The pilot's inability to maintain altitude due to excessive airframe ice accretion, resulting in an in-flight collision with trees and terrain. Additional causes were the pilot's continued flight in icing conditions once they had been encountered, and his failure to promptly initiate evasive action in order to exit the icing conditions. A contributing factor was the pilot's pre-flight decision to operate into an area of forecast moderate icing conditions in an aircraft not equipped with in-flight icing protection.