This paper studies wavelength assignment algorithms on WDM all-optical trees of rings under different models: static, incremental and dynamic. It is shown that 5L/2 is the tight bound of the number of required wavelengths for static trees of rings with load L. This paper also proposes an O[log₂(t+1)]-approximation and a ∑_i=1^hmax_r∈Ri[log | V(r)|] +h-approximation algorithm for incremental and dynamic trees of rings respectively, where t,h and R_i are the number of rings, the number of the layers of the underlying tree and the set of rings of layer i in the network respectively.