Karthik, as part of the Eastern Himalaya Program, studied forest recovery following shifting cultivation in the Upper Siang district, Arunachal Pradesh. Besides looking at the patterns of vegetation recovery, he undertook multi-factor field experiments to understand factors that affect the recovery process. He also attempted to understand factors that affect the spread of Mikania micrantha, an invasive climber species at local and landscape scales. His research also includes documenting the shifting cultivation system of the Adi community in the Siang region in Arunachal Pradesh.