Hun Sen said Tuesday Ty Sokhun had made “grave mistakes” as head of the forestry administration.
More than 20 million people living along the Mekong River are reportedly facing water shortages this dry season.
A drought across southern China and Southeast Asia has brought the Mekong River to its lowest level in 50 years.
More than 10 companies have invested in jatropha plantations.
Since 2001, nearly 1,000 hectares of mangrove have been replanted at the Koh Kong Wildlife Sanctuary.