Relative Growth of Themeda Australis and Poa Labillardieri in Pots in Response to Phosphorus and Nitrogen.

Abstract

Results are presented of a pot experiment on the responses of Themeda australis and Poa labillardieri to increasing levels of substrate phosphorus and nitrogen. The species were grown either alone or together. When grown alone both species responded to increasing phosphorus and nitrogen and there was a significant interaction between the two elements in shoot growth per half-pot. There was no effect of increasing phosphorus without added nitrogen and no effect of increasing nitrogen without added phosphorus. At intermediate and high levels of substrate phosphorus there was a significant competitive effect between the shoot weight per half-pot and the tiller number of Themeda and Poa plants when grown together, irrespective of the nitrogen level. Poa always had a greater shoot dry weight and more tillers when grown with Themeda than when grown alone, and conversely Themeda a lower shoot dry weight and less tillers when grown with Poa than when grown alone. Chemical analyses of 22-week-old plant tissue showed that in Poa plants grown with Themeda the phosphorus and nitrogen contents were significantly greater than in Poa plants grown alone, at both intermediate and high levels of substrate phosphorus. The enhanced ability of Poa plants to absorb nutrients and grow in response to a low level of added phosphorus at the expense of Themeda plants when the two species are mixed in pots is similar to their response in the field in southern New South Wales. A description of the statistical treatment of the results, especially the competitive effect between Themeda and Poa when grown together, is appended.