Disclosed are apparatus and methods for generating synthesized utterances. A computing device can receive speech data corresponding to spoken utterances of a particular speaker. Textual elements of an input text corresponding to the speech data can be recognized. Confidence levels associated with the recognized textual elements can be determined. Speech-synthesis parameters of decision trees can be adapted based on the speech data, recognized textual elements, and associated confidence levels. Each adapted decision tree can map individual elements of a text to individual of the speech-synthesis parameters. A second input text can be received. The second input text can be mapped to speech-synthesis parameters using the adapted decision trees. A synthesized spoken utterance can be generated corresponding to the second input text using the speech-synthesis parameters. At least some of the speech-synthesis parameters are configured to simulate the particular speaker.