Responses of legume versus nonlegume tropical tree seedlings to elevated CO₂ concentration

We investigated responses of growth, leaf gas exchange Carbon-isotope discrimination, and whole-plant water-use efficiency (W_P) to elevated CO₂ concentration ([CO₂]) in seedlings of five leguminous and five nonleguminous tropical tree species. Plants were grown at CO₂ partial pressures of 40 and 70 Pa. As a group, legumes did not differ from nonlegumes in growth response to elevated CO₂]. The mean ratio of final plant dry mass at elevated to ambient [CO₂] (M_E/M_A) was 1.32 and 1.24 for legumes and nonlegumes, respectively. However, there was large variation in M_E/M_A among legume species (0.92-2.35), whereas nonlegumes varied much less (1.21-1.29). Variation among legume species in M_E/M_A was closely correlated with their capacity for nodule formation, as expressed by nodule mass ratio, the dry mass of nodules for a given plant dry mass. WP increased markedly in response to elevated [CO₂] in all species. The ratio of intercellular to ambient CO₂ partial pressures during photosynthesis remained approximately constant at ambient and elevated [CO₂], as did carbon isotope discrimination, suggesting that WP should increase proportionally for a given increase in atmospheric [CO₂]. These results suggest that tree legumes with a strong capacity for nodule formation could have a competitive advantage in tropical forests as atmospheric [CO₂] rises and that the water-use efficiency of tropical tree species will increase under elevated [CO₂].