Effect of elevated CO2 and soil fertilization on whole-plant growth and water use in seedlings of a tropical pioneer tree, Ficus insipida Willd.

Seedlings of the tropical pioneer tree species Ficus insipida were cultivated at present-ambient and elevated (about twice-ambient) CO2 concentrations in open-top chambers located in a forest clearing near Panama City, Republic of Panama. To examine potential chamber-specific effects on growth and transpiration, plants were also studied outside chambers at ambient CO2 levels. Plants were grown individually in 38 litre pots containing a mixture of soil and leaf litter, either in the absence or presence of a slow-release fertilizer. Data from three experiments. lasting 7 to 9 weeks each, are presented. Transpirational water loss of plants was determined gravimetrically. Fertilized plants grew more rapidly than unfertilized plants. Elevated CO2 strongly enhanced biomass accumulation in fertilized plants. In unfertilized plants, elevated CO2 enhanced growth in two experiments, but not in a third. Transpiration ratios (TR, g water lost: g dry mass accumulated) of plants grown in open-top chambers ranged from 176 (elevated CO2 plus fertilizer) to 336 (ambient CO2, minus fertilizer). The addition of fertilizer decreased TR by 15 to 20%, irrespective of the CO2 concentration, and elevated CO2 reduced TR by 27 to 35%, irrespective of whether fertilizer was present or not. The reduction in TR in response to elevated CO2, was independent of whether biomass accumulation was enhanced by elevated CO2 or not. In all experiments in which biomass accumulation was increased at elevated CO2, absolute water expenditure at elevated CO2 was greater or similar to that at ambient levels - despite the lower TR at elevated CO2 In the single experiment in which elevated CO2 diet not lead to increased growth, the absolute water expenditure of plants was lower at elevated than at ambient CO2. There was no chamber effect on biomass accumulation, but TR of both fertilized and unfertilized plants was 19 to 31% higher inside compared to outside the open-top chambers.