Soil-nutrient availability modifies the response of young pioneer and late successional trees to elevated carbon dioxide in a Brazilian tropical environment

Abstract

The aim of this work was to determine the impact of three levels of [CO 2] and two levels of soil-nutrient availability on the growth and physiological responses of two tropical tree species differing in their ecological group: Croton urucurana Baillon, a pioneer (P), and also Cariniana legalis (Martius) Kuntze, a late succession (LS). We aimed to test the hypothesis that P species have stronger response to elevated [CO 2] than LS species as a result of differences in photosynthetic capacity and growth kinetics between both functional groups. Seedlings of both species were grown in open-top-chambers under high (HN) or low (LN) soil-nutrient supply and exposed to ambient (380 μmol mol −1) or elevated (570 and 760 μmol mol −1) [CO 2]. Measurements of gas exchange, chlorophyll a fluorescence, seedling biomass and allocation were made after 70 days of treatment. Results suggest that elevated [CO 2] significantly enhances the photosynthetic rates ( A) and biomass production in the seedlings of both species, but that soil-nutrient supply has the potential to modify the response of young tropical trees to elevated [CO 2]. In relation to plants grown in ambient [CO 2], the P species grown under 760 μmol mol −1 [CO 2] showed increases of 28% and 91% in A when grown in LN and HN, respectively. In P species grown under 570 μmol mol −1 [CO 2], A increased by 16% under HN, but there was no effect in LN. In LS species, the enhancement of A by effect of 760 μmol mol −1 [CO 2] was 30% and 70% in LN and HN, respectively. The exposure to 570 μmol mol −1 [CO 2] stimulated A by 31% in HN, but was no effect in LN. Reductions in stomatal conductance ( g s) and transpiration ( E), as a result of elevated [CO 2] were observed. Increasing the nutrient supply from low to high increased both the maximum rate of carboxylation ( V cmax) and maximum potential rate of electron transport ( J max). As the level of [CO 2] increased, both the V cmax and the J max were found to decrease, whereas the J max/ V cmax ratio increased. In the LS species, the maximum efficiency of PSII ( F v/ F m) was higher in the 760 μmol mol −1 [CO 2] treatment relative to other [CO 2] treatments. The results suggest that when grown under HN and the highest [CO 2], the performance of the P species C. urucurana, in terms of photosynthesis and biomass enhancement, is better than the LS species C. legalis. However, a larger biomass is allocated to roots when C. legalis seedlings were exposed to elevated [CO 2]. This response would be an important strategy for plant survival and productivity of the LS species under drought stresses conditions on tropical environments in a global-change scenario.