Climatic conditions for seedling recruitment within perennial grass swards in south-eastern Australia

Abstract

Recruitment of new perennial grass plants within existing grassland ecosystems is determined by seed availability, suitable microsites, nutrients and climatic conditions, water and temperatures. This paper reports on the development of criteria to predict recruitment events using modelled soil moisture conditions associated with recruitment of species in five field experiments at Orange (Phalaris aquatica), Trunkey Creek (Austrodanthonia spp.), and Wellington (Bothriochloa macra) in central New South Wales, Australia, and the frequency of those conditions during the past 30 years. Recruitment events were recorded when a rainfall event (median 68 mm across the three sites) kept the surface volumetric soil moisture (0–50 mm) above the permanent wilting point for at least 15 continuous days, allowing for, at most, two ‘dry days’ in between. A key finding from our study is that rainfall events creating favourable soil moisture conditions for seedling emergence typically occurred in the second half of February, sometimes extending to early March. Previously it was thought that recruitment would more likely occur through autumn, winter, and spring when rainfall in southern Australia is more reliable. The 30 years’ data (1975–2004) showed that the P. aquatica site had a median of 20 continuous moist days each year in February–March, whereas, there were 16 and 10 days for the Austrodanthonia and B. macra sites, respectively. The probabilities of exceeding seven or 15 continuous days of moist surface soil were 98% and 78% at the P. aquatica site, 91% and 49% at the Austrodanthonia site, and 73% and 30% at the B. macra site, and indicated that some recruitment is possible in most years. These analyses were extended to several sites across New South Wales, Victoria, and Tasmania to estimate the frequency with which recruitment could occur within natural swards. Across these sites, the probabilities of exceeding seven continuous days of soil moisture were >55% and of exceeding 15 continuous days were lower, which showed that suitable climatic conditions exist during late summer–early autumn across south-eastern Australia for a recruitment event to occur. Future research may show that the criteria developed in this paper could have wider regional application.