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The spectral and dynamic properties of InAs-GaAs MOCVD-grown vertically stacked self-organized quantum-dot lasers are studied experimentally. A strong mode grouping effect (quasi-periodic modulation of the lasing spectrum) is observed and interpreted as a result of wavelength-dependent losses in the laser waveguide associated with substrate leakage and reflection. Some samples also display a broader spectral modulation which may be attributed to lasing from different groups of dots, or energy levels. Experimental observations are in agreement with a theoretical explanation involving increased optical nonlinearities due to the localized nature of carriers. In relaxation oscillation pulse trains, a substructure is observed which we believe to be a dynamic manifestation of the same carrier localization effects; a preliminary rate-equation simulation supports this interpretation.