63 巻 4 号（2011） SEISAN-KENKYU

研　　究　　速　　報

51

443

1. Introduction

The importance of plantation is increasing because of the

global demand for forest products are increasing, and hence it

put heavy pressure on the forest resources thus the supply from

forests are diminishing (FAO, 1997; Dorais, 2011). Monocul-

ture plantations like rubber, teak, cashew, palm oil, etc. have an

adverse impact on the biodiversity and ecosystem services (Bun-

ker et al., 2005; Lal, 2008). Plantations have some advantages

like ability to generate greater fi nancial returns because of fast

growth than natural forests regeneration (Lamb, 2011). More-

over, plantation becomes an interesting topic of research in the

present scenario of climate change regime. Recent report of FRA

(2010) shows plantation accounts for 7% of total forest area.

Therefore, estimation of carbon sequestration by plantation spe-

cies becomes very important. Estimation of carbon sequestration

by plantation species requires information about their biophysi-

cal parameters.

Cashew and rubber plantation are the major cash crop for

Cambodian farmers. Cambodia has a suitable soil and weather

condition for these plantation species (Moorthy, 2006). These

plants have structural differences in their canopy cover, tree size,

orientation of branches, tree distribution etc. Satellite imageries

have been used for decades for plantation monitoring. However,

in tropical countries, the frequent cloud cover often restrains the

acquisition of the optical imagery. This limitation can be over-

come by using Synthetic Aperture Radar (SAR) data. Previous

studies show the dependency of the SAR data on various prop-

erties of radar system (wavelength, polarization and incident

angle) and properties of target (soil moisture, surface roughness,

vegetation structure properties, vegetation water content and

geometric properties) (Ulaby et al., 1982, Jensen, 2000). The

backscattering depends on the distribution in size, orientation

and density of the scatterers (Toan Le et al., 1992; Dobson et al.,

1992; Paloscia et al., 1999; Macelloni et al., 2001). Therefore,

polarimetric application of PALSAR is useful for discrimination

of different types of scattering mechanisms from the targets (Toan

Le et al., 1992; Dobson et al., 1992). Cashew and rubber plant

shows different types of the backscattering behavior because of

differences in their biophysical parameters. In this study, we are

presenting the results of preliminary study of variation in PAL-

SAR σ 0 with ground based biophysical parameters of cashew

and rubber plants.

2. Study Area

Both cashew and rubber plantation areas are situated in Kam-

pong Cham Province of Cambodia. It is about 90 km from Phnom

Penh and lies between 1202’17”N to 1206’7”N latitude and 104059’51”E to 10504’23”E longitude (cashew plantation area) and 1205’36’’N to 1208’20’’N latitude and 105041’9’’E to 105044’11’’E longi-

tude (rubber plantation area) (Figure 1). Cambodia is a tropical

country with the rainy season from May to October and dry sea-

son from November to April. The minimum and maximum tem-

peratures of the study areas are about 210C and 350C, respectively.

The mean annual rainfall ranges from 150 to 180 cm. The study

areas are homogeneous and fl at, where PALSAR data can be used

without limitation of topographic effects.

3. Materials And Methods

3.1 Satellite data

Two scenes of PALSAR FBD 1.5 level data acquired on

December 05th, 2010 (cashew area) and November 18th, 2010

(rubber area) with 34.30 look angle were used in this study. We

selected PALSAR data in dry season to minimize the effects of

soil moisture. Landsat and AVNIR-2 were also used to identify

the spectral signature of plantation area.

3.2 Forest inventory data

On the basis of the analysis of Landsat satellite spectral sig-

nature, the stratifi ed random samplings were done in November,

2010 and January, 2011. The purpose of stratification in sam-

pling was to reduce the variation in different stages of plantation.

Twenty two and twenty sample plots with area of 30 × 30 m

Assessment of Cashew and Rubber Plants Biophysical Parameters

Based on ALOS/PALSAR Data

研　究　速　報

Ram AVTAR*, Wataru TAKEUCHI* and Haruo SAWADA*

＊ Institute of Industrial Science, The University of Tokyo

研　　究　　速　　報

52

444 63 巻 4 号（2011） SEISAN-KENKYU

were selected in cashew and rubber plantation, respectively. We

tried to collect the samples on the basis of growth stage and age.

Forest inventory data (DBH, height, age, crown diameter and

tree density) for both plantation areas were collected.

3.3 Data processing

PALSAR data was processed using ENVI4.4 software and

digital number (DN value) was converted to σ 0 of intensity im-

age using the revised calibration coefficients (Shimada et al.,

2009). Then the data were processed with the frost filter with

window size of 5 × 5 to reduce the speckle noises. The mean σ 0

was calculated for each sampling plot with 3 × 3 pixel size and

the relationships between σ 0 and biophysical parameters were

analysed. The allometric equation for calculation of biomass of

cashew and rubber plants was used from Ressona (2008) and

Ketterings (2001) respectively. The method is described in the

following fl ow chart: (fi gure 2）

4. Results And Discussion

The full-polarimetric PALSAR data with 23.10 look angle was

available in the study area but in this study, we used only dual-

polarimetric PALSAR data because full-polarimetric data with

23.10 look angle is not calibrated yet (personal communication

with Dr. M. Shimada, JAXA). PALSAR data with HH and HV

polarization were analyzed for their relation with the cashew and

rubber plants biophysical parameters (biomass, height, DBH,

age). High penetration capability of L-band SAR shows a posi-

tive relation with cashew and rubber plants biophysical param-

eters. Figure 3 (a, b, c, d) and 4 (a, b, c, d) represent the relation

between PALSAR backscattering and biophysical parameters

(biomass, height, DBH and age) of sampling plots of cashew and

rubber plantation area respectively. Although HH and HV polar-

ization have positive relations, the sensitivity of σ 0 HV is higher

than σ 0 HH in both plantation areas because of the increase in

volume scattering with the growth of cashew and rubber plants.

Low backscattering and its high variation are seen at juvenile

stage of both cashew and rubber plants, and they are mainly due

to the high growth rate during the juvenile stage. After certain

growth (5 years), the cashew and rubber plants become mature

and show high backscattering. Because of the fast growth during

juvenile stage, the gaps between the plants decrease considerably

within 3 to 4 years after plantation (fi gure 3d, 4d). At juvenile

stage, plants have smaller leaves and fewer branches. This might

be a reason why σ 0 in HH and HV are lower in juvenile stage.

As growth proceeds, the gaps between canopies gradually de-

crease making the surface less visible hence; high backscattering

and low variations were observed at mature stage (8-12 years)

of cashew and rubber plants. In the juvenile stage, the like-

polarized (HH) and cross-polarized (HV) shows less backscat-

Figure 1 Location of the study area

Figure 2 Flow chart of the methedology

63 巻 4 号（2011） SEISAN-KENKYU

研　　究　　速　　報

53

445

tering because of specular phenomena. However in mature stage

the σ 0 HV increases because of the contribution from the volume

scattering. This increase in σ 0 is mainly because of growth in

biophysical parameters of plantation area.

σ 0 in HV shows saturation at more than 100 t/ha biomass

for cashew whereas at 50 t/ha for rubber plants. This saturation

might be because of saturation of PALSAR σ 0 after 13 years of

growth in cashew plants. However in case of rubber plants the

saturation might be because of saturation of growth of rubber

plants. The reasoning behind the fact might be a symmetrical

crown and their crown diameter became constant after a certain

growth in rubber plants. The crown might be the main contribut-

ing factor for volume scattering.

The rubber plant has a simple and symmetrical canopy with

homogeneous crown cover, whereas the cashew plant shows

branching from the ground level with a big crown diameter

which overlaps with adjacent trees and it increases with the age

of cashew plant (figure 5a, b). Therefore, cashew causes high

volume scattering in mature stage than rubber.

5. Conclusion

The relationship between biophysical parameters and PAL-

SAR σ 0 were investigated for cashew and rubber plantation area.

The σ 0 HV has shown a high positive correlation with various

Figure 3 PALSAR σ 0 (05/12/2010) and various biophysical parameters of Cashew plants (a) biomass, (b) height, (c) DBH, (d) age

Figure 4 PALSAR σ 0 (18/11/2010) and various biophysical parameters of Rubber plants (a) biomass, (b) height, (c) DBH, (d) age

研　　究　　速　　報

54

446 63 巻 4 号（2011） SEISAN-KENKYU

biophysical parameters of cashew and rubber plants and could

be useful for comparing biophysical parameters. High backscat-

tering and low variations were observed at mature stage (8-12

years) of plantation area. Saturation in backscattering has shown

from the age of 13 and 10 years of growth for cashew and rubber

plants respectively. The estimation of various biophysical param-

eters based on PALSAR data would be useful for estimation of

biomass, age, height and other biophysical parameters which can

be useful for management of plantation area and further estima-

tion of carbon sequestration in future research.

Acknowledgement

The authors are highly thankful to the Monbukagakusho

(MEXT) Japanese Government Fellowship to pursue research

in The University of Tokyo, Japan. Authors also want to put on

record contribution of Institute of Industrial Science, The Uni-

versity of Tokyo for facilitating data analysis in its labs and also

to the Department of Civil Engineering, University of Tokyo and

Global Centers of Excellence (GCOE Program) for providing

the requisite fund during the fi eld visit.

(Manuscript received. June 13, 2011)

References

1) Bunker, D. E., DeClerck, F., Bradford, J. C., Colwell, R. K., Per-

fecto, I., Phillips, O. L., Sankaran, M. & Naeem, S. 2005 Species

loss and aboveground carbon storage in a tropical forest. Science

310, 1029–1031.

2) Dobson M. C., Ulaby F.T., Toan Le T., Beaudoin A., Kasischke E.

S. and Christensen N., 1992. Dependence of radar backscatter on

coniferous forest biomass. IEEE Transactions on Geoscience and

Remote Sensing. 30, 2, 412–415.

3) Dorais A., Cardille J., 2011. Strategies for incorporating high-

resolution google earth databases to guide and validate classifi ca-

tions: Understanding deforestation in Borneo. Remote Sensing, 3,

2257–1176.

4) FAO, 1997. Asia-Pacific forestry sector outlook study working

paper series. Forestry policy and planning division, Rome.

5) FRA, 2010. Global forest resource assessment country report

Cambodia. Rome.

6) Jensen J. R., 2000. Remote sensing of the environmental an earth

resource perspective. Prentice Hall U. K.

7) Ketterings, Q. M., R. Coe, M. v. Noordwijk, Y. Ambagau and

C. A. Palm, 2001. Reducing uncertainty in the use of allometric

biomass equations for predicting aboveground tree biomass in

mixed secondary forests. Forest Ecology and Management 146,

199–209.

8) Lal R., 2008. Carbon sequestration. Philosophical Transactions

of The Royal Society B, 363, 815–830.

9) Lamb D., 2011. Regreening the bare hills: Tropical forest restora-

tion in the Asia-Pacifi c region, Springer, New York.

10) Macelloni G., Paloscia S., Pampaloni P., Marliani F., Gai M.,

2001. The relationship between the backscattering coeffi cient and

the biomass of narrow and broad leaf crops. IEEE Trans. Geosci.

Remote Sens. 39, 873–884.

11) Moorthy V. K., 2006. Organic cashew project in Cambodia. Na-

tional seminar on cashew, Raipur, India.

12) Paloscia S., Macelloni G., Pampaloni P., Sigismondi S., 1999.

The potential of C- and L-Band SAR in estimating vegetation

biomass: The ERS-1 and JERS-1 experiments. IEEE Trans. Geo-

sci. Remote Sens. 37, 2107–2110.

13) Rajaona, A. M., (2008). Comparative study of allometric param-

eters of Cashew tree (anacardium occidentale) In North East

Brazil, Master thesis, University of Bonn.

14) Shimada M., Isoguchi O., Tadono T., Isono K., 2009. PALSAR

polarimetric calibration and geometric calibration. IEEE, Trans-

actions on Geoscience and Remote Sensing, 47, 12, 3915–3932.

15) Toan Le. T., Beaudoin A., Riom J., Guyon D., 1992. Relating for-

est biomass to SAR data, IEEE Transactions on Geoscience and

Remote Sensing, 30, 2, 403–411.

16) Ulaby, F. T., Moore, R. K., Fung, A. K., 1982. Microwave Re-

mote Sensing: Active and Passive. Volume 2: Radar Remote

Sensing and Surface Scattering and Emission Theory, Addison-

Wesley, Advanced Book Program: Reading, Massachusetts, 609.

Figure 5 (a) Cashew plantation area (b) rubber plantation area

(a) (b)