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Relationships among photosynthesis, foliar nitrogen and stomatal conductance in tropical rain forest vegetation. Tomas Domingues; Joe Berry; Luiz Martinelli; Jean Ometto & Jim Ehleringer 1. Results IV Stomatal Conductance
Tomas Domingues; Joe Berry; Luiz Martinelli; Jean Ometto & Jim Ehleringer1
There is a strong positive correlation between Amax 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 d13C.
Nitrogen Use Efficiency
Photosynthetic nitrogen use efficiency (PNEU) is the ratio of Amax 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).
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.
Significant correlations have been observed between maximum
carbon assimilation rates (Amax) 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.
Amax, respiration rates, stomatal conductance to water vapor at Amax, and Ci at Amax
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 6400.
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 d13C were
determined at Laboratório de Ecologia Isotópica of the Centro de Energia Nuclear na
Agricultura (CENA) of the University of São Paulo, Piracicaba, Brazil.
Results II - CO2 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 C4 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.
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.