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

Abstract

Summary Seedlings of the tropical pioneer tree species Ficus insipida were cultivated at present-ambient and elevated (about twice-ambient) CO 2 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 CO 2 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 CO 2 strongly enhanced biomass accumulation in fertilized plants. In unfertilized plants, elevated CO 2 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 CO 2 , plus fertilizer) to 336 (ambient CO 2 , minus fertilizer). The addition of fertilizer decreased TR by 15 to 20%, irrespective of the CO 2 concentration, and elevated CO 2 reduced TR by 27 to 35%, irrespective of whether fertilizer was present or not. The reduction in TR in response to elevated CO 2 was independent of whether biomass accumulation was enhanced by elevated CO 2 or not. In all experiments in which biomass accumulation was increased at elevated CO 2 , absolute water expenditure at elevated CO 2 was greater or similar to that at ambient levels – despite the lower TR at elevated CO 2 . In the single experiment in which elevated CO 2 did not lead to increased growth, the absolute water expenditure of plants was lower at elevated than at ambient CO 2 . 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.