4.5.1 Photosynthesis
The light saturated net photosynthesis rate per unit leaf area (Amax-area) varied fivefold among the 19 species studied, ranging from 3.6 µmol m-2 s-1 in Meliosma sumatrana in the natural forest, to 21.3 µmol m-2 s-1 in the secondary forest species Trema orientalis (Table 4.8, Figure 4.22 A). The three studied land use types differed significantly from each other.
The natural forest mean reached only 62% of the mean rate of the agroforestry system (7.6 compared to 12.2 µmol m-2 s-1), which in turn showed a lower rate than the highly
productive secondary forest (mean: 17.5 µmol m-2 s-1). The dark respiration rates measured at daytime at constant temperature (28°C) varied between -2.35 and -0.54 µmol m-2 s-1 (Table 4.8). The secondary forest mean was 52% higher than the natural forest mean.
Particularly high Rd rates were found in Erythrina sp. in the agroforestry system (-2.35 µmol m-2 s-1).
4.5.2 Maximum stomatal conductance for water vapour
Maximum of stomatal conductance for water vapour (gsmax) as recorded in daily courses of 10 – 20 leavesshowed a wide range of species means, from the lowest mean observed in Meliosma sumatrana to the more than ten times higher rate in Mallotus mollissimus (68 and 734 mmol s-1 m-2, Table 4.8, Figure 4.22 B). The three studied land use types differed significantly concerning their mean gsmax of their respective species (p < 0.05, means: 367 (NF), 609 (SF), 520 (AF) mmol s-1 m-2). Several groups of significantly different species
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Table 4.8 Means and standard deviation of light saturated net photosynthesis (Amax), daytime dark respiration rate (Rd) and maximum stomatal conductance for water vapour (gsmax) in mature sun leaves of 19 species occurring in three land use types. Significantly different species means within each land use type are indicated by different letters (p < 0.05). Capital letters are used for means on land use type level.
A max
(µmol m-2 s-1) Rd
(µmol m-2 s-1) gsmax
(mmol m-2 s-1) Land use type
Species mean sd mean sd mean sd
Natural forest
Aglaia argentea 8.5bc 1.2 -0.86ab 0.54 305b 130
Pimelodendron amboinicum 5.1cd 1.0 -1.15ab 0.47 248b 36
Bishofia javanica 9.6b 3.7 -1.71b 0.84 583a 135
Cananga odorata 13.2a 2.4 -1.01ab 0.37 547a 206
Litsea sp.1 5.4cd 2.0 -0.57a 0.49 148b 71
Meliosma sumatrana 3.6d 0.8 -0.54a 0.20 68b 49
Semecarpus forstenii 9.1b 1.9 -1.11ab 0.41 198b 63
Siphonodon celastrineus 5.3cd 2.5 -0.56a 0.42 147b 78
Mean 7.5C 3.7 -0.94A 0.61 368C 232
Secondary forest
Acalypha caturus 16.7abc 1.8 -1.46ab 0.58 684ab 134
Grewia glabra 20.3a 2.3 -1.87b 0.49 625abc 203
Hommalanthus populneus 18.3ab 1.8 -1.41ab 0.44 532abc 191
Macaranga hispida 14.3abc 2.6 -0.99ab 0.34 457c 131
Macaranga tanarius 14.2abc 2.8 -1.31a 0.21 484bc 190
Mallotus mollissimus 15.9c 2.6 -1.30ab 0.35 734a 115
Pipturus argentus 20.2a 2.2 -1.51ab 0.26 676abc 166
Trema orientalis 20.3a 1.9 -1.60ab 0.56 682ab 192
Mean 17.5A 3.3 -1.43B 0.47 609A 205
Agroforestry system
Erythrina sp. 13.0b 3.4 -2.35b 1.31 646a 149
Gliricidia sepium 19.9a 2.1 -1.82ab 0.36 711a 185
Theobroma cacao 7.4c 2.3 -0.90a 0.55 205b 71
Mean 13.2B 5.9 -1.69B 1.02 521B 267
means were also found within these land use types. For example, Cananga odorata and Bischofia javanica had significantly higher gsmax than the other six species studied in the natural forest. The two Euphorbiaceae species Macaranga hispida and Mallotus mollissimus represented the lowest and highest extremes, respectively, within the secondary forest, differing significantly from the other six secondary forest species. In the agroforestry
4 RESULTS
system, the leguminous shadow tree species showed a threefold higher maximum stomatal conductance than Theobroma cacao.
Figure 4.22 A. Means and standard deviation of light saturated net photosynthesis per leaf area (Amax-area) in 19 species, covering three land use types, based on measurements on 10 mature sunlit leaves in at least two mature trees per species. B. Maximum stomatal conductance for water vapour (gsmax) of 19 species. Daily courses on between 10 and 20 mature sunlit leaves from 2 - 4 mature trees of each of the 19 species were measured and the peak gs values selected. Only values recorded at relative humidity levels < 80% were selected for calculation of the means and standard deviations of gsmax presented.
4.5.2.1 gsmax as related to VPD
Stomatal conductance (gs) showed a significant negative correlation with water vapour pressure deficit (VPD) in all secondary forest species studied, but only in two of the eight natural forest species. All three species in the agroforestry system showed a significant negative correlation (Table 4.9). The mean slope (b) of the relation gs = VPD * b + A was significantly larger in the eight secondary forest species (-30.0 ± 11.0 (µmol m-2 s-1) / Pa) than the eight natural forest species (-10.8 ± 8.5 (µmol m-2 s-1) / Pa), indicating a higher sensibility of stomatal response to VPD in the secondary forest species. Among the 13 species with significant humidity dependence of gs, b showed a significant negative correlation with gsmax (p < 0.01, r2 = 0.56, n = 13. Figure 4.23).
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0
Figure 4.23 Maximal stomatal conductance (gsmax) versus the slope (b-value) from gs versus VPD regressions for 13 species of NF, SF and AF. (Only b-values from significant gs -VPD regressions were considered. gs = VPD * b + A).
Table 4.9 Correlation parameters for the dependence of stomatal conductance on water vapour pressure deficit (VPD) in all 19 species studied (gs = VPD * b + A). Means of b for each land use type presented, with different letters indicating significant differences. (n - sample size, A - intercept, b - slope, r2 - degree of explanation, p – probability)
Species n A b r2 (-) p
Natural forest
Aglaia argentea 8 316 -3.8 0.02 0.76
Bischofia javanica 43 594 -5.6 0.02 0.40
Cananga odorata 30 984 -28.9 0.59 <0.01
Litsea sp.1 7 326 -18.1 0.53 0.06
Meliosma sumatrana 5 165 -9.5 0.43 0.23
Pimelodendron aboinicum 22 315 -6.5 0.53 <0.01
Semecarpus forstenii 13 269 -8.0 0.11 0.26
Siphonodon celastrineus 11 233 -6.0 0.14 0.24
-10.8 ± 8.5B Secondary forest
Acalypha caturus 47 660 -15.6 0.11 0.02
Grewia glabra 66 907 -27.2 0.31 <0.01
Homalanthus populneus 53 904 -25.1 0.18 <0.01
Mallotus mollissimus 73 1150 -48.7 0.53 <0.01
Macaranga hispida 66 638 -17.8 0.40 <0.01
Macaranga tanarius 48 804 -33.7 0.52 <0.01
Pipturus argentus 42 985 -39.7 0.58 <0.01
Trema orientalis 55 1036 -32.5 0.44 <0.01
-30.0 ± 11.0A Agroforestry system
Erythrina sp. 114 839 -16.7 0.13 <0.01
Gliricidia sepium 105 966 -21.7 0.62 <0.01
Theobroma cacao 101 293 -6.9 0.19 <0.01
-15.1 ± 7.5AB
4 RESULTS
4.5.3 Water use efficiency
Photosynthetic water use efficiency (WUE) as measured under light saturation, ambient CO2-concentration, 28°C leaf temperature and 1.4 kPa VPD in the 19 in-depth studied species is presented in Figure 4.24.
The natural forest species showed the largest span of water use efficiency values, ranging from 34 µmol mol-1 achieved in Bischofia javanica to 84 µmol mol-1 in Meliosma sumatrana. The eight secondary forest species showed a slightly shorter, somewhat lower range, from 28 µmol mol-1 (Grewia glabra) to 64 µmol mol-1 (Acalypha caturus). Water use efficiency rates of the agroforestry species were high, ranging from 53 (Gliricidia sepium) to 61 µmol mol-1 (Theobroma cacao).
Natural forest mean was 52 ± 17 µmol mol-1, secondary forest mean 45 ± 12 µmol mol-1 and mean within the agroforestry system was 56 ± 4 µmol mol-1. There was no significant difference between the means of the three land use types.
0
Figure 4.24 Photosynthetic water use efficiency at light saturation (2000 µmol m-2 s-1, leaf temperature: 28°C, VPD: 1.4 kPa, ambient CO2-concentration: 369±6 ppm)for each of the 19 species included in the physiological study, covering three different land use types. The values were measured with the Li-6400 system on 10-15 leaves from two trees per species.
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4.5.4 The relation between δ13C and water use efficiency
No significant correlation was found between δ13C and water use efficiency in the sample of 19 species covering three land use types (p = 0.18, r2 = 0.10 (-), n = 19, Figure 4.25). The agroforestry system showed a significant negative relation between these
parameters (p < 0.01, r2 = 1 (-), n = 3), whereas no correlation was found in the secondary (p
= 0.21, r2 = 0.24 (-), n = 8) or the natural forest samples (p = 0.57, r2 = 0.06 (-), n = 8).
-34 -32 -30 -28 -26
δ13 C[‰]
20 30 40 50 60 70 80 90
Water use efficiency [µmol mol ]-1
Figure 4.25 δ13C plotted as a function of water use efficiency. No significant correlation was found in the sample of 19 species (r2 = 0.13 (-), p = 0.13, a = -26.9, b = -0.06).