The potential of deep-rooted species to mitigate the impacts of heatwaves and declining rainfall on pastures in southeast Australia

Adapting to a variable climate will become increasingly important as growing-season rainfall declines and frequency of heatwaves increase across southern Australia. This research used biophysical and economic modelling to determine the value of establishing a deeper-rooted species compared to a shallower-rooted species at a short and a longer growing seaon site in southeast Australia. The Sustainable Grazing Systems Pasture Model (SGS) was used to simulate the monthly growth of two pasture species differing only in root depth, root distribution, and a recently developed method for estimating associated canopy temperature under 3 different climate scenarios. Modelling incoporated updated estimates of heat stess recovery. The metabolisable energy intake (MJ) output from the biophysical modelling, was used in a partial discounted net cash flow budget. Both the biophysical and economic modelling suggest that deep roots were advantageous in all climate scenarios at the long growing season site, but in no scenarios at the short growing season season site likely due to the the deep-rooted species drying out the soil profile earlier. Production of the deep-rooted species in future scenarios at the long growing season site averaged 698 kg/ha/year more than the more shallow-rooted species, while at the short growing season site it averaged 164 kg/ha/year less than the shallower-rooted species. Timing of the extra growth and the persistence of the pasture were key characteristics determining the extent of the benefit of the deeper-rooted species. At the lower rainfall site other adaptation options such as summer dormancy will likely be necessary.