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

The aim of this work was to determine the impact of three levels of [CO2] 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 [CO2] 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) [CO2]. Measurements of gas exchange, chlorophyll a fluorescence, seedling biomass and allocation were made after 70 days of treatment. Results suggest that elevated [CO2] 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 [CO2]. In relation to plants grown in ambient [CO2], the P species grown under 760 μmol mol−1 [CO2] showed increases of 28% and 91% in A when grown in LN and HN, respectively. In P species grown under 570 μmol mol−1 [CO2], 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 [CO2] was 30% and 70% in LN and HN, respectively. The exposure to 570 μmol mol−1 [CO2] stimulated A by 31% in HN, but was no effect in LN. Reductions in stomatal conductance (gs) and transpiration (E), as a result of elevated [CO2] were observed. Increasing the nutrient supply from low to high increased both the maximum rate of carboxylation (Vcmax) and maximum potential rate of electron transport (Jmax). As the level of [CO2] increased, both the Vcmax and the Jmax were found to decrease, whereas the Jmax/Vcmax ratio increased. In the LS species, the maximum efficiency of PSII (Fv/Fm) was higher in the 760 μmol mol−1 [CO2] treatment relative to other [CO2] treatments. The results suggest that when grown under HN and the highest [CO2], 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 [CO2]. 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.

► Under highest CO2, pioneers species performed better than late sucessional species. ► More biomass is allocated to roots in late succesional species under elevated CO2. ► Larger root biomass is important strategy under stress in a global-change scenario.