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In an attempt to monitor the oil-palm growth stages effectively, the most suitable SAR frequency and polarisation should be employed. This frequency and polarisation should be the one with the highest sensitivity to detect changes in the best oil-palm growth parameters. The best growth parameters are the tree parameters which show regular increase with age, and these are: i) leaflet radius, which varies regularly from around 1.85 cm at 4 years to 2.25 cm at 12 years old, ii) mean frond radius, which varies from around 2.1 cm at 4 years to 2.90 cm at 12 years old and iii) trunk height, which varies from around 1.8 metres at 4 years to around 6 metres at 12 years old. The leaflet and frond moisture contents are not good growth parameters as they do not appear to increase regularly with age. While Oil-palm crown structure with unique leaflet and frond orientations may provide a good parameter to discriminate oil-palm from other types of trees, such parameters are not good growth parameters as they vary only slightly throughout the growth stage. The trunk diameters vary irregularly between 0.5 to 0.7 metres throughout the growth stage, thus providing no good indicator for oil-palm age.

In addition to choosing the most sensitive frequency and polarisation to capture the increase of these best growth parameters, SAR configurations should also be chosen such that they minimise the occurrence of scattering mechanisms which can disturb the desired sensitivity. The oil-palm plantation floor under present study varies significantly within a single block and from one block to another; with varying percent cover of soil surface, understory and plantation litters. At young plantation (< 4 years), around 90% of plantation floor is covered by understory whereas at older plantation, the floor comprises the mixture of the following: i) around 50-60% comprising understory, ii) 2-3 % dead leaves and fruit bunches remain and iii) soil surface. There is also a small area within the plantation with swamp underneath.

Results from both ERS C w and JERS Lh h observation in chapter 6 have demonstrated

that C w band at 23° incidence angles is more sensitive than JERS Lhh at 35° to detect oil-palm age from 4 to 12 years old. The sensitivity analysis in this chapter has

polarisation is indeed a more suitable SAR configuration to be employed, with Ch v

shows significantly better sensitivity than Ch h or Cw- This analysis also shows that C-

band higher sensitivity has been due to the following factors:

i) Throughout the growth stage, the leaflet radius and moisture content is within the range at which C-band is expected to interact and scattered very strongly by such leaflets: i) leaflet radius is within the range of 1.5 to 2.5 cm, where the resonance effect occurs and very high sensitivity to even a very small increase in radius can therefore be expected, and ii) leaflet gravimetric moisture content is greater than 0.5; this high leaf moisture ensures that most incoming signal is scattered by the leaflets, thus leaving no signal passing through the crown to give rise to any disturbances from unwanted surface related scattering. This results in C-band consistently high sensitivity to the increase in leaflet radius independent of the ground conditions. This relative independence from ground surface effects has proved to be very useful while monitoring oil-palm plantation where between 50-90 percent of the plantation floor is covered by the mixture of mostly understory, dead leaves and fruit bunches remains.

ii)At mature plantations (> 6 years old), the dimension of leaves and branches of the understory and litters are usually larger than C-band wavelength. As a result, despite the increasingly less dense crown structure at older plantations (as the number of frond decreases), less disturbance has been observed from direct scattering from understory and litter. At L-band, results from JERS Lhh observation and model

analysis have shown that the decrease in the number of fronds has resulted in both the reduction in CVS and CGS (which is responsible for L-band expected sensitivity to frond parameters), while at the same time increasing the "disturbance" scattering from the ground surface. Coupled with L-band weak interaction with oil-palm frond with relatively high moisture content, these factors have significantly hampered L-band expected sensitivity to frond radius, hence to oil-palm age.

iii)Results from chapter 6 have also shown that C-band appears to be more sensitive to changes in leaflet radius than the number of leaflets. CVS appears to increase significantly following the increase in radius from age 6 to 12, despite a decrease in the number of fronds and leaflets. While oil-palm LAI saturates at about 8 years old

(Corley, 1976), the leaf radius still increases to over 12 years old. C-band can therefore provides good potential to detect oil-palm age via its high sensitivity to changes of leaf radius, despite the decrease in LAI at older plantations.

Results from this sensitivity analysis have also shown that out of the all frequencies and polarisations investigated, Chv appears to be best frequency/polarisation to be employed for the retrieval of oil-palm age. As the sole contributor to the total backscatter comes from CVS, this polarisation is totally independent of ground surface roughness. Coupled with its particularly high sensitivity following the increase of oil-palm leaflet radius from

1.0 to 2.5 cm, the use of Chv to detect oil-palm age can therefore provide a very powerful tool.

Despite its good sensitivity to the increase of small leaflet radius, this sensitivity analysis has shown that X-band is less sensitive to oil-palm age than C-band. X-band signals interact mostly with the leaflets at the upper part of the tree crowns, where the leaflet radius does not increase regularly with age, (the range of leaf radius is around 1.30 to 1.7 cm throughout the growth stage under consideration). Despite the "resonance effect" at X-band following the increase of leaflet radius from 0.5 to 1.5 cm, this irregularity has made X-band less sensitive to oil-palm age.

The sensitivity analysis shows that L-band does not appear to "see" leaves within oil- palm leaflet range of size, and is therefore not sensitive to this growth parameter. It is however, more sensitive to larger leaflet radius. At rough surface, the less occurrence of TGS has made L-band sensitivity become extended to smaller radius (>3.0 cm). However, this range of radius is still larger than that of oil-palm leaflets, thus the use of L-band to detect oil-palm age via its sensitivity to oil-palm leaflet radius is never feasible. L-band however, shows slight sensitivity following the increase in frond radius at rough surfaces. Following the increase of mean frond radius from 2.0 to 3.0 cm, Lhh

and Lvv shows slight sensitivity, whereas the use of Lhv can significantly improve this sensitivity. The low sensitivity to the increase in frond radius may also be due to the fact that oil-palm fronds are relatively wet compared to other tree branches. While the common gravimetric moisture content for the branches are around 0.5, that of oil-palm

that while the backscattering from the frond increases following the increase in gravimetric moisture content up to 0.5, further increase has resulted in increasing signal absorption instead, hence the decrease in backscatter. This result may explain L-band low sensitivity to oil-palm frond parameters.

L-band sensitivity to trunk parameters depends significantly on the condition of the ground surfaces. Highest sensitivity is achieved at smooth and medium surfaces when TGS occurs very strongly. In most real forests/plantations, however, the floors can be represented as rough surfaces, thus the use of L-band to retrieve trunk parameters can be severely limited. Results from the sensitivity analysis, however, show that some sensitivity can still be achieved when trunk size is large enough. The large diameter appears to have increased the occurrence of TGS significantly even at rough surface. When the height is around 6 metres, TGS starts to become a dominant mechanism when trunk diameter is larger than 0.5 metres. When the trunk diameter is consistently large (>0.5 m) like that in oil-palm, TGS starts to dominate at even quite low tree height (> 2 metres). As from 4 to 12 years old, oil-palm trunk diameter stays relatively constant between 0.5 to 0.7 m and the trunk height increases from around 1.8 to 6 metres, this result suggests that there is still some potential for L-band to detect oil-palm age via its sensitivity to trunk height. However, this result is based on the representation of relatively smooth trunk surface roughness. The inclusion of rough trunk surface may reduce the occurrence of TGS, thus changing the limit of the trunk size at which L-band can still be expected to provide observable response. A larger trunk diameter or tree height than those suggested from this analysis may be required if a sufficiently high TGS magnitude capable of overcoming CVS is to be expected . With support from JERS observation which shows very bright areas over oil-palm plantation with swamp underneath, the present study suggests that the rough oil-palm trunk surface, to a certain extent, may still give rise to the occurrence of TGS mechanism. The oil-palm trunk surface RMS height is only around 4 cm, which is still much smaller than L-band 23 cm wavelength. At this range of roughness, the scattering pattern from a relatively rough surface where a large proportion of the signal is scattered in the specular direction can still be expected (Ulaby et al., 1986). Results from the sensitivity analysis to varying ground surface roughness has also demonstrated that at ground RMS height around 4 cm, the TGS mechanism is still quite high, albeit lower than those from GS and CVS (see

fig u re 7.3).

The variation of soil moisture within the plantation may also result in high variation of X-, C- and L-band sensitivities to oil-palm growth parameters as it varies the strength and occurrences of GS, CGS and TGS. There is a variation of gravimetric soil moisture content from 0.1 to 0.35 within the plantation. Both C- band at 23° and L-band at 25° are particularly sensitive to changes of soil moisture within this range figure 7.22a and h). When this sensitivity to soil moisture is comparable to the desired sensitivity to the growth parameters, this may seriously harm the utility of both C- and L-band to detect oil-palm age. In this case, L-band is more affected than C-band as over the entire range of surface roughness, TGS, CGS and GS mechanisms dominate, thus resulting in high backscatter variation due to unwanted soil moisture variation ( see figure 7.22h)

T able 7.5. The sen sitivity o f C w a t 23“ in ciden ce angle.

iM y e rs P a r a m e te r s R a n g e o f C h a n g e s D y n a m ic R a n g e O b s e r v e d (d B ) S m o o th M e d iu m

1

1

₁

1

1

Table 7.5 summarises the dynamic range achieved by C w at 23° from model simulation to changes in oil-palm growth parameters. Cyv appears to be particularly sensitive to changes in leaflet radius. A reasonably high dynamic range achieved at all range of surface roughness, with the highest sensitivity achieved at medium surfaces (18.9 dB). A reasonably high dynamic range of around 7-8 dB can still be achieved at smooth and rough surface. Cyv is also sensitive to leaflet gravimetric moisture content, which throughout the growth stage varies irregularly with age between 0.6 to 0.7. This range of moisture content results in a quite significant changes in backscatter of between 3 to 7 dB at different surface roughness. However, this variation appears to be around half or

less that that due to the increase in leaflet radius, thus C w sensitivity to leaflet radius may still be useful to detect oil-palm age. In addition to irregular variation in leaflet moisture content, there is also irregular variation in soil moisture content within the plantation. A high dynamic range following this range of variation has been observed at smooth and rough surfaces. This dynamic range is comparable to that due to the increase in leaflet radius, hence may seriously hamper C w utility to detect oil-palm .age. At medium surfaces, however, C w appears to be independent from the effect of soil moisture variation. Provided the ground condition can be represented as having medium surface roughness (i.e. well-managed plantation where the understory and litters are cleared regularly), C w can provide a powerful tool to detect oil-palm age.

T a b le 7.6. The s e n s itiv ity o f Lhh a t 35" in c id e n c e a n g le.

Layers P aram eters R ange o f C h anges D y n a m ic R a n g e O bserved (dB) Sm ooth M ediu m Rough Crown L e a f L e a f R adius 1 .5 - 2 .5 cm

L e a f M oisture 0 .6 - 0 .7 Frond F ron d R adius 2 .0 - 3 .0 cm

F ron d M oisture 0.6—0.7 - 1.2

Trunk Trunk H eigh t I.S —6.5 m 7.6 6.1

Trunk D B H 0 .5 -0 .6 5 m 4 .7 4.2

1.4 0.5

G roun d S o il M oisture 0.1 - 0.35 7.4 4.5 6.8

Table 7.6 summarises the dynamic range achieved by L h h at 35° incidence angle from

model simulation. L h h shows only slight sensitivity to frond radius, with a small

dynamic range of round 2 dB achieved at rough surfaces when TGS is minimised. Coupled by its almost comparable sensitivity to frond moisture content variation, which varies irregularly with age, the sensitivity of L h h to detect oil-palm age via its sensitivity

to frond radius appears to be ineffective. Lhh sensitivity to trunk parameters, however, is

somewhat better. At at smooth and medium surfaces, the sensitivity to trunk height is reasonably good. This sensitivity to trunk height, however, may be disturbed by its quite comparable sensitivity to trunk diameter, which varies irregularly with age. As can be seen from the table, that at smooth and medium surfaces, the backscatter due to irregular variation in trunk diameter is about the third of that due to the increase of the trunk height throughout the growth stage under consideration. This may seriously hamper the

utility of L h h to detect oil-palm age. In addition to this, L h h also appear to be very

sensitive to soil moisture variation within the plantation. A dynamic range obtained due to such variation is indeed comparable and even higher (at rough surfaces) than that due to the increase of the trunk height. For rough surfaces, the changes in soil moisture appears to have more effect to the total Lhh backscatter than the changes in trunk height. All these factors will seriously hamper the utility of L h h at 35° incidence angle to detect

oil-palm age.

T a b le 7.7. The b e st frequ en cy a n d p o la risa tio n to be u tilise d f o r d etectin g o il-p a lm grow th stages.

Layers Parameters Range of Changes Most Sensitive Bands

Dynamic Range Observed (dB)

smooth m ediu m rough

C row n

L e a f R adius 1 .5 - 2 .5 cm Ch v 1 9.5

L e a f L e a f M oisture 0.6-0.7 Chv

F ron d R adius 2 .0 - 3 .0 cm Lh v

F rond F ron d M oisture 0.6-0.7 - 5 . 5

Trunk H eigh t 1 .8 - 6 .5 m Lhh 7 .6

Trunk Trunk D ia m eter 0.5—0 .7 ni 4 .7

19.4 4.1 3 .9 - 5 6.1 4.2 1 9 .4 4.1 - 3 1.4 0.5

While L h h and C w may not be the best configurations to be employed to detect oil-palm

age, table 7.7 shows the summary of the recommended frequencies/polarisations to detect oil-palm age via their best sensitivities to detect different growth parameters. As can be seen in table 7.7, the most sensitive bands to detect changes in leaflet radius, frond radius, and trunk height are C h v , L h v and L h h respectively. C h v appears to be the

most feasible to be utilised compared to L h v and L h h The sensitivity of L h v and L h h

appears to be disturbed by almost comparable sensitivity to frond moisture content (for

L h v ) and trunk diameter (for L h h ) which do not vary regularly with age. With a very

high dynamic range achieved by C h v due to even the small increase in leaflet radius, the

backscatter variation due to irregular variation in leaf moisture content is much smaller. Coupled with the fact that this polarisation is most independent of the ground surface condition, C h v can provide a very powerful single frequency/polarisation to detect oil-

Chapter 8.

THE USE OF THE MODEL TO ASSESS THE IMPROVEMENT