For the forest growth and yield calculations presented here, the process-based model FORMIX was applied in combination with a geographic information system. The study was carried out for the 55 084 ha of the Deramakot Forest Reserve, which consists of heavily logged and degraded lowland dipterocarp forest. FORMIX was used to assess forest development with and without timber harvesting. Three scenarios were compared: undisturbed forest growth and two reduced-impact timber harvesting scenarios ("textbook" and "borderline"). The latter differ in the number of seed trees retained after harvesting. The simulations reveal that timber harvesting is feasible, although stands are presently degraded. Achievable harvesting levels differ widely from 200 to 10 000 m³·year^–1 for the next 40 years for the textbook and borderline scenario, respectively. A future increase up to 80 000 m³·year^–1 is possible if all silvicultural and management standards are strictly observed. An analysis of stand development reveals that the portion of commercial dipterocarps decreases with time, while the portion of pioneer and understorey species increases. Future stands do not contain as many trees >60 cm DBH (diameter outside bark at 1.3 m height above ground), whereas they contain more trees in lower diameter classes, ensuring adequate potential for future harvests.