Climatic conditions associated with successful perennial grass recruitment events in south-eastern Australia

Recruitment of new perennial grass plants within existing ecosystems is determined by seed availability, suitable microsites, resources and climatic conditions. This paper reports on the modelled soil moisture conditions associated with successful recruitment events in five field experiments at Orange (Phalaris aquatica), Trunkey Creek (Austrodanthonia spp.) and Wellington (Bothriochloa macra) in Central New South Wales (NSW), and the frequency of those conditions during the past 30 years. High seedling numbers were recorded when mature germinable seed was present and when a rainfall event (median 68 mm across the three sites) kept the surface soil moisture (0-50 mm) above the permanent wilting point for at least 15 days. Rainfall events typically occurred in the second half of February, sometimes extending to early March. No recruitment occurred if the surface soil was only moist for 7 days or less. The past data (1975 - 2004) showed that Orange had a median of 20 moist days each year at this time, whereas it was 16 days for Trunkey Creek and 10 days for Wellington. The probabilities of exceeding 7 or 15 days of moist surface soil were 98% and 78% at Orange, 91% and 49% at Trunkey Creek, and 73% and 30% at Wellington. These results show some recruitment is possible in most years, provided seasonal conditions enable adequate seed set over summer.