Demography of three perennial grasses in a central Queensland eucalypt woodland

Abstract

The population dynamics of the palatable, perennial grasses Bothriochloa ewartiana (Domin) C.E.Hubb. (desert Mitchell grass), Chrysopogon fallax S.T.Blake (golden beard grass) and Heteropogon contortus (L.) P.Beauv. ex Roem. & Schult. (black speargrass), were studied in an extensive grazing study conducted in a eucalypt woodland within the Aristida–Bothriochloa pasture community in central Queensland between 1994 and 2000. Treatments were three grazing pressures based on light, medium and heavy utilisation of forage available at the end of summer and two timber treatments (trees intact and trees killed). Seasonal rainfall throughout this study was generally favourable for plant growth with no severe drought periods. Grazing pressure had a greater overall impact on plant dynamics than timber treatment, which had minimal impact. Grazing pressure had a large impact on H. contortus dynamics, an intermediate impact on B. ewartiana and no impact on C. fallax. Fluctuations in plant density of both B. ewartiana and C. fallax were small because both species were long lived with low levels of seedling recruitment and plant death, whereas fluctuations in H. contortus density were relatively high because of its relatively short life span and higher levels of both recruitment and death. Heavy grazing pressure increased the recruitment of B. ewartiana and H. contortus in some years but had no impact on that of C. fallax. Heavy grazing pressure reduced the survival of the original plants of both B. ewartiana and H. contortus but not of C. fallax. For H. contortus, the size of the original plants was larger where trees were killed than where trees were left intact and plants of the 1995 seedling cohort were larger in 1998 at heavy compared with those at light and medium grazing pressure. Grazing had a minor negative impact on the soil seed bank of H. contortus. Populations of all three species remained stable throughout this study, although the favourable seasonal rainfall experienced and the short duration of this study relative to the life span of these species may have masked longer term, deleterious impacts of heavy grazing pressure.

Managing for rainfall variability: impacts of grazing strategies on perennial grass dynamics in a dry tropical savanna

“…The effect of grazing pressure was greatest on B. ewartiana and C. fallax and this grazing impact was accentuated by severe drought. Results reported here are similar to those for B. ewartiana and C. fallax elsewhere in northern Australia (McIvor 2007;Jones et al 2009) although the results for H. contortus differ markedly from those in other geographical regions (Orr et al 2004a(Orr et al , 2010aJones et al 2009).…”

“…Heavy grazing increased B. ewartiana turnover while both moderate grazing and rotational spelling increased C. fallax turnover. This increased B. ewartiana turnover under heavy grazing is consistent with similar results for B. ewartiana under heavy grazing in central Queensland (Jones et al 2009). However, increased C. fallax turnover in the present study contrasts with no grazing impact on this species in central Queensland (Jones et al 2009).…”

“…This increased B. ewartiana turnover under heavy grazing is consistent with similar results for B. ewartiana under heavy grazing in central Queensland (Jones et al 2009). However, increased C. fallax turnover in the present study contrasts with no grazing impact on this species in central Queensland (Jones et al 2009). This difference probably reflects the longer time span of our study (13 years) compared with that (6 years) for the central Queensland study.…”

“…Complete turnover of Aristida spp., P. effusum and H. contortus populations in the present study reflects their short life spans relative to the length of this study. In comparison, Jones et al (2009) recorded a turnover for H. contortus of 67% over 6 years with no grazing pressure impact while a longer, 13-year study (Orr et al 2010a) recorded a turnover of 95% also with no effect of grazing pressure. These differences again reflect different study durations relative to the life span of H. contortus.…”

“…Studies of the dynamics of a range of perennial grasses including Astrebla spp. (Orr 1998), Heteropogon contortus (Orr 2004;Orr et al 2004aOrr et al , 2010aMcIvor 2007;Jones et al 2009), Bothriochloa ewartiana and Chrysopogon fallax (McIvor 2007;Jones et al 2009) provide an overall understanding of sustainable grazing management in some important pasture communities in northern Australia.…”