In situ responses to elevated CO2 in tropical forest understorey plants

1. Plants growing in deep shade and high temperature, such as in the unders torey of humid tropical forests, have been predicted to be particularly sen sitive to rising atmospheric CO2. We tested this hypothesis in five species whose microhabitat quantum flux density (QFD) was documented as a covariab le. After 7 (tree seedlings of Tachigalia versicolor and Beilschmiedia pend ula) and 18 months (shrubs Piper cordulatum and Psychotria limonensis, and grass Pharus latifolius) of elevated CO2 treatment (c. 700 mu l litre(-1)) under mean QFD of less than 11 mu mol m(-2) s(-1), ail species produced mor e biomass (25-76%) under elevated CO2. 2. Total plant biomass tended to increase with microhabitat QFD (daytime me ans varying from 5 to II mu mol m(-2) s(-1)) but the relative stimulation b y elevated CO2 was higher at low QFD except in Pharus. 3. Non-structural carbohydrate concentrations in leaves increased significa ntly in Pharus (+ 27%) and Tachigalia (+ 40%). 4. The data support the hypothesis that tropical plants growing near the ph otosynthetic light compensation point are responsive to elevated CO2. An im proved plant carbon balance in deep shade is likely to influence understore y plant recruitment and competition as atmospheric CO2 continues to rise.