Restoring with forest monocultures in Hawaii
A reforestation area in Hawaii, planted with a monoculture of the native Nitrogen-fixing tree Acacia koa. The understory is dominated by dense, healthy invasive pasture grasses.
A large portion of tropical forest has been cleared for agriculture and pasture. In the case that land managers desire to reforest, planting trees is one method to reduce grass growth through shading and accumulation of leaf litter, thereby leading to forest succession and a more diverse ecosystem. Nitrogen-fixing trees are often planted in tropical reforestation work because of their fast growth rates and ability to enrich soil nutrients. However, nitrogen (N) contributed to the soil from N-fixing trees can also facilitate growth of competitive exotic grasses, creating a paradox.
We explored whether, across sites in Hawaii that varied in climate, high density plantings of Acacia koa, a native N-fixing tree, would be able to decrease grass biomass to a level to allow for secondary succession. While high density monoculture plantings in some sites were able to lower grass biomass substantially, it was never enough to allow for native woody seedlings to naturally recruit into these areas. In the case where a diverse forest is the management goal, planting with koa monocultures may not be the best strategy; instead, high density multi-species plantings may achieve goals more quickly. This research is important to highlight because koa monocultures are an often-used forest restoration strategy across Hawaii.