Photosynthesis-nitrogen relationships in pioneer plants of disturbed tropical montane forest sites

Tropical forest disturbances lead to the establishment of secondary success ional plant communities constituted by light demanding species with high re lative growth rate that conduct to rapid canopy closure. Two main strategie s for N nutrition are: (a) mineral N acquisition in the form of NH4 and NO3 , and (b) symbiotic atmospheric N-2 fixation. Given the high N requirement for maximization of leaf area and radiant energy absorption, we hypothesize that contrasting strategies of N nutrition in these environments are refle cted in leaf photosynthetic characteristics. We compared the N-photosynthes is relationships and carbon balance parameters per unit leaf area as they v ary with age in two species with contrasting N acquisition strategies: a N- 2-fixer Crotalaria anagyroides HBK (Papilionoideae), and a mineral-N user V erbesina turbacensis HBK (Asteraceae). N-2 fixation capacity was associated to higher specific leaf area (SLA), higher photosynthetic capacity (P-max) per unit leaf area and leaf mass, and higher N content per unit leaf mass. The N-2-fixer species showed higher slope in the relationship P-max-N per unit leaf mass and area when compared to the leaves of non-fixer species. M oreover, the intrinsic photosynthetic N use efficiency (P-max/N) was higher in the N-2 fixer than in leaves of the non-fixer species. Changes in N due to leaf age resulted in larger changes in CO2 flux density at the leaf lev el in the N-2-fixer species. The higher photosynthetic capacity of the N-2- fixer species was mechanistically related to higher stomatal conductance, i nternal CO2 concentration (c(i)) values closer to atmospheric CO2 concentra tion (c(a)), and lower intrinsic water use efficiency than the mineral N-us er species. Despite their higher P-max per unit leaf area, total non-struct ural saccharides concentration was lower in mature leaves of the N-2-fixer plant as compared to the non-fixer counterpart. This might be caused by the presence of a larger root sink (symbionts) stimulating saccharides export and higher diurnal respiration rates.