Pines/Conifers
Imhoff, 1993 : Biomass = 866.75 TCBI - 89.69        (9)
r²= 0.4329
Karam et al., 1995: Biomass = 873.28 TCBI - 78.78        (10)
r²= 0.3588
Hsu et al., 1993 : Biomass = 1476.90 TCBI - 131.00        (11)
Souyris et al., 1995 : Biomass =2609.60 TCBI - 323.09        (12)
r²= 0.6835
Dobson et al., 192 : Biomass = 1495.00 TCBI - 209.59        (13)
r² = 0.7689
Dobson et al., 1995a : Biomass = 1176.20 TCBI - 22.57        (14)
r² = 0.1866
BROAD-LEAVED STANDS
Imhoff, 1993 : Biomass = 973.50 TCBI - 74.48             (15)
r²=0.6616
Dobson et al., 195a: Biomass = 5396.50 TCBI - 455.58    (16)
r²= 0.2857
While the regression equation for pines/conifers generated from the results of results of Souyris et al. (1995) had a somewhat higher multiplicative coefficient, similarities could be observed between the equations from Imhoff (1993) and Karam et al. (1995), and from Hsu et al. (1993), Dobson et al. (1992) and Dobson et al. (1995a). The discrepancies between these groups of equations could be attributed to radar system differences and other reasons specified in Section 3.2. The differences in the equations generated for broad-leaved stands from Imhoff (1993) and Dobson et al. (1995a) are significant. However, it should be noted that the latter equation was based on four biomass values only. The highest correlation with biomass was exhibited by that from Dobson et al. (1992), with the equation explaining 77% of the biomass variation.

Given in Table 3 are the TCBI values averaged over the entire biomass range for each of the studies. A comparision of the average biomass between coniferous stands from the different studies, and between coniferous and broad-leaved stands, could be made by using these values as inputs to the corresponding biomass formulas given in equations (9) to (16).

It can be discerned from the table below that the TCBI values from the results of Imbhoff (1993) and Dobson et al. (1995a) for pines/conifers and broad-leaved stands are nearly identical. This was most probably caused by the averaging process, which took into consideration the extreme values within the 0-300 tons/ha biomass range. Moreover, in the case of Dobson et al. (1995a), the insufficiency of available data may also have contributed to the similarity of the averaged results.

Authors/s	Average TCBI Value
	Pines/conifers	Broad-leaved stands
1. Imhoff, 1993	0.2224	0.2187
2. Karam et al., 1995	0.1761
3. Hsu et al., 1993	0.1426
4. Souyris et al., 1995	0.1514
5. Dobson et al., 1992	0.2242
6. Dobson et al., 1995a	0.1106	0.1125

Table 3. Average TCBI values for coniferous and broad-leaved stands as computed from interpolated results of different studies
3.3 Radar backscatter versus forest stand structure

Radar backscatter is mainly influenced by the geometric properties of the target. As stated above, forest stands with the same biomass but dissimilar morphology may produce different backscatter readings. Hence, radar data-based equations to estimate total aboveground biomass should be tailored according to the general structure of the forest stand. Although the importance of determining the stand structure prior to generating radar-derived estimates of forest biomass was emphasized in some of the studies (e.g. Imhoff, 1993; Dobson et al., 1995a), not one of the investigations reviewed here have considered stand structure determination through the use of radar backscattering data. In the absence of a priori information on stand structure, radar data-based techniques, such as the application of TCMI, is deemed essential.

3.3.1. TCMI and stand structure
The application of TCMI, which is the true ratio between the L-HH and C-HV backscatter, as a possible measure of stand morphology is premised on the differences in the sensitivity of the two wavelength-polarization combinations to the various tree components. Higher TCMI values are expected for conifers given their bigger trunk and smaller crown component volume compared to their broad-leaved counterpart. This theory has been proven to be generally true based on the test applied using interpolated backscatter and biomass values from related investigations albeit an overlapping of some of the values was observed. The overlaps between the conifer and broad-leaved stand TCMI points, as can be seen from Figure 3, occur at the lower and higher part of the biomass range. This is quite expected due to the minor differences in vegetation structure at low biomass levels and the saturation of radar measurements at high amounts of biomass. The usefulness of TCMI is thus at its optimum when there is a distinct difference in structure between the broad-leaved and needle-leaved trees and the backscattering data are taken at biomass levels below the radar saturation limits.

To further illustrate the usefulness of the TCMI in accounting for the difference in structure between broad-leaved and needle-leaved stand, given in Figure 3b are the graphs corresponding to those in figure 3a but with the L-HH and C-HV data limited to those taken within a biomass range of 20 to 150 tons/ha. By adjusting the lower and higher ends of the biomass range to these levels, the probability that broad-leaved and needle-leaved trees are of the biomass range to these levels, the probability the broad-leaved and needle-leaved trees are more structurally defined, and that the radar measurements are less affected by structurally limits, is increased - and so is the effectiveness of the TCMI. As can be discerned from the figure, the overlapping TCMI points which exist in Figure 3a have been eliminated in the case of Dobson (1995a) and were greatly reduced in the case of Imhoff (1993). For a similar purpose, given below are the average TCMI values within the 0 to 300 tons/ha and 20 to 150 tons/ha biomass range for the different studies. The 20 to 150 tons/ha range caused a higher dynamic range between the TCMI values, and hence a better separation, of the pine and broad-leaved stands. Interestingly, the average TCMI values generated from the different investigations for pines/conifers, except those form Hsu et al. (1993), are similar though the data were taken from different study sites and conditions. The same observation holds true in the case of the values from the broad-leaved stands.

Autors/s	Average TCMI Value
	Pines/conifers		Broad-leaved stands
	0-300	20-150	0.300	20-150
1. Imhoff, 1993	3.3695	3.7013	2.5168	2.3430
2. karam et al., 1995	3.2902	3.4119
3. hsu et al., 1993	5.4144	5.5431
4. Souyris et al., 1995	3.6145	3.2267
5.Dobson et al., 1992	3.4657	3.5937
6. Dobson et al., 1995a	3.7642	3.9058	2.5882	2.5119

Table 4. Average TCMI values within biomas ranges of 0 to 300 tons/ha and 20 to 150 tons/ha for the different studies