Discrepancies between observed and predicted climate-driven net herbage accumulation

The decline in net herbage accumulation (NHA) on the high phosphorus (P) fertilizer farmlet (HF) of a long-term P fertilizer and associated sheep grazing experiment in the last 25 years, aligns with the necessity to reduce the on-site nominal sheep stocking rates over the same period on this farmlet. This finding appears at odds with projected climate change driven modelling that forecast a largely positive outcome on pasture growth in summer moist environments. In this paper we explore the apparent discrepancies between the observed and predicted climate-driven NHA by using a climate-driven pasture growth module within a larger process-based model (AgPasture in APSIM) to simulate NHA, legume growth, nitrogen fixation and water balance across three slopes [Low (LS; 0°-12°), Medium (MS; 13°-25°) and High slope (HS;>25°)] from 1980-2021. To assess the ability of the model to capture the influence of spatiotemporal climate variables on pasture growth, the model output for 1972-1981 was compared with NHA measurements collected across the three slope classes for that same period. A good relationship was found between modelled and measured NHA across the three slopes classes giving confidence in the model’s ability to capture the influence of both spatial and temporal climate variation on plant growth. A comparison of the modelled NHA for the three slope classes during 1982-88 with 2012-2018 indicates a significant (p<0.01) decline in NHA over time. There has been no clear trend in annual rainfall since 1982, however, mean daily maximum temperature has increased 1.5°C. The average modelled summer soil moisture deficit (January to March) has increased from -41mm between 1982-1988 to -55 mm between 2012-2018. Our modelling work suggests that the summer soil moisture deficit and temperature stress are having a greater effect on NHA than the predicted benefits of higher [CO₂] and winter and early spring temperatures, leading to long-term reductions in NHA, rather than an overall increase.