Tropical forests are important storehouses of carbon and biodiversity. In isolated island ecosystems such as the Hawaiian Islands, relative dominance of native and nonnative tree species may influence patterns of forest carbon stocks and biodiversity. We determined aboveground carbon density (ACD) across a matrix of lava flows differing in age, texture, and vegetation composition (i.e., native or nonnative dominated) in wet lowland forests of Hawaii Island. To do this at the large scales necessary to accurately capture the inherent heterogeneity of these forests, we collected LiDAR data across areas of interest and developed relationships between LiDAR metrics and field-based estimates of forest ACD. This approach enabled us to inventory, rather than merely sample, the entire populations (i.e., forests) of interest. Native Hawaiian wet lowland forests exhibited ACD values similar to those of intact tropical forests elsewhere. In general, ACD of these forests increased with increasing lava flow age, but patterns differed between native and nonnative forest stands. On the youngest lavas, native-dominated forest ACD averaged <60 Mg/ha, compared to ∼100 Mg C/ha for nonnative-dominated forests. This difference was due to the presence of the nonnative, N2-fixing trees F. moluccana and C. equisetifolia in the nonnative-dominated forest stands, as well as the corresponding absence of N2-fixing trees in native-dominated forest stands. Following ∼500 years of primary succession and thereafter, however, both forest types exhibited ACD values averaging ∼130 Mg C/ha, although it took nonnative forests only 75–80 years of post-establishment succession to reach those values. Given the large areas of early-successional M. polymorpha-dominated forest on young lava flows, further spread of F. moluccana and C. equisetifolia populations would likely increase ACD stocks but would constitute a significant erosion of the invaluable contribution of Hawaii's native ecosystems to global biodiversity.