Allometric Relationships for Estimating Above Ground Biomass in Six Mangrove Species

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Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Allometric Relationships for E s t i m a t i n g Above- Ground B i o m a s s in Six M a n g r o v e Species

B.F. C L O U G H and K. S C O T T

Australian Institute of Marine Science, PMB No. 3, TownsviUe MC, Qld. 4810 (Australia) (Accepted 6 July 1989)

A B S T R A C T

Clough, B.F. and Scott, K., 1989. Allometric relationships for estimating above-ground biomass in six mangrove species. For. Ecol. Manage., 27: 117-127.

Allometric relationships are described for estimating leaf biomass, branch biomass, stem biomass and total above-ground biomass from measurements of stem diameter (DBH) in the mangrove species Rhizophora apiculata, R. stylosa, Bruguiera gymnorrhiza, B. parviflora, Ceriops tagal var. australis and Xylocarpus granatum. A linear relationship was found when the biomass of each above-ground component was plotted against DBH on a log-log scale. The two Rhizophora species were found to have the greatest stem and total above-ground biomass for a given DBH, followed by B. parviflora, B. gymnorrhiza, C. tagal var. australis, and X. granatum, the last having a significantly lower biomass for a given DBH than the other five species. However, there was much less variation in stem volume for a given DBH amongst the six species, owing to differences in the specific gravity of their stems.

I N T R O D U C T I O N

Mangroves are widely distributed along warmer coastlines of the world where, with favourable geomorphological, climatic and edaphic conditions, they often form extensive tidal forests. Over the last decade or so there has been increasing recognition that tidal forests are important as a nursery for a number of commercial fisheries species (Lindall et al., 1973; Robertson and Duke, 1987;

Thayer et al., 1987). They also represent a significant forest resource for many countries with limited terrestrial forest resources. Mangroves are already managed for timber production in a number of countries in the Indo-Pacific region, notably Bangladesh (A.M. Choudhury, Bangladesh Space Research and Re- mote Sensing organization, personal communication, 1986 ), Malaysia (Noakes, 1955; Tang et al., 1984) and Thailand (Aksornkoae, 1987), and this practice is likely to become more widespread as terrestrial forest resources become fur- ther depleted.

Contribution No. 463 from the Australian Institute of Marine Science.

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Reliable estimates of biomass and growth rates of mangroves are essential for estimating total net primary production in ecological studies, for assessing the yield of commercial products from mangroves, and for the development of sound silvicultural practices. Non-destructive allometric methods are used widely to estimate the biomass of trees in terrestrial forests (e.g. see reviews by W h i t t a k e r and Marks, 1975, and Causton, 1985 ). Such methods involve the establishment of a relationship between the biomass of whole trees, or their c o m p o n e n t parts, and some readily measured p a r a m e t e r such as the diameter of the stem at breast height (DBH). Allometric relationships between above- ground biomass and DBH have been reported for the mangroves

Rhizophora apiculata

(Ong et al., 1985; Putz and Chan, 1986) and

Bruguiera parviflora

(Ong et al., 1985), but not for other c o m m o n species. This paper presents allometric relationships, derived from studies in northern Australia, that can be used to estimate above-ground biomass and stem volume of six species of mangrove from m e a s u r e m e n t s of DBH.

METHODS

Trees of

Bruguiera gymnorrhiza, B. parviflora, Ceriops tagal

var.

australis, Rhizophora apiculata, R. stylosa

and

Xylocarpus granatum

were sampled at sites on Hinchinbrook Island (18°16'S, 1 4 6 ° 1 3 ' E ) , the M u r r a y River (18°04'S, 1 4 6 ° 0 2 ' E ) and the Daintree River (16°16'S, 1 4 5 ° 2 5 ' E ) in north-eastern Queensland. The two

Rhizophora

species were sampled from closed-canopy, mixed forests where they were the dominant trees (800 stems ha -1 for DBH

> 10 cm) overlying an understory ofB.

gymnorrhiza

and

Rhizophora

spp.

Cer- iops tagal

var.

australis

was sampled from a largely monospecific forest of this species (600 stems h a - 1 for DBH :> 10 cm ), with occasional trees of

Lumnitzera racemosa. Bruguiera gymnorrhiza, B. parviflora

and

X. granatum

were sampled from mixed forests where all three species were co-dominant (2000 stems h a - ¹ for DBH > 10 cm) over a mixed understory comprised chiefly of the same species.

The diameter of the stem at breast height (DBH) of trees from a range of size-classes was measured at a height of approximately 1.5 m, or above the highest prop-root of the two

Rhizophora

species when this arose from the stem at a height above 1.5 m. The trees were then felled and divided into leaves, branches, stem and, where possible, above-ground roots. It was not practicable to separate the buttress roots of

B. parviflora, C. tagal

var.

australis

and X.

granatum

from the basal part of the stem, so these were included in the stem fraction of these three species. The total harvested fresh weight of each com- p o n e n t was measured in the field, and representative subsamples were re- moved to the laboratory where they were oven-dried to constant weight at 80 ° C. The total harvested dry-weight of each c o m p o n e n t was calculated from the ratio of dry-weight to fresh weight of the corresponding subsamples. The specific gravity of the stem (heartwood, sapwood and bark) was determined

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from the dry-weight, and the length and mean diameter of short (0.5-1 m), fresh sections of the main trunk of each species.

RESULTS

Linear relationships were obtained when dry-weight was plotted against D B H

on a log-log scale for each component of all species (e.g. Fig. 1). The least- squares line of best fit is described by:

LEAF

10 o

S 1

o

i , , , , i i i I , i F I I I F F I

10 10 2

1 0 2 =

10 -I 10=-_-

I0

I

I 0 -~

BRANCH

b 0 1 i i 1 , 1 1 1 i

1 0

, I , , I , , E

1 0 2

I 0 = =

v 1 0

1 0 3

10 .s . STEM

10 =

10

1

i f ~ 1 , i l l I 10 -1

10 10 =

ROOT

/

i , , , 1 1 , 1 1 ,

1 0

i : i t , i i 1

1 0 =

DBH ( c m ) TOTAL

1 0 =

1 0

1 i o i i i l l l l ] i i i i 1 ~ 1 1 1

1 0 1 0 =

DBH ( c m )

Fig. 1. Log-log plots of dry-biomass against ^{D B H} for different parts of Rhizophora apiculata and R. stylosa. Both species, each represented by at least 10 trees, fit the same regression line, and for the sake of clarity data for the two species have not been identified with separate symbols. The corresponding regression eoeffieients are given in Table 1.

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120

loglo Biomass : A +B log10 D B H (1)

where A and B are constants. T h e regression constants (A, B), the correlation coefficient (r 2) and the s t a n d a r d error of the biomass estimate (E) for all components of each species are summarised in Table 1. No significant difference could be detected between the regression coefficients for the two Rhizo- phora species, which were therefore fitted to the same regressions (Fig. 1 ). T h e regression equations were all highly significant at the P < 0.0001 level. Values for r 2 were generally greater t h a n 0.9 for all components other t h a n leaves, where data were more variable (Table 1 and Fig. 1). This is to be expected, since leaf biomass is subject to seasonal variation (Gill and Tomlinson, 1971;

Christensen and Wium-Andersen, 1977; Duke et al., 1984) and is also more susceptible to losses from wind gusts. However, leaf biomass represents less t h a n 10% of the above-ground biomass for trees of all species with DBH greater

T A B L E 1

A l l o m e t r i c r e g r e s s i o n s o f a b o v e - g r o u n d b i o m a s s o n DBH for s i x m a n g r o v e s p e c i e s

S p e c i e s V a r i a b l e A B r 2 E

B. gymnorrhiza L e a f - 1.1679 1.4914 0.854 1.57

n - - 17 B r a n c h - 1.5012 2.2789 0.926 1.60

2 - 2 4 - c m DBH S t e m - 0.6482 2.1407 0.977 1.29

T o t a l - 0.7309 2.3055 0.989 1.19

B. parviflora L e a f - 1.5716 1.407 0.621 2.32

n -- 16 B r a n c h - 1.9403 2.4639 0.885 1.88

2 - 2 1 - c m DBH S t e m * - 0.8661 2.4037 0.992 1.18

T o t a l - 0.7749 2.4167 0.993 1.17

C. tagal var.australis L e a f - 1.9300 2.1294 0.927 1.42

n - - 26 B r a n c h - 1.7061 2.5516 0.938 1.47

2 - 1 8 - c m DBH S t e m * - 0.8333 2.3393 0.977 1.24

T o t a l - 0.7247 2.3379 0.989 1.16

R. apiculata, L e a f - 1.8571 2.1072 0.857 1.59

R. stylosa B r a n c h - 1.8953 2.6844 0.912 1.57

n=23 S t e m - 1.0528 2.5621 0.991 1.14

3 - 2 3 - c m DBH R o o t - 2.1663 3.1353 0.968 1.32

T o t a l - 0.9789 2.6848 0.995 1.11

X. granatum L e a f - 2.2380 2.3966 0.951 1.39

n - - 15 B r a n c h - 2.3315 3.0975 0.959 1.47

3 - 1 7 - c m DBH S t e m * - 1.0879 2.4624 0.988 1.18

T o t a l - 1.0844 2.5883 0.994 1.13

A a n d B a r e c o n s t a n t s i n t h e e q u a t i o n log B i o m a s s -- A + B log DBH, r 2 is t h e c o r r e l a t i o n c o e f f i c i e n t , E is t h e s t a n d a r d e r r o r o f t h e b i o m a s s e s t i m a t e , a n d n is t h e n u m b e r of t r e e s s a m p l e d w i t h i n t h e i n d i c a t e d DBH r a n g e . B i o m a s s is i n k g a n d DBH i n c m .

* i n c l u d e s b u t t r e s s r o o t s .

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than 10 cm (Table 2), so that variation in leaf biomass did not significantly affect the regressions for total above-ground biomass.

A separate allometric relationship for above-ground roots was obtained only for the two Rhizophora species. As indicated earlier, the buttress roots of B.

parr±flora, C. tagal car. australis and X. granatum were included in the stem fraction; however, several independent checks indicated that the biomass of buttress roots was less than 10% of the stem biomass in these species. The negatively geotropic knee-roots of B. gymnorrhiza, which protrude a few cm above the soil surface, were not included in the allometric relationships for this species.

While the log-log plots shown in Fig. 1 provide a convenient method for

T A B L E 2

Distribution of above-ground biomass I amongst different c o m p o n e n t s for trees of specified size classes

Species DBH Leaf B r a n c h S t e m Root

(cm)

B. gymnorrhiza < 5 23.1±3.4 25.8±4.7 51.1±2.4 n.a.

5-10 5.2__1.6 9.5_+1.3 85.3_+_0.8 n.a.

10-15 5 . 3 ± 1 . 9 19.2±5.6 75.5 ±6.4 n.a.

15-20 3 . 7 ± 0 . 4 19.0±4.2 77.3±4.6 n.a.

20-25 3.2 19.8 77.0 n.a.

B. parviflora < 5 2 . 5 ± 1 . 4 7.1±1.7 90.5±3.1 n.a.

5-10 1.6±0.8 8 . 1 ± 1 . 7 90.3-2.1 n.a.

10-15 1.9±0.7 7 . 2 ± 3 . 4 91.0±3.9 n.a.

15-20 1.9 ± 0.5 15.9 ± 7.8 82.2 ± 7.6 n.a.

20-25 3.5 3.5 96.0 n.a.

C. tagal car. australis < 5 4.9 ± 1.9 14.4 ± 5.5 80.7 ± 7.1 n.a.

5-10 3 . 7 ± 0 . 7 13.2±3.1 83.2+_3.6 n.a.

10-15 4 . 1 ± 0 . 5 20.0± 1.4 75.9±1.7 n.a.

15-20 3.9___0.4 24.7±1.2 71.4±1.2 n.a.

R. apiculata < 5 6.2 ± 1.6 12.0 ± 2.5 73 ± 6.9 8.6 ± 5.4

R. stylosa 5-10 4 . 9 ± 1.2 15.0 ± 4.2 64.0±7.5 16.1±3.4

10-15 2.6_+1.2 9 . 0 ± 3 . 8 62.8±5.4 25.6±2.2

15-20 3.1 18.2 57.9 20.8

20-25 1.7 12.3 68.8 17.2

X. granatum < 5 7.0 ± 1.9 13.5 ± 3.3 79.5 ± 3.3 n.a

5-10 4.0±1.1 12.8±3.1 83.2 ± 3.7 n.a

10-15 4.9_+1.3 20.0± 5.3 75.1_+4.3 n.a.

15-20 4.4 41.7 53.9 n.a.

20-25 4.8 31.4 63.8 n.a.

1Values are percentages ± s t a n d a r d deviation. Values w i t h o u t a s t a n d a r d deviation represent a single tree in t h a t size class.

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1 2 2

4 0 0 -

3 0 0 -

2 0 0 -

1 O0 -

0 BO

T

B

I I q k

5 10 15 20 25

5 0 0 -

4 0 0 -

3 0 0 -

2 0 0 -

1 0 0 -

0 / BP

T

B

I'0 I15 210 215 L

4 0 0

3 0 0

2 0 0

1 O0

0 [

4 0 0 -

3 0 0 -

2 0 0 -

1 O0 -

0 0

C A

/T

B L

I I

10 15 2 0 2 5

X G

T

B

k

I I I I /

5 1 0 1 5 2 0 2 5

D B H ( c m )

6 0 0

5 0 0 400

3 0 0

2 0 0

1 O 0

0

T

RH L

S

R B

I I

lO 1'~ ~'o ~'~

DBH ( c m )

Fig. 2. Allometric relationships between DBH and biomass for each component of above-ground biomass. The relationships were calculated from Eqn. (2) using transformations (see text) of the appropriate coefficients (Table 1 ). Species codes: BG, B. gymnorrhiza; BP, B. parviflora; CT, C.

tagal var. australis; RH, R. apiculata and R. stylosa; XG, X. granaturn. Component codes: T, total dry weight; L, leaf dry weight; B, branch dry weight; S, stem dry weight; R, above-ground root dry weight.

determining values for the constants A and B, they are not particularly useful for displaying the allometric relationships for individual components of the above-ground biomass. Equation (1) can be rewritten as:

Biomass ^{= A p} ^{• D B H} ^B (2)

where B has the same numeric value as in Eqn. ( 1 ), and Ap is the antilog of A

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TABLE 3

Stem specific gravity (kg dry weight m - 3 fresh volume) for six species of mangrove

Species n Mean SD

B. gymnorrhiza 17 665 42

B. parviflora 15 650 35

C. tagal var. australis 26 752 46

R. apiculata, R. stylosa 24 810 100

X. granatum 10 486 34

in Eqn. (1). Figure 2 shows the allometric relationship between biomass and DBH for each component of the six species, calculated according to Eqn. (2).

Individual data-points have been omitted, for clarity. Interspecies comparisons show that the two Rhizophora species yield the greatest above-ground biomass for a given DBH, followed by B. parviflora, B. gymnorrhiza, C. tagal var.

australis, and X. granatum (Fig. 2). A similar trend was obtained for stem biomass (Table 2 ), although the differences between species were smaller than those for total above-ground biomass.

The specific gravity of the stem (wood+bark) for all six species is shown in Table 3. The stem of the two Rhizophora species had the highest specific gravity while that of X. granatum had a significantly lower specific gravity than the other species (Table 3).

D I S C U S S I O N

While it is possible to use polynomial or other equations to estimate biomass from easily measured parameters such as DBH, the power curve (Eqn. (2) ) and its linear transformation (Eqn. (1)) provide an acceptably good description of the relationship between above-ground biomass and ^{D B H}in a wide range of forest types (Whittaker and Marks, 1975). Other workers have used relationships which include height as a variable, usually in the forms:

V = b ( D B H 2 H ) c o r

V = a + b ( D B H 2 H ) c

where V is stem volume, H is height, and a, b and c are constants (see review by Causton, 1985). Although height was measured in several of the species, it was not included in the allometric relationships reported here, chiefly because it is not a parameter that can be estimated rapidly for each tree over relatively large areas of mangrove forest. In consequence, the inclusion of height as a variable in the present allometric relationships was considered to reduce the

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124

practicality of their use. Furthermore, as discussed below, the simple form of relationship used in this study appears to provide an accurate estimate of above- ground biomass without the need to include the additional variable, height.

This study confirms the earlier observations of Ong et al. (1985) and Putz and C h a n ( 1986 ) that Eqns. ( 1 ) and (2) provide a reliable means of estimating above-ground biomass from DBH for mangrove species. The allometric relationship obtained in this study for total above-ground biomass of the two Rhi- zophora species agrees well with those obtained by Ong et al. ( 1985 ) and Putz and Chan (1986) for R. apiculata in mangrove forests at Matang in Malaysia (Table 4). On the other hand, the allometric relationships obtained for B. par- viflora in this study differed substantially from those obtained by Ong et al.

(1985). Some possible reasons for this discrepancy will be considered later.

The results of this study indicate that there is considerable interspecies variation in above-ground biomass for a given stem diameter. Whereas no significant difference could be detected in the allometric relationships for the two Rhizophora species, these relationships differed significantly amongst the other species, including the two species of Bruguiera (Fig. 2). These differences did not appear to be site-specific because three of the species, B. gymnorrhiza, B.

parviflora and X. granatum, were sampled from mixed forests in which all three species were co-dominant. This finding contrasts with the observations of Ogawa et al. (1965), who found similar allometric relationships for a number of tropical terrestrial tree species in Thailand.

One way of checking the internal consistency of the allometric relationships for a given species is to sum estimates of biomass for individual components

T A B L E 4

C o m p a r a t i v e e s t i m a t e s of total above-ground b i o m a s for trees of R. apiculata a n d B. parviflora, derived from different sources.

Species DBH Total dry weight (kg)

(cm)

A B C

R. apiculata

B. parviflora

5 8 11 10

10 51 58 56

15 151 154 156

2O 327 309 321

25 595 530 562

5 8 9 n.a.

10 44 58 n.a.

15 117 167 n.a.

20 234 358 n.a.

25 401 637 n.a.

A, t h i s study; B, Ong et al. (1985); C, P u t z a n d C h a n (1986).

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(derived from their allometric relationships ) at a particular DBH, and compare this value with the estimate obtained for total above-ground biomass at the

s a m e D B H . The independent estimates of total above-ground biomass thus obtained agreed to within 10% for all species except B. parviflora over the range of size-classes sampled. The somewhat larger differences (ca. 15%) between the two estimates for the latter species reflects the greater variability (up to 50% of the mean) in the relative proportions of leaves and branches (Table 2).

For all size-classes, stem accounted for an unusually high proportion (ca.

90%) of the total above-ground biomass in B. parviflora compared with the other species (Fig, 2 and Table 2 ). Assuming that the dry-weight per unit area for B. parviflora leaves is not too different from that for other species, it follows that the crown of this species has 10-20% less leaf-area for photosynthetic carbon assimilation for a given D B H t h a n the other five species. Whether this is reflected in a slower growth rate, or whether photosynthetic efficiency is higher in B. parviflora t h a n in other species, is not yet clear, although prelim- inary unpublished data suggest that its photosynthetic capacity is not substantially greater than that of other species.

In silviculture, the stem yields most of the commercially useable timber. The two Rhizophora species clearly have the greatest stem biomass for a given DBH, followed by B. parviflora, whereas X. granatum has a significantly lower stem biomass for a given DBH (Table 5). However, owing to variation in stem specific gravity between the six species (Table 3), there was remarkably little difference in green-stem volume for a given DBH amongst the six species (Ta- ble 5 ). Although differences in relative growth rate between species may be the chief criterion for selection of species for silviculture in a particular area, the results shown in Table 5 provide information that might be useful in selecting

T A B L E 5

S t e m biomass (dry weight) a n d volume (green) for trees of selected sizes

Species B i o m a s s (kg) Green Volume ( m 3)

DBH (cm) DBH (cm)

15 20 25 15 20 25

B. gymnorrhiza 80 154 268 0.114 0.232 0.403

B. parviflora 91 183 312 0.141 0.281 0.480

C. tagal var australis 83 162 274 0.110 0.215 0.364

R. apiculata

R. stylosa 91 191 338 0.113 0.235 0.417

X. granatum 64 131 226 0.132 0.269 0.465

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126

species for silviculture, depending on whether volume or biomass is the yield index of particular interest.

The general question of the reliability of allometric relationships for estimating biomass, and their application to forests of similar species in a range of environments, has been considered in reviews by Whittaker and Marks (1975) and, more recently, by Causton (1985). In the present study, the trees were sampled from relatively dense, closed-canopy forests where lower branches were absent owing to self-thinning. This may account for the relatively low biomass of branches and leaves in some species, especially

B. parviflora.

The allometric relationships described in this paper for these two components of the crown may not be appropriate in more open mangrove forest stands, where horizontal expansion of the crown of individual trees is less restricted by the crowns of their neighbours. The effect of tree population density on the allometric relationship between stem biomass and D B H is not clear, but it seems likely be less affected by stand structure than those for the branches and leaves.

Furthermore, care should be taken in extrapolating the present allometric relationships to trees outside the range of DBH from which the relationships were obtained. Prospective users of these allometric equations would therefore be wise to check several trees in their plots to confirm that they conform to the allometric relationship for that species.

ACKNOWLEDGEMENTS

The allometric relationships for

R. apiculata

and

R. stylosa

were obtained during the James Cook II Expedition of the Australia-New Zealand Scientific Exploration Society, on which the voluntary assistance of B. Anastasi, J. Bills, T. Evans, C. Giacometti, D. Morris, S. Roach, M. Sands, E. Seymour, P. Wat- son and A. Date is gratefully acknowledged. The remainder of the work would not have been possible without support from the Australian Government through its CCEP Programme, and the valuable assistance of T. Bienkiewicz, C. Bone, J. Jones, T. Tyson and D. Whittle. A. Robertson kindly provided estimates of stem density for the two

Rhizophora

species. The author thanks K. Boto and A. Robertson for their valuable comments on an earlier draft of the manuscript.

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