An individual-based, multiple-growthform gap model of forest stand development and succession (ZELIG.MGF) of the JABOWA and FORET lineage was modified to simulate long-term changes in 'ōhi'a-hāpu'u montane rain forests on the island of Hawai'i. Based on the autecology, architecture, and life history of the two dominant species, we were able to re-create some of the stand dynamics and population structures observed in these forests. The phenomenon of displacement dieback, which occurs only on rich sites with persistent cloud cover, is portrayed as a natural successional consequence of 'ōhi'a lehua Metrosideros polymorpha Gaud.) senescence and shade intolerance, which contrasts with the shorter stature, clonal reproduction, and shade tolerance of hāpu'u pulu tree fern Cibotium glaucum (Sm.) Hook. & Arn.). ZELIG.MGF predicts that 'ōhi'a will achieve a maximum basal area of 27 m2/ha in stands 80–90 years old, after which basal area is projected to decline to levels of 8–11 m2/ha that persist after 220 years. Alternating phases of 'ōhi'a and hāpu'u may dominate individual gaps, but overall old-growth 'ōhi'a populations do not recover to earlier levels. 'Ōhi'a overstory mortality is consistent with senescence or a growing imbalance of respiratory to photosynthetic tissue in large trees. Understory mortality as modeled is largely due to shading by adults and by hāpu'u tree ferns, although mechanical damage from dead ha-pu'u fronds, which was not modeled, may also be important. 'Ōhi'a stand rejuvenation can occur when the density of hāpu'u is reduced by harvesting or wind storms.