EFFECTS OF ELEVATED CO2 ON CAPE-FYNBOS SPECIES ADAPTED TO SOILS OF DIFFERENT NUTRIENT STATUS - NUTRIENT-RESPONSIVENESS AND CO2-RESPONSIVENESS

The combined effects of elevated atmospheric CO2 and nutrient supply r ate on plant biomass accumulation were determined for four Leucadendro n species (Proteaceae) of the mediterranean climate Fynbos Biome, Sout h Africa. Juvenile individuals were grown for 6 months in experiments comprising 2X2 factorial combinations of substrate nutrient supply rat e and atmospheric CO2 concentration in open-top chambers in a greenhou se. The four selected Leucadendron species included one pair of specie s common on extremely nutrient-poor acid sands (typical of the Fynbos Biome), and another pair associated with more nutrient rich substrates (rare in the Fynbos Biome). Plant biomass accumulation data were anal ysed to explore the determinants of plant CO2 responsiveness, particul arly the role of plant sink strength characteristics. Results lead us to speculate that the nitrogen:phosphorus supply ratio may have limite d plant CO2 responsiveness in three of the four species under conditio ns of higher nutrient supply rate. Intrinsic plant growth characterist ics, possibly related to the relative ability of the species to genera te sinks, may ultimately have limited the capacity of all species to r espond to elevated CO2.