Response of five dry tropical tree seedlings to elevated CO<SUB>2</SUB>: impact of seed size and successional status

The impact of seed size and successional status on seedling growth under elevated CO2 was studied for five dry tropical tree species viz. Albizia procera, Acacia nilotica, Phyllanthus emblica, Terminalia arjuna and Terminalia chebula. Seedlings from large (LS) and small seeds (SS) were grown at two CO2 levels (ambient and elevated, 700-750 ppm). CO2 assimilation rate, stomatal conductance, water use efficiency and foliar N were determined after 30 d exposure to elevated CO2. Seedlings were harvested after 30 d and 60 d exposure periods. Height, diameter, leaf area, biomass and other growth traits (RGR, NAR, SLA, R:S) were determined. Seedling biomass across species was positively related with seed mass. Within species, LS seedlings exhibited greater biomass than SS seedlings. Elevated CO2 enhanced plant biomass for all the species. The relative growth rate (RGR), net assimilation rate (NAR), CO2 assimilation rate, R:S ratio and water use efficiency increased under elevated CO2. However, the positive impact of elevated CO2 was down regulated beyond 30 d exposure. Specific leaf area (SLA), transpiration rate, stomatal conductance declined due to exposure to elevated CO2. Fast growing, early successional species exhibited greater RGR, NAR and CO2 assimilation rate. Per cent enhancement in such traits was greater for slow growing species. The responses of individual species did not follow functional types (viz. legumes, non-legumes). The enhancement in biomass and RGR was greater for large-seeded species and LS seedlings within species. This study revealed that elevated CO2 could cause large seeded, slow growing and late successional species to grow more vigorously