Carbon values of environmental tree plantings at the farm and catchment scales, and their economic implications to farming systems in the central wheatbelt of Western Australia

This study was conducted to gain estimates of carbon sequestration rates in farmland tree plantings located within the central wheatbelt region of Western Australia (WA). Because the central wheatbelt is a low rainfall area (ca. 320 mm mean annual rainfall), the expectation has been that tree growth rates and maximum carbon carrying capacity (CCC) would be low. To test these assumptions, a field based carbon research exercise was initiated. Four new predictive equations were developed to predict biomass from easily measured field based inventory data. This auxiliary data was collected at 21 sites across the catchment area. Findings from this research indicate that farmland revegetation projects in this region can sequester significant amounts of carbon over the short to medium term, with sites sequestering an average of 38 t C ha-1 (139 t CO2e ha-1) after 15 years. These values were significantly higher than the 5 t C ha-1 (18 t CO2e ha-1) predicted by the National Carbon Accounting Toolbox FullCAM model. Using the site based data developed in this study, a local carbon yield model was entered into MIDAS, a whole of farm economic optimisation model, to assess the carbon permit price required to make these land use options economically viable. Results from the MIDAS, with the specific input assumptions made for this study, and the locally collected carbon data, indicate that environmental tree plantings would meet the opportunity cost of the poorest lands at a carbon market price of $47 per tonne of carbon dioxide equivalents (CO2 e). If this value was to reach $65 t-1 CO2 e, all of the lowest quality soils (LMU 1 and LMU 2) could be transitioned into environmental trees in an economically viable manner.