Relationships among photosynthesis, foliar nitrogen and stomatal conductance in tropical rain forest vegetation Tomas Domingues; Joe Berry; Luiz Martinelli; Jean Ometto & Jim Ehleringer 1 Introduction Significant correlations have been observed between maximum carbon assimilation rates (A max ) and nitrogen content of leaves for several ecosystems. This relationship has been used to simplify a number of ecosystem-scale carbon balance models. The amount of nitrogen plants allocate to photosynthetic activities is a function of the light levels experienced by a particular leaf. Tropical rain forests displays complex canopies with high species richness. We tested the hypothesis that carbon assimilation rates can be predicted based on leaf nitrogen content for plant functional groups from an evergreen tropical rain forest ecosystem and a pasture site near Santarém (PA), Brazil. Methods A max, respiration rates, stomatal conductance to water vapor at A max, and Ci at A max where measured on 25 plant species grouped into 6 functional groups as follows: Both the primary forest and the pasture sites were located about 70 km south of Santarém (2° 25’ S, 54° 43’ W). Sampling period ranged from November 1999 through July 2003 covering both wet and dry season. Gas-exchange measurements were collected with a Li-Cor The environmental conditions within the Li-Cor chamber were held constant. Leaf area was determined by drawing the leaf contour on paper, just after the gas- exchange measurements, and calculating this area using NIH-Image software. After drying, leaf dry weight, nitrogen content and  13 C were determined at Laboratório de Ecologia Isotópica of the Centro de Energia Nuclear na Agricultura (CENA) of the University of São Paulo, Piracicaba, Brazil. Site Plant Functional Group Number of species Primary ForestTop canopy lianas3 Primary Forest Top canopy trees5 Primary ForestMid canopy trees6 Primary ForestUnderstory trees6 PastureGrass1 PastureSaplings4 Results I - Canopy structure A tall, dense canopy characterizes the primary forest site used in this study. Canopy height varies between 30 and 40 meters. Figure A represents the profile of Leaf Area Index (LAI) for the primary forest. The distribution of leaves inside the canopy causes the light environment to change with height. Associated with such changes, leaves exhibit variations in area-to-mass ratio (Specific Leaf Area - SLA), shown in Figure B. The figures contain data from three locations. A B Acknowledgements We are grateful for the financial support provided by NASA LBA-ECO. We also appreciate the help from the LBA-office at Santarém and from our friends at the Ehleringer Lab. Results II - CO 2 fluxes and Foliar Nitrogen Leaves allocate considerable portion of available nitrogen to the protein pool responsible for photosynthesis. Figure C shows a significant positive correlation between Amax and leaf nitrogen (F=9.02; P=0.007). Grass was omitted in this regression because they use the C 4 photosynthetic pathway and thus depart from the expected nitrogen-assimilation relationship. Daytime dark respiration rate is determined by leaf metabolism. The species showed a significant positive correlation with leaf nitrogen content (F=11.6; P=0.003) (figure D). When data were averaged into functional groups neither regressions was significant. CD Results III Nitrogen Use Efficiency Photosynthetic nitrogen use efficiency (PNEU) is the ratio of A max over foliar nitrogen. Lianas showed the lowest PNEU among all groups (Figure E), indicating that this group is perhaps more susceptible to water stress. The Grass functional group showed the highest PNEU (= 27.3) (not shown in this figure). Conclusions Leaf distribution creates a heterogeneous environment inside the forest canopy. Species showed strong correlation of Leaf Nitrogen to both A max and Respiration. Nitrogen use efficiency, stomatal conductance, Ci/Ca and  13 C data suggest that lianas are more susceptible to water stress. Boundaries between functional groups were not obvious in this study. Results IV Stomatal Conductance There is a strong positive correlation between A max and stomatal conductance at the species level (F=25.5, P<0.001). However, this is not significant when calculated for functional groups (Figure F) (F=6.28, P=0.09). Figure G shows measured Ci/Ca values and its correlation with  13 C. G F E