Responses of model communities of two tropical tree species to elevated atmospheric CO2 : growth on unfertilized soil

Abstract

Summary Biomass accumulation of model seedling communities of two tropical tree species was studied at ambient and elevated CO 2 levels (four replicates per CO 2 concentration), using open-top chambers situated in a cleared area at the edge of a tropical forest near Panama City, Republic of Panama. Each chamber (diameter about 2 m) contained a mixture of 18 plants of Ficus insipida , a fast-growing pioneer species, and 18 plants of Virola surinamensis , a slow-growing late-successional species. Plants grew in well-drained, well-watered, loose, unfertilized soil. During the 30 week experimental period, biomass accumulation by F. insipida , which increased in height from about 20 to 180 cm, predominated, whereas V. surinamensis grew only slowly in the strongly shaded understorey. Neither community biomass accumulation (above- plus belowground) nor the biomass ratio ( F. insipida: V. surinamensis ) of the two species was significantly affected by elevated CO 2 , although community biomass accumulation was on average slightly higher at elevated than at ambient CO 2 , and the biomass of F. insipida increased on average relative to V. surinamensis at elevated CO 2 . Plants grown at elevated CO 2 contained greater levels of starch and lower levels of nitrogen per unit leaf dry mass, and, therefore, showed higher ratios of C: N than plants at ambient CO 2 . Specific leaf area, the area per unit leaf dry mass; and the leaf area ratio (LAR), the total leaf area per total plant dry mass, decreased in response to elevated CO 2 . This decrease in LAR counteracted the potential effect on biomass gain of increases in both net assimilation rate (the rate of dry mass accumulation per unit leaf area) and rates of photosynthetic leaf net CO 2 uptake at elevated as compared to ambient CO 2 , particularly in Ficus insipida .