Dicranopteris linearis (Gleicheniaceae), a climbing, mat-forming fern, dominates >42,000 ha of Hawaiian rainforests. One main objective was to determine the relative importance of this native species in intact, natural rainforest ecosystems on windward Mauna Loa, Hawai'i. The second goal was to examine this species' functional attributes and genetic diversity, and to relate them to natural distribution patterns over an elevational range of 290 to 1660 m on primary successional substrates of two different ages, 136 and ~3400 years old;Dicranopteris' clonal growth form, marcescent leaves, and high phosphorus use efficiency, combined with a short, <1 year, leaf segment lifespan make this species highly competitive in colonizing open canopy, low soil fertility, and intermittently waterlogged sites, i.e., conditions typical of early primary successional and post-dieback sites on the pahoehoe and a'a lava substrates;Colonization of a primary successional site by this species probably consists of two phases: (1) initial establishment by sexual reproduction via outcrossing during the first ~100 years, followed by (2) vegetative growth in the two to four clones established over a 0.5-ha area;Dicranopteris contributes up to 74% of the aboveground net primary productivity (ANPP), whereas it contains only 0-14% of the biomass. Dicranopteris litter decomposition rates are very low, with ≥50% of the original leaf mass remaining after two years. Therefore, Dicranopteris is a substantial contributor to soil detrital pools. At the ecosystem-level, Dicranopteris accounts for up to 57% and 47% of the nitrogen and phosphorus uptake, respectively, whereas it contains only 0-24% and 0-30% of the respective masses of N and P. Dicranopteris facilitates small inputs of nitrogen via asymbiotic nitrogen fixation to these nitrogen-limited sites;Growth in the overstory dominant, Metrosideros, increased upon removal of Dicranopteris from a young-flow site at 290 m. However, 43% of the understory area was bare soil three years after removal. Therefore, Dicranopteris may be the only native species capable of vegetating the understory. In the absence of Dicranopteris, an apparent vacuum is created which could be filled by a non-native invader. Thus, Dicranopteris' most important role may be to resist the invasion of exotic species into unique Hawaiian rainforests.