The effect of grassland gap size on establishment, growth and flowering of the endangered Rutidosis leptorrhynchoides (Asteraceae).
Abstract
To determine the effect of grassland gap size on patterns of seedling recruitment and juvenile establishment of the endangered composite Rutidosis leptorrhynchoides, seed and transplants were introduced into artificially created canopy gaps in both a 'short' and 'tall' temperate Themeda triandra tussock grassland in southern Victoria, Australia. Square gaps of 0 cm (undisturbed control), 15 cm, 30 cm, 50 cm and 100 cm width were created by removing the dominant grass, and the emergence, survival and growth of R. leptorrhynchoides were followed for 1 year. Survival and performance were compared with light quantity at ground level and soil moisture differences between the gaps. Emergence of seedlings was greatest in large gaps (30, 50 and 100 cm) but survival was restricted primarily to the 100-cm gaps in both grasslands. Survival of transplants to 1 year occurred in 30-, 50- and 100-cm gaps, suggesting that juvenile plants tolerate competition for resources better than germinating seedlings do. Survival, rate of growth and total number of inflorescences produced, however, was significantly greater in 100-cm gaps. There were few differences in the seasonal pattern of soil moisture between gaps in both grasslands. The amount of light at ground level was significantly greater in the short-grassland and in large gaps (30, 50 and 100 cm) at most times during the year. Differences between gaps in total soil moisture and light levels, however, only partly explain the patterns of transplant survival and growth observed. Soil moisture variability is suggested to have been an important factor influencing transplant survival over summer. These results confirm that R. leptorrhynchoides is a gap-sensitive species, with recruitment and survival unlikely to occur in canopy gaps less than 30-50 cm in diameter. Management of remnant populations needs to ensure that large canopy gaps are regularly maintained to maximize successful seedling recruitment and maintain the standing population. This may be achieved by burning the grassland at 3-year intervals. In the absence of the frequent burning of productive grasslands, localized extinction is likely. Reintroduction of the species into secure grassland reserves is likely to be problematic given the paucity of large Themeda-free gaps in these grasslands. Alternative strategies for conserving the species include its introduction into grassy woodlands where gap closure rates are likely to be slower.