Application of optical remote sensing technology for oil palm management
Discriminant Analysis of Landsat TM Spectral Radiance on Oil Palm Plantation
The relative strength of the relationship between oil palm age classes and the radiance data acquired from Landsat TM bands were examined. The Wilks's Lambda coefficients (Johnston 1978, Klecka 1980, and Boyd et. al. 1996) adopted from discriminant analysis in SPPS were used to analyse the relative importance of six Landsat TM bands as discriminating variables. The magnitude of the Lambda coefficient is inversely related to the ability of wavebands to discriminate between classes (Boyd et. al. 1996). Table 3.1 shows the rank order of the six individuals Landsat TM bands according to discriminant analysis. It illustrates that the data acquired by band 5 and band 7 (middle infrared, MIR) of both the study sites showed a considerably more discriminating powers, with band 7 providing the highest.
| Lands at TM |
Wilks's Lambda (a) |
Correlation ( rs ) |
Wilks's Lambda (b) |
Correlation ( rs ) |
| TM1 |
0.665 |
-0.71683 |
0.856 |
0.9857 |
| TM2 |
0.381 |
-0.70300 |
0.806 |
-0.01492 |
| TM3 |
0.371 |
-0.65986 |
0.809 |
-0.14758 |
| TM4 |
0.864 |
-0.50710 |
0.772 |
-0.06242 |
| TM5 |
0.283 |
-0.79930 |
0.459 |
-0.84384 |
| TM7 |
0.273 |
-0.73718 |
0.383 |
-0.65027 |
Table 3.1: Discriminant analysis for TM bands and age classes for Tuan Mee Estate
(a) and Balau Estate (b). Spearman rank correlation coefficients (p <0.05) between
radiance and age is also shown (source: ibrahim, 2000)
The Spearman rank correlation coefficient (r
s) of each Landsat TM bands was calculated. The visible and near infra red bands displayed insignificant correlation (p < 0.05). The relationships obtained from the data acquired in MIR bands were stronger and statistically more significant than those achieved in the visible (band1, band 2 and band 3) and near infrared band (NIR) (band 4). The highest correlation coefficients between oil palm age classes and TM radiance were obtained from TM7 and TM5 [(r
s = -0.737 and -0.799 (Tuan Mee), rs
= -0.650 and -0.844 (Balau Estate)], respectively which were significant at the 99% confidence level. A study by Boyd et. al. (1996) also found that Landsat TM band 5 and 7 showed higher Spearman rank correlation coefficient and better power to discriminate tropical regenerative stages in even-aged forest. The effects of interrelated factors such as scattering within leaves, canopy shadow, chlorophyll absorption, canopy transpiration and under storey reflectance (McWilliam et al. 1993) can be used to explain the relationships resulting from data acquired in respective TM Wavebands and oil palm age classes. However, the influence of each factor may be changed due to the canopy development of oil palm. Other factors that influence the relationship may be oil palm management practices in pruning, harvesting, fertilising and weeding of the respective study areas. TM bands had higher discrimation power and Spearman rank correlation coefficients between oil palm age and Landsat TM wavebands for Tuan Mee than Balau Estate. The result will be discussed in section 3.3.
The information provided by MIR bands 7 and 5 can be used effectively by incorporating other image processing techniques such as Advanced Vegetation Indices (AVI), Bare Soil Indices (BIO) and Shadow Indices (SI). The combination of the information provided by data acquired in the Landsat TM bands and its indices it is believed can strengthen the correlations obtained between the remotely sensed image and oil palm age classes and properties.
The study also illustrates that, of the indices, the Soil Index had the strongest Spearman rank correlation coefficients over the whole 1-26 or 4-21 year age range at 99% confidence level (r
s = -0.78 and -0.82 for Tuan Mee and Balau estate, respectively). Other indices such as NDVI and the Advanced Vegetation Index had insignificant correlations and an inability to discriminate between age classes (r
s = 0.47 and -0.05 for Tuan Mee Estate and r
s = 0.13 and -0.01 for Balau Estate). However, the Spearman rank correlation coefficient for the Shadow Index (SI) was strong at Tuan Mee (r
s = 0.70) and weak for Balau (r
s = 0.02).
Spectral response and the development of oil palm canopy
A decrease in relative radiance with stand age was identified in all Landsat TM bands (table 3.1). McMorrow (1995) found the same trend for Landsat TM spectral response of oil palm stand age classes in Sabah Malaysia, and that oil palm spectral response was most age-sensitive for stands less than 5 years old. In this study, oil palm spectral radiance was also most age-sensitive for younger stands, between 1 to 4 years old (figure 3.1). The higher discriminatory power and significant correlation at Tuan Mee relative to Balau Estate is explained by the fact that in the age range was 4 to 21 years old, whereas at Tuan Mee Estate it is between 1 to 26 years old, so Balau estate excludes the young stands that have the stronger correlation between age and radiance.
The factors explaining the spectral response of vegetation are numerous. They include interaction of radiation with the plant canopy, under storey plant effects, attenuation coefficient for radiation in the canopy and it's underlying soil background (Price and Bausch 1995). Corley (1973) explained that oil palm canopy closure starts at 4 years old and canopies reach a constant size by 9 to10 years old. The percentage of incident light intensity below canopy is a function of LAI: Log I = -0.44 L
where I is light intensity below the canopy and L is leaf area index (Corley 1973). Light intensity below the canopy is important because although the leaf area will be constant by 9 to 10 years old, by 20 years old the change of leaflets horizontal angle increases from 20 degrees to probably 40 to 60 degrees (Corley, 1973). The leaflets angle becoming less horizontal and more vertical and thus it mean that the canopy gap fraction is greater and so the ground cover contributes more.
The magnitude of the contribution of the canopy and soil are indicated in figure 3.3. Based on the contribution of the canopy and soil to the relative Landsat TM radiance, 6 types of oil palm plantation spectral response were identified: (1). One year old after planting, (2). 2 years old after planting, (3). 3 years old after planting, (4). 4 to 8 years after planting, (5). 9 to 19 years old after planting and (6). More than 20 years old after planting. The factors operating at each stage are:
-
At 1 to 3 years old, the oil palm canopy LAI development increases rapidly and at the same time the contribution of soil and under layer vegetation decrease, i.e. as crown diameter increases.
- The LAI expands gradually until 8 years old.
- The LAI reaches a constant at 9 to 10 years old, depending on the genotype and oil palm management practices. The contribution of oil palm canopy, soil and under-storey vegetation is believed to be equal at 9 to 19 years old.
- The leaflet angles are believed to increase up to 60° (starting at 20 years old) and allow more light penetration to under-storey vegetation and soil.
- After 21 years old, the under-storey vegetation is believed to increase rapidly and at the same time the soil effect reduces until constant under-storey vegetation was achieved depending on oil palm plantation management practices
The unsupervised classification based on a ratio of the Shadow Index and Bare Soil Index from were computed. It contained 5 classes: (1) 1 year old oil palm after planting, (2) 2 years old after planting, (3). 3 years old after planting, (4) 4 to 8 years old after planting and (5) 9 to 26 years old after planting. The accuracy of the TM image classification was visually compared against the age map complied from AARSB maps. The results were acceptable and useful, however further study are required especially on data calibration and spatial variability of Landsat TM data.
