Gas exchange measurements were undertaken at field trials to investigate the impact of thinning of 12 year old gmelina plantations located at Bato. Amax for leaves at the thinned site were significantly higher (pnarra>mahogany>mayapis. This pattern is also reflected in the average height of each species but not in the pattern for average biomass (number of leaves) and dbh which followed the pattern of falcata>mahogany>narra>mayapis. The reversal of mahogany and narra suggests a different pattern of resource allocation. Narra has high rates of Amax with a smaller number of leaves compared with mahogany which has a lower rate of Amax but a much greater number of leaves and total leaf biomass. There was a substantial difference between the light curves for 11 month old gmelina trees in the three experiment plots with a 3 m x 3 m spacing compared with gmelina trees in the one experimental plot with a 4 m x 4 m spacing (Figure 6). The preliminary interpretation of these results is that at even a very early age, resources are becoming limiting for photosynthesis at a 3 m x 3 m spacing. This unexpected finding will be investigated further with a variable spacing trial

Gregorio, N. O.

Herbohn, J. L.

Vanclay, J.

University of Queensland eSpace

11. INITIAL GAS EXCHANGE RESULTS FROM 
FIELD TRIALS 
 
John Herbohn, Nestor Gregorio and Jerry Vanclay 
 
 
Gas exchange measurements were undertaken at field trials to investigate the impact of 
thinning of 12 year old gmelina plantations located at Bato. Amax for leaves at the thinned site 
were significantly higher (p<0.05) than for leaves at the unthinned site. In addition, trees at the 
thinned site had an average dbh increment over the past 15 months of 5.6 cm compared with 
an average 3.5 cm dbh increment for trees at the unthinned site. It appears that the 
photosynthetic capacity of leaves of the trees at the thinned site has increased in response to 
an increase in resources made available to individual trees by the removal (thinning) of 
competing trees. Measurement of Amax at for four species in a mixed species trial revealed the 
following pattern: falcata>narra>mahogany>mayapis. This pattern is also reflected in the 
average height of each species but not in the pattern for average biomass (number of leaves) 
and dbh which followed the pattern of falcata>mahogany>narra>mayapis. The reversal of 
mahogany and narra suggests a different pattern of resource allocation. Narra has high rates 
of Amax with a smaller number of leaves compared with mahogany which has a lower rate of 
Amax but a much greater number of leaves and total leaf biomass. There was a substantial 
difference between the light curves for 11 month old gmelina trees in the three experiment 
plots with a 3 m x 3 m spacing compared with gmelina trees in the one experimental plot with 
a 4 m x 4 m spacing (Figure 6). The preliminary interpretation of these results is that at even a 
very early age, resources are becoming limiting for photosynthesis at a 3 m x 3 m spacing. 
This unexpected finding will be investigated further with a variable spacing trial. 
 
 
INTRODUCTION 
 
Field trials are long term experiments, which typically take many years to produce 
publishable scientific results. In the case of the trials established under ASEM/2003/052, 
publishable results have already been achieved for the variable spacing trial in Leyte Leyte, 
the gmelina trial at Mahaplag and the thinning experiments at Bato and Maasin. These early 
results are largely due to the well thought out experimental designs which were designed to 
produce results from an early age, and further enhanced by the high growth rates of the 
species planted. The field trials will be maintained as part of ACIAR project ASEM/2006/092 
and regular data collection undertaken and further results will be presented as part of the 
end of project report of that project. 
 
Photosynthesis is a physiological process involving the fixation of CO2 from the atmosphere 
and its conversion into organic matter and at the same time capturing energy from the sun. It 
is the key process that drives plant growth. Photosynthesis can be measured in a number of 
ways, one of the most common is to use a portable infrared gas analyser (IRGA) to measure 
changes in CO2 concentration as leaves photosynthesize. This paper briefly presents some 
of the early results from photosynthesis measurements made at the various field trials sites 
established as part of ASEM/2003/052. 
 
 
 
 
 
 83
Initial Gas Exchange Results from Field Trials 
 
METHODS 
 
Study Sites and Experimental Design 
 
Gas exchange measurements were undertaken at each of field trials established as part of 
ACIAR project ASEM/2003/052. These field trials comprise of thinning trials for gmelina and 
mahogany located at Bato and Maasin; mixed species, variable spacing and shade/fertiliser 
trials in Leyte Leyte; and a gmelina germplasm and early age silviculture trial at Mahaplag. 
 
Trees in the Leyte Leyte and Mahaplag sites were generally between 0.3 m and 3 m and as 
such there was easy access to leaves. At the Bato and Maasin sites, the trees were mature 
and ranged in height from 20 m to 31 m. In order to gain access to canopy leaves at these 
sites, four bamboo towers (Figure 1) were constructed (see Herbohn et al. in preparation for 
further details).   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1. Canopy tower in thinned 
gmelina plot at Bato (top left), 
Canopy tower in unthinned gmelina 
plot at Bato (top right) and team 
members undertaking gas exchange 
measurements (bottom) on platform 
at unthinned site at Bato 
 
 84
Improving Financial Returns to Smallholder Tree Farmers in the Philippines    
Gas Exchange Measurements  
 
Light response curves were determined for trees occurring at each site. In the case of 12 
year old gmelina at Bato, response curves were determined on consecutive days for 5 trees 
at the unthinned and thinned sites. Environmental conditions were similar on both days. Leaf 
gas exchange was measured using a LI-6400XT (LI-COR, Lincoln, Nebraska) portable gas 
exchange system with a 6400-02B LED leaf chamber. Light response curves were 
constructed using the automatic light curve function available with the LI-6400XT version 6 
software. Photosynthetic Photon Flux Density was initially set at 2000 μmol m-2 s-1 and then 
stepped down in nine increments to 0 μmol m-2 s-1. A flow rate of 500 μmol and the 
reference CO2 concentration was set at 400 μmol CO2 mol-1 air. Leaf chamber humidity was 
maintained at 60 to 80 % and leaf temperature at 29–31 oC. All measurements were made 
between 9 am and 3 pm and were suspended if leaves exhibited signs of wilting or if 
conditions became excessively hot (i.e. above 33 oC) or wet. Light saturated photosynthesis 
(Amax) was measured at 2000 μmol m-2 s-1 for additional individual trees at each site. Unless 
otherwise stated, measurements were made on fully expanded sun leaves. 
 
For the gmelina trial at Mahaplag, average light response curves for each of the 
experimental plots were obtained by measuring three leaves on each of five randomly 
selected trees. One leaf measurement was discarded from two sites due to apparent 
inconsistencies in the data once plotted. For 5 randomly selected trees in each plot, the tree 
was divided into vertical quadrants (Figure 2) and Amax was measured for three leaves in 
each quadrant. The first leaf was at random in the quadrant, the next two leaves sampled in 
the quadrant were then selected by moving 120o in both a clockwise and anti-clockwise 
direction.  
 
 
 
 
 
 
 
Q1
Q3
Q2
Q4
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 2. Identifying and measuring quadrants on 8 month old gmelina trees at Mahaplag 
 
RESULTS AND DISCUSSION 
 
Light response curves for 12 year old gmelina at the thinned and unthinned sites at Bato are 
presented in Figure 3. The light response curves at the thinned site are generally higher than 
those for trees at the unthinned site, with only a slight overlap. T-tests revealed that Amax for 
leaves at the thinned site were significantly higher (p<0.05) than for leaves at the unthinned 
 85
Initial Gas Exchange Results from Field Trials 
 
site. In addition, trees at the thinned site had an average DBH increment over the past 15 
months of 5.6 cm compared with an average 3.5 cm DBH increment for trees at the 
unthinned site. It appears that the photosynthetic capacity of leaves of the trees at the 
thinned site has increased in response to an increase in resources made available to 
individual trees by the removal (thinning) of competing trees. This response is probably due 
to the increased availability of a limiting resource such as light, water, a macronutrient such 
as N or P or a combination of these. It is also interesting to note that this impact starts to 
become statistically significant at quite low levels of PPFD. 
 
Figure 3. Light response curves for Gmelina trees at thinned and 
unthinned plots at Bato
-5
0
5
10
15
20
25
30
0 500 1000 1500 2000
Light intensity
P
ho
to
sy
nt
he
si
s
 
 
 Amax = 18.6 
 Thinned diameter 
increment = 5.6  
 
 
 
 Amax = 14.0 
 Unthinned 
diameter  
 
 
 
 
 
 
 
 
 
 
 
The average light response curves for the four species used in the mixed species trial in Leyte 
Leyte are given in Figure 4. Amax follows the pattern of falcata>narra>mahogany>mayapis. 
This pattern is also reflected in the average height of each species but not in the pattern for 
average biomass (number of leaves) and dbh which followed the pattern of 
falcata>mahogany>narra>mayapis. The reversal of mahogany and narra suggests a different 
pattern of resource allocation. Narra has high rates of Amax with a smaller number of leaves 
compared with mahogany which has a lower rate of Amax but a much greater number of 
leaves and total leaf biomass. There is a far greater variability in both the shape of the 
individual light response curves and Amax for mayapis compared with the faster growing 
falacta, mahogany and narra (Figure 5).  
 
 
 
 
 
 
 
 
 86
Improving Financial Returns to Smallholder Tree Farmers in the Philippines    
-5
0
5
10
15
20
25
0 500 1000 1500 2000
Light intensity (μmol m-2 s-1) 
P
ho
to
sy
nt
he
si
s 
(μ
m
ol
 C
O
2 
m
-2
 s
-1
)
Narra
Mayapis
Falcata
Mahogany
 
 
Figure 4. Average light curves for species used in mixed species trial 
 
 
(a) Light response curve for Mayapis 
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
0 500 1,000 1,500 2,000
Light intensity (μmol m-2 s-1)
P
ho
to
sy
nt
he
si
s 
(μ
m
ol
 C
O
2 
m
-2
 s
-1
) 
Tree 75
Tree 59
Tree 48
Tree 45
Tree 24
 
 
(b) Light response curves for Narra 
-5.0
0.0
5.0
10.0
15.0
20.0
25.0
0 500 1,000 1,500 2,000
Light intensity (μmol m-2 s-1) 
P
ho
to
sy
nt
he
si
s
(μ
m
ol
 C
O
2 
m
-2
 s
-1
)
Tree 12
Tree 35
Tree 44
Tree 61
Tree 76
 
 
Figure 5. Individual light response curves for individual trees for each species used in the 
mixed species trial in Leyte Leyte 
 
 87
Initial Gas Exchange Results from Field Trials 
 
(c) Light response curve for Falcata (unadjusted) 
-5
0
5
10
15
20
25
30
0 500 1000 1500 2000 2500
Light intensity (μmol m-2 s-1)
P
ho
to
sy
nt
he
si
s 
(μ
m
ol
 C
O
2 
m
-2
 s
-1
)
Tree 85 (1)
Tree 66 (1)
Tree 35 (1)
Tree 21
Tree 35
Tree 66
Tree 85
Tree 72
 
 
(d) Light response curve for Mahogany (unadjusted) 
-5
0
5
10
15
20
0 500 1000 1500 2000 2500
Light intensity (μmol m-2 s-1) 
P
ho
to
sy
nt
he
si
s
(μ
m
ol
 C
O
2 
m
-2
 s
-1
) Tree 83
Tree 77
Tree 56
Tree 24
Tree 12
Tree 24 - 2
 
 
Figure 5 (Cont.). Individual light response curves for individual trees for each species used 
in the mixed species trial in Leyte Leyte 
 
There was a substantial difference between the light curves for 11 month old gmelina trees 
in the three experiment plots with a 3 m x 3 m spacing compared with gmelina trees in the 
one experimental plot with a 4 m x 4 m spacing (Figure 6). The differences were statistically 
significant, as evidenced by the non-overlap of standard error bars. The preliminary 
interpretation of these results is that at even a very early age, resources are becoming 
limiting for photosynthesis at a 3 m x 3 m spacing. This unexpected finding will be 
investigated further with a variable spacing trial. 
 
 88
Improving Financial Returns to Smallholder Tree Farmers in the Philippines    
-5
0
5
10
15
20
25
0 500 1000 1500 2000
Light intensity (μmol m-2 s-1)
Ph
ot
os
yn
th
es
is
(μ
m
ol
 C
O
2 m
-2
 s
-1
)
Gmelia - Improved (4X4 M
spacing)
Gmelina unimproved and
unmanaged
Gmelina improved (3x3 m
spacing)
Gmelina unimproved
seed and managed
 
 
Figure 6. Light response curves for 11 month old gmelina treatments at Mahaplag 
 
For each of the four experimental plots at Mahaplag there was a trend of increasing Amax 
from the lower Quadrant 1 to the upper Quadrant 4 (Figure 7). This is consistent with the 
greater proportion of older (and hence less photosynthetically active) leaves being located in 
the lower two quadrants and the upper quadrants containing a higher proportion of younger 
and highly photosynthetically active leaves. It is also interesting to note that there is much 
greater variability in Amax across the quadrants in sites from sites two and four with 
unselected germplasm.     
 
 
 
 
 
 
 
 
 
 
 
 
 
 89
Initial Gas Exchange Results from Field Trials 
 
 90
 
Figure 7. Amax by quadrant for 8 month old gmelina trees at Mahaplag 
 
REFERENCE  
 
Herbohn JL, Mangaoang E, Gregorio N and Vanclay J (in preparation). Using bamboo scaffolding to 
access forest canopies: some lessons from their use in the Philippines 
 
 
 
 
 
 
 
 
Site 3: Improved germplasm (3X3 M spacing) 
photosynthesis by quadrant
0.0
5.0
10.0
15.0
20.0
25.0
0.9 1.9 2.9 3.9
Quadrant
Ph
ot
os
yn
th
es
is
Tree 18
Tree 35
Tree 45
Tree 65
Tree 76
Site 4 Unimproved germplasm (managed) 
photosynthesis by quadrant
0.0
5.0
10.0
15.0
20.0
25.0
0.9 1.9 2.9 3.9
Quadrant
Ph
ot
os
yn
th
es
is
Tree 14
Tree 38
Tree 44
Tree 68
Tree 73
Site 1: Improved germplasm (4X4 M spacing) 
photosynthesis by quadrant
0.0
5.0
10.0
15.0
20.0
25.0
0.9 1.9 2.9 3.9
Quadrant
Ph
ot
os
yn
th
es
is
Tree 15
Tree 28
Tree 54
Tree 67
Tree 84
Site 2: Unimproved germplasm (unmanaged) 
photosynthesis by quadrant
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
0.9 1.9 2.9 3.9
Quadrant
Ph
ot
os
yn
th
es
is
Tree 24
Tree 28
Tree 47
Tree 65
Tree 85


Initial gas exchange results from field trials

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Initial gas exchange results from field trials

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