C-13 NATURAL-ABUNDANCE IN PLANT-COMMUNITIES ALONG A RAINFALL GRADIENT - A BIOLOGICAL INTEGRATOR OF WATER AVAILABILITY

Carbon isotope natural abundance (delta(13)C) has been previously used as a powerful tool in the study of water-using processes at the leaf, individual and within-community levels. We analysed 348 species from 12 plant communities along a 900 km-long rainfall gradient in southern Queensland. Although the range of delta(13)C values found in a given community was large, variability in the delta(13)C signature of plants within a community was relatively small given the large numbers of species sampled (mean n per site of 29) and the taxonomic diversity in each. The community-averaged delta(13)C signature ranged from -25.6 parts per thousand in a brigalow (Acacia harpophylla) dominated community in western Queensland to -31.2 parts per thousand in subtropical rainforest in eastern Queensland. A strong relationship was found between the delta(13)C value averaged for each site and rainfall within the range 350-1700 mm per annum. Foliar delta(13)C was also related to the number of rain days per annum and moisture balance (rainfall - evaporation). The strength of these relationships varied only slightly according to the rainfall parameter used, with values for r(2) of 0.78, 0.70, 0.70 and 0.74 for the relationship between delta(13)C and long-term rainfall average, 5-year rainfall average, number of rain days and moisture balance, respectively. Despite considerable taxonomic variability within a given plant community, the averaged delta(13)C signature for that community gives a strong indication of moisture availability.