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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