Effects of elevated CO2 on Cape Fynbos species adapted to soils of different nutrient status: Nutrient- and CO2-responsiveness

The combined effects of elevated atmospheric CO2 and nutrient supply rate on plant biomass accumulation were determined for four Leucadendron species (Proteaceae) of the mediterranean climate Fynbos Biome, South Africa. Juvenile individuals were grown for 6 months in experiments comprising 2X2 factorial combinations of substrate nutrient supply rate and atmospheric CO2 concentration in open-top chambers in a greenhouse. The four selected Leucadendron species included one pair of species 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 analysed to explore the determinants of plant CO2 responsiveness, particularly the role of plant sink strength characteristics. Results lead us to speculate that the nitrogen:phosphorus supply ratio may have limited plant CO2 responsiveness in three of the four species under conditions of higher nutrient supply rate. Intrinsic plant growth characteristics, possibly related to the relative ability of the species to generate sinks, may ultimately have limited the capacity of all species to respond to elevated CO2.