EFFECTS OF RAINFALL, MOISTURE STRESS, AND STOCKING RATE ON THE PERSISTENCE OF WHITE CLOVER OVER 30 YEARS

The effects of spring rainfall; critical levels of summer moisture str ess, and sheep stocking rates on the persistence of white clover (Trif olium repens cv. Huia) have been evaluated in a 30-year experiment (19 64-93) based on sown, well-fertilised pasture. Plant species presence was measured each year as basal cover using a vertical 10-pin frame. H its at ground level from 800 points/plot were recorded in late Septemb er on duplicate plots, which were set-stocked at 3 rates (10, 20 reduc ed to 15, 30 reduced to 20 d.s.e./ha). A soil-water model based on rai nfall and tank evaporation was calibrated against on-site soil water m easurements (0-260 mm) and used to predict soil water (mm) for weekly time steps over 30 years. Smoothing of long-term rainfall data (SYSTAT , Lowess) showed an overall decline in warm-season rainfall (October-M arch), which was punctuated by above-average (1969-74) and average run s of years (1983-90). Flexible smoothing splines (SAS) were used to in dicate patterns of yearly white clover presence. For all stocking trea tments, there were significant declines in the presence of white clove r over 3 decades. At the highest stocking rate, the recovery of white clover following the 1965 drought was poor. Late summer (January-March ) moisture stress, defined as the number of weeks when soil water (0-2 60 mm) was <15 mm, was critical in determining white clover presence i n the following spring (September). Rainfall received from October to December generally had a positive effect. These climate-based relation ships reinforce the importance of stolen growth and survival as a rege nerative strategy for white clover. However, over the 30 years, the sp ecies showed decreasing resilience post drought, which suggests a long -term failure of seed-based regeneration. Annual rates of soil nitroge n build-up ranged from 29 to 54 kg N/ha.year and were poorly related t o white clover presence in the stocking treatments. Governing mechanis ms, based on interactions between seasonal moisture stress, sheep stoc king rate, interspecific plant competition, and seed pool dynamics, ar e proposed to explain the nature of long-term decline in white clover presence in well-fertilised, sown pastures in the Northern Tablelands of New South Wales.