2.1.2. Saturation of radar measurements
The amount of density of the scattering components within the crown, in addition to wavelength and dielectric constant, also influences the penetration depth achieved by the transmitted energy. The influence stems from the changes in path length and the degree of attenuation imposed by the crown components on the incident radiation. As the energy travels through the canopy, it is gradually depleted by the scattering caused by dielectric discontinuities in the different canopy layers such that there is less energy available to the next layer (UNSW course notes, 1996). If we assume a continuous increase in the dimensions and amount of canopy components from left to right of a forest profile, there will be a point in the profile wherein even the longer wavelength of the L-band would no longer be able to pass through the crown layer due to attenuation.

The increase in the dimensions, amount or density of the different tree components is generally directly correlated with an increase in the total aboveground biomass. That is, as the total biomass increases in value, lesser canopy penetration is expected from the forward radiation. With L-band, the amount of biomass at which the energy no longer passes through the canopy is called the saturation level for that band. Since type of polarization also exerts influence on the penetration depth, the same band with different polarizations will also have different of saturation. These influences are illustrated in Figure 1.

Point be in Figure 1 corresponds to the amount of biomass where the sensitivity of both the L-HH and LHH + C-HV backscattering to biomass measurements becomes saturated. The saturation point for C-HV is represented by point a. The flattening of the lines beyond these points indicates that the backscattering coefficients no longer respond to an increase in biomass of the forest components. Thus, any derived relationship between biomass and backscatter would no longer be valid.

This notion of saturation has important implications for the application of radar to forest areas of high biomass. Returning to figure 1, it can be deduced that equation (5) is no longer applicatibe to biomass levels to the right to b. A possible solution for the estimation of biomass at ranges beyond the soL-HH + soC-HV saturation point is the inclusion of the backscattering from a band with a longer wavelength. Equation (5) could then be modified to

where k is the unit vector corresponding to P-band with HH polarization backscattering coefficient, s^o_P-HV.

Figure 1. Sensitivities to forest components and theoretical saturation levels for C-HV, L-HH, and LHH + C-HV
2.2 Stand morphology/structure determination
In this study, two general categories are used to differentiate stands according to morphology: those composed of needle-leaved pines/conifers and broad-leaved deciduous/evergreen trees. Pines/conifers are characterized by a pattern where the bole outgrows the lateral branches thus resulting in a well-defined cylindrical trunk and a usually conical crown. The lateral branches thus resulting in a well -defined cylindrical trunk and a usually conical crown. The lateral branches extend up to the relatively lower portion of the central stem producing a narrow and deep crown but the bole is visible almost throughout the entire tree height due to the small size and density of the branches and the needle-lke leaves. Broad-leaved trees are characterized by a less-pronounced trunk but bigger and more voluminous leaves, as well as a wider and thicker crown compared that of the conifers. The lateral branches grow at the same rate, or even faster, than the bole and the definition of the bole can be completely lost due to repeated forking (Dobson et al., 1995a).

As radar backscattering behaviour is directly influenced by he geometric properties of the target, it follows that backscattering from stands of varying structural attributes will be relatively different. A stand of needle-leaved trees, with the small size and density f the leaves and branches as well as the long trunks of the trees, will have a strong L-HH return. On the other hand, the greater attenuation imposed by the bigger and more dense foliage components, coupled with the less conspicuous trunks, will results in a lower L-HH but higher C-HV backscatter from stands of broad-leaved trees. This change could also be related to, say, bit-trunked trees and mallee or scrub type vegetation, or mangroves and trees with more conspicuous trunks. This theory is illustrated in Figure 2.

It is suggested that the above theory can be useful in the determination of tree structure. Given that the sensitivity of the L-HH and C-HV so is a is a function of the trunk and crown components, respectively, then the true ratio of these two backscattering coefficients is a possible measure of tree morphology. This could be illustrated by expression

Where TCMI is the Trunk-Canopy Morphology Index. A high TCMI implies a tree structure with more trunk and less crown component while a low TCMI indicates the opposite.

Figure 2. Sensitivity of L-HH and C-HH s° to stand structure properties