Pasture production and composition response after killing Eucalypt trees with herbicides in central Queensland

Abstract

Clearing woodlands is practised worldwide to increase crop and livestock production, but can result in unintended consequences including woody regrowth and land degradation. The pasture response of two eucalypt woodlands in the central Queensland rangelands to killing trees with herbicides, in the presence or absence of grazing and regular spring burning, was recorded over 7 or 8 years to determine the long-term sustainability of these common practices. Herbage mass and species composition plus tree dynamics were monitored in two replicated experiments at each site. For 8 years following herbicide application, killing Eucalyptus populnea F. Muell. (poplar box) trees resulted in a doubling of native pasture herbage mass from that of the pre-existing woodland, with a tree basal area of 8.7 m2 ha–1. Conversely, over 7 years with a similar range of seasons, killing E. melanophloia F. Muell. (silver-leaved ironbark) trees of a similar tree basal area had little impact on herbage mass grown or on pasture composition for the first 4 years before production then increased. Few consistent changes in pasture composition were recorded after killing the trees, although there was an increase in the desirable grasses Dichanthium sericeum (R. Br.) A. Camus (Queensland bluegrass) and Themeda triandra Forssk. (kangaroo grass) when grazed conservatively. Excluding grazing allowed more palatable species of the major grasses to enhance their prominence, but seasonal conditions still had a major influence on their production in particular years. Pasture crown basal area was significantly higher where trees had been killed, especially in the poplar box woodland. Removing tree competition did not have a major effect on pasture composition that was independent of other management impositions or seasons, and it did not result in a rapid increase in herbage mass in both eucalypt communities. The slow pasture response to tree removal at one site indicates that regional models and economic projections relating to tree clearing require community-specific inputs.

Ground-layer vegetation of cleared woodlands (pastures) has lower biodiversity and different invertebrate assemblages to remnant woodlands in grazed landscapes of eastern Australia

“…Another study, also using suction samplers, found no effect of clearing on biodiversity (order richness), total abundance, abundance of trophic groups or composition of invertebrate assemblages (Houston and Melzer 2018). However, that study took place in recently cleared paddocks (< 5 years) with native pastures of comparable biomass that resembled the composition of the ground-layer vegetation of the original woodlands (Hall et al 2016). In contrast, clearing in the current study had occurred many years ago (> 20 years) and pastures were dominated by introduced pasture grasses such as Buffel Grass.…”

“…Grazing is the dominant land use in Australia (40% of the land mass) and almost 80% of Queensland's rangelands are grazed (Department of Environment and Resource Management 2011). Graziers throughout the world traditionally manipulate the landscape to enhance pasture production, primarily by clearing trees or altering grazing pressure by varying stock numbers (Hall et al 2016). Consequently, it is important that understanding of impacts of grazing management, including those associated with land clearing, on biodiversity of native fauna of woodlands be obtained.…”