Review of woody biomass recovery following chemical clearing: implications for modelling of carbon sequestration

Abstract

Clearing of woody regrowth for pasture management has implications for livestock production, carbon sequestration and biodiversity. Clearing activity includes mechanical and chemical clearing. Chemical clearing is through selective or broadscale application of herbicide, while mechanical clearing typically is through ‘pushing’ or ‘pulling’ trees over or blade ploughing. Anecdotally the use of broadscale chemical clearing of regrowth for pasture establishment has increased over the past decade, however, there is limited information on changes in woody biomass recovery following chemical clearing which may be slower relative to mechanical clearing. This paper will focus on a review of existing knowledge to inform modelling of the carbon sequestration consequences of chemical clearing in Queensland. Existing information was synthesised from the literature and case study data from two grazing properties. There was relatively limited data on biomass recovery post-chemical clearing and blade ploughing. Rates of aboveground biomass carbon recovery varied depending on the herbicide used. Recovery rates following application of the herbicide tebuthiuron (Graslan®) were typically lower than those from other herbicides. Further, recovery rates of aboveground biomass carbon post chemical clearing with tebuthiuron were typically 2-28% of those after mechanical pulling, while those from other herbicides ranged from 38-76%. In addition to herbicide type, factors such as the soil type, timing of application (season), grazing pressure and use of fire can affect the recovery rates of post-herbicide application. Our findings suggest that models calibrated for mechanical clearing will overestimate the recovery of biomass following chemical clearing, particularly when the herbicide tebuthiuron has been applied. We recommend additional field surveys and monitoring, preferably through establishing permanent sampling plots to quantify the rate of carbon sequestration in regrowth of woody vegetation specific to post-chemical and blade plough clearing in rangelands.