Extensive clearing of tropical and subtropical forests for livestock grazing has altered plant communities globally. Cleared areas are often planted with exotic pasture grasses that remain persistent, even after grazers are removed. Efforts to restore forests often depend on remnant and planted canopy trees jumpstarting succession by competing with exotic grasses, changing microhabitats, and encouraging bird visitation and thus seed dispersal. To date, there is little mechanistic information on how different canopy species affect community trajectories through these different trophic interactions. We explored the effects of the two dominant canopy trees species-- Acacia koa (koa) and Metrosideros polymorpha (ohia)—on understory development via effects on litter and avian seed dispersal in mid-elevation mesic forest in Hawaii. Koa is a nitrogen (N)-fixing tree that is commonly used in restoration projects due to its high survivorship and growth. Ohia, by comparison, has slow growth rates, high C:N litter, and offers different food resources to birds. We used a series of plant and soil surveys, seed rain traps, and seed addition experiments, to ask how these two canopy trees affect understory recruitment.
Our data show that the identity of tree species used in restoration of these degraded pastures had strong effects on the development of native plant understory communities, and that this was mediated by tree-specific effects on litter and understory environment. Remnant and/or reforestation koa on its own did not facilitate passive regeneration of native woody understory, but rather increased soil N and exotic grass biomass. In contrast, ohia tended to have a greater build-up of high C:N litter, lower grass biomass and a higher number of native seedlings. Grass removal and seed addition experiments showed that seed addition only functioned to increase seedling recruitment if grass was removed. In addition, the amount of bird-mediated seed rain was similar between ohia and koa, suggesting that while seed may be limited overall, it is not the tree species effects on avian seed dispersal that drives this pattern. Overall, the greatest barrier to the succession of these forests appears to be priority effects established by the exotic grasses which seem to hold koa restoration areas in a koa-grass stable state.