In this work, we discuss the use of diagonal representation (DR) of the macro-domain basis functions, in the context of the characteristic basis function method (CBFM) applied to forest scattering problem. We examine the suitability of this enhancement technique for heterogeneous natural forests. Results show that the use of the DR for the MBFs, according to certain criteria, enables us to significantly reduce the CPU time needed to generate the CBFs with little loss of accuracy, and without increasing memory costs. Consequently, the strategy of using the DR for the computing of the CBFs enables us to simulate larger forest scenes and to do so at higher frequencies.