Relationships among photosynthesis, foliar nitrogen and stomatal conductance in tropical rain forest

1
Relationships among photosynthesis, foliar
nitrogen and stomatal conductance in tropical
rain forest vegetation
Tomas Domingues Joe Berry Luiz Martinelli Jean
Ometto Jim Ehleringer1
Results IV Stomatal Conductance There is a strong
positive correlation between Amax and stomatal
conductance at the species level (F25.5,
Plt0.001). However, this is not significant when
calculated for functional groups (Figure F)
(F6.28, P0.09). Figure G shows measured Ci/Ca
values and its correlation with
d13C.
Results III 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.
Introduction 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.
E
F
A
B
G
Methods 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 (F9.02 P0.007). 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
(F11.6 P0.003) (figure D). When data were
averaged into functional groups neither
regressions was significant.
C
D

• Conclusions
• Leaf distribution creates a heterogeneous
  environment inside the forest canopy.
• Species showed strong correlation of Leaf
  Nitrogen to both Amax and Respiration.
• Nitrogen use efficiency, stomatal conductance,
  Ci/Ca and ?13C data suggest that lianas are more
  susceptible to water stress.
• Boundaries between functional groups were not
  obvious in this study.

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.