Relationships among photosynthesis, foliar nitrogen and stomatal Relationships among photosynthesis, foliar nitrogen and stomatal conductance in tropical rain forest vegetationconductance in tropical rain forest vegetation
Tomas Domingues; Joe Berry; Luiz Martinelli; Jean Ometto & Jim Ehleringer1
IntroductionSignificant 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.
MethodsAmax, 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 13C were
determined at Laboratório de Ecologia Isotópica of the Centro de Energia Nuclear na
Agricultura (CENA) of the University of São Paulo, Piracicaba, Brazil.
SitePlant Functional
GroupNumber of
speciesPrimary Forest Top canopy lianas 3
Primary Forest Top canopy trees 5
Primary Forest Mid canopy trees 6
Primary Forest Understory trees 6
Pasture Grass 1
Pasture Saplings 4
Results I - Canopy structureA 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
AcknowledgementsWe are grateful for the financial support provided by NASA LBA-ECO.
We also appreciate the help from the LBA-office at Santarémand from our friends at the Ehleringer Lab.
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.
C D
Results III
Nitrogen Use EfficiencyPhotosynthetic 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).
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.
Results IV
Stomatal ConductanceThere 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 13C.
G
F
Nitrogen use efficiency
0
1
2
3
4
5
6
Shrub Mid Under Up Liana
Functional Groups
A/N
Leaf area index
y = -19.6x + 106.6
R2 = 0.85
0
25
50
75
100
0 2 4 6 8
LAI (leaf m2 per ground m2)
Relative canopy height
Specific leaf area
y = -61Ln(x) - 224
R2 = 0.57
0
25
50
75
100
0.00 0.01 0.02 0.03 0.04
SLA (m2 g-1)
Relative canopy height
Assimilation vs. Stomatal conductance
0
5
10
15
20
25
0.0 0.1 0.2 0.3
g (mol m -2 s-1)
Amax
(μ mol m
-2 s
-1)
Grass
Shrub
Up
Liana
Mid
Under
Ci/Ca vs. 13 C
0.4
0.5
0.6
0.7
0.8
0.9
-38 -35 -32 -29 -26
13 C ( )per mil
/Ci Ca
Shrub
Up
Liana
Mid
Under
Assimilation vs. Leaf [N]
y = 2.99x + 1.78
R2 = 0.300
5
10
15
20
25
0 1 2 3 4 5
Leaf [N] (g m-2)
Amax
(μ mol m
-2 s
-1)
GrassShrubUpLianaMidUnder
Respiration vs. Leaf [N]
y = 0.51x - 0.17
R2 = 0.340
1
2
3
4
0 1 2 3 4 5
Leaf [N] (g m-2)
Respiration
(μ mol m
-2 s
-1)
GrassShrubUpLianaMidUnder
E
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