legume-oversown native pasture. Plant population dynamics is the study of indi- Severe drought conditions throughout much of vidual plants within a population and how plant the study necessitated ongoing adjustments to the numbers change with time (Harper 1977). Little original stocking rates and, as a result, drought is known of the dynamics of H. contortus popula- was the major influence on the dynamics of tions under grazing despite their economic H. contortus populations. Plant density and basal importance. Mott et al. (1985) reviewed the area in the silver-leaved ironbark landscape were dynamics of perennial grass populations in Aus- consistently higher than those in the narrow- tralian savannas and reported that 60% of H. con- leaved ironbark landscape. There was limited evi- tortus plants die within 5 years in southern dence of any impact by either light or moderate Queensland and that high seed production allows stocking rate but there was evidence of an impact many seeds to germinate, establish and so main- at the heaviest stocking rate. There was minimal tain populations. impact of legume oversowing. Prior to white settlement the dominant per- Relatively large fluctuations in plant density ennial grass throughout what is now H. contortus occurred during this study resulting from the death pastures was Themeda triandra (kangaroo grass). of existing plants, due mainly to drought, and The shift from T. triandra to H. contortus in seedling recruitment. Similarly, there were rela- southern Queensland resulted from increased tively large fluctuations in basal area caused grazing pressure together with changes in burning mainly by changes in plant size. Rates for turnover regimens between 1840 and 1880 (Fox 1967). of plant numbers were relatively high whereas More recently, grazing pressures have increased plant turnover rates of basal areas were relatively further because of economic pressures on com- low. Regular seedling recruitment appeared nec- mercial producers, the introduction of Bos indicus essary to ensure the persistence of this species. cattle, the use of dietary supplements and pro- Despite the high turnover, populations were longed drought (Tothill and Gillies 1992). Despite maintained at reasonable levels indicating the the economic importance of these pastures, little overall resilience of H. contortus. is known on the impacts of increasing grazing pressure on population dynamics of H. contortus. Correspondence: Dr D.M. Orr, QDPI, PO Box 6014, Rockhampton Mail Centre, Qld 4702, Australia. E-mail: Extensive native pastures are usually highly [email protected] organised landscape systems that range from a 18 D.M. Orr, C.J. Paton and D.J. Reid scale of metres to kilometres and this structure The overall study (described by MacLeod and functions to conserve water and nutrients (Ludwig McIntyre 1997) consisted of 4 land classes based and Tongway 1997). In southern Queensland, on the dominant tree species and 3 stocking rates landscapes supporting H. contortus pastures are on either native pasture or legume–oversown often organised as a topographic sequence of native pasture. This paper will focus on a subset Eucalyptus maculata (spotted gum) on ridge of data including 2 land classes (narrow-leaved crests, E. crebra (narrow-leaved ironbark) on ironbark and silver-leaved ironbark), at 3 nominal upper slopes, E. melanophloia (silver-leaved iron- stocking rates (0.3, 0.6 and 0.9 beasts/ha) in both bark) on lower slopes with E. tereticornis (blue native pasture and legume-oversown native pas- gum) in creek lines (Taylor and Cook 1993). ture. There were 2 replicates of the 0.3 and 0.6 Despite this, little is known of how landscape beasts/ha stocking rates for the native pasture and position influences H. contortus populations. legume-oversown native pasture in both land Oversowing H. contortus pastures with classes. However, there was only 1 replicate for grazing-tolerant, introduced legumes is a potential the 0.9 beasts/ha treatment in each of the 2 land avenue for increasing animal production from classes and only for the legume-oversown native these pastures. However, such legume oversowing pasture. may lead to an imbalance in the proportion of the The depth of the soil A horizon varied but was legume and perennial grasses in the pasture typically 20–30 cm in the narrow-leaved ironbark (Miller and Stockwell 1991). and 40–50 cm in the silver-leaved ironbark This paper reports the dynamics of H. con- (A. Barton, personal communication). Fences tortus populations in response to landscape posi- were located so that paddocks contained only tion and stocking rate in both native pasture and 1 land class. Paddock sizes (6.6, 3.3 and 2.2 ha) legume-oversown native pasture in an extensive were varied so that 2 animals grazed each grazing study conducted between 1990 and 1996. paddock. Initially, 2 weaner Belmont Red steers Further papers in this series report seed produc- grazed each paddock for the first year and one of tion and soil seed banks of H. contortus (Orr et these was replaced at the end of this first year. al. 2004a), dynamics of Aristida spp. populations The remaining steer was replaced after 2 years (Orr et al. 2004b) and impacts of spring burning with another weaner and this procedure con- on H. contortus and Aristida spp. populations tinued with animals remaining for 2 years. (Orr 2004). Other papers from this grazing study include MacLeod and McIntyre (1997) and MacLeod and Cook (2004) which report animal Legume oversowing production and economic aspects of legume over- sowing, respectively. The 6 legumes, sown by band seeder (Cook et al. 1993), were Macroptilium atropurpureum (Siratro), Chamaecrista rotundifolia (Wynn Materials and methods cassia), Stylosanthes guianensis var. intermedia (Fine stem stylo), S. scabra (Seca stylo), Grazing study Aeschynomene falcata (Bargoo joint-vetch) and Lotononis bainesii (lotononis). Molybdenised A grazing study was established in 1988 on superphosphate fertiliser was applied at 45 kg/ha “Glenwood” station, 50 km west of Mundubbera at sowing. The legumes were oversown twice: (25°41′S, 150°52′E), which has a 100-year initially in November 1989 and again in February history of commercial cattle grazing. The soils 1993 because severe drought (see below) had are granite-derived, mainly yellow podzolics with precluded satisfactory legume establishment from a coarse-textured surface (Barton 1991). The the 1989 sowing. vegetation is E. crebra–E. melanophloia wood- land with an understorey of H. contortus. Average annual rainfall is 708 mm with 70% Stocking rates falling in October–March, inclusive (Cook and Russell 1983). The site was burnt in spring 1988 The severe drought conditions experienced and remained ungrazed until grazing treatments throughout this study and the need to resow the commenced in December 1989. Grazing treat- legumes in February 1993 necessitated changes ments continued until March 1996. to the notional stocking rates of 0.3, 0.6 and Dynamics of H. contortus populations 19 0.9 beasts/ha (Table 1). Stocking rates were Previous attempts to monitor individual halved in all paddocks over 4 periods, for a H. contortus plants (J. C. Tothill, personal com- cumulative total of 22 months from December munication) indicated that plants fragment with 1989–February 1993. All paddocks were increasing age (Samuel and Hart 1995) and it is destocked from February–April 1993 when some difficult to follow the fate of plant segments. grazing treatments were altered. Consequently, we delineated individual plants in From June 1993 until grazing concluded in the permanent quadrats using a pantograph March 1996, all native pasture treatments were (Williams 1970) to record these plants together grazed at the original stocking rates. Treatments with any plant segments. designated as 0.9 beasts/ha with legume-oversown Commencing in autumn 1990, the position of native pasture were grazed at 0.9 beasts/ha. individual H. contortus plants in each quadrat However, treatments designated as 0.3 and was charted and the diameter measured firstly 0.6 beasts/ha with oversown native pasture were along the widest diameter and secondly the stocked at half the originally proposed stocking