Restoring with forest monocultures: A mechanistic look at invasive species suppression

Abstract

To create conditions suitable for the recruitment of native forest species, tools are often needed to suppress grasses when restoring forests from abandoned pastureland. Planting trees is one method to reduce grass growth through shading and accumulation of leaf litter, while benefiting woody seedlings by moderating the microclimate. 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 the growth of competitive invasive grasses. The balance between grass suppression versus facilitation may depend upon tree density and site conditions. Using planted stands of a native N-fixer, Acacia koa (koa), on Hawai'i Island, we evaluated the efficacy of invasive grass suppression by koa and what mechanisms might be responsible. We identified a range of tree densities and sites spanning the island. We found consistent effects of grass suppression by koa via shading and litter accumulation, though total grass suppression rarely occurred. There was a positive relationship between soil N and koa basal area, but this did not lead to more grass. It is unclear if the conditions created by dense koa thickets facilitate diverse forest regeneration. If grass suppression to levels that allow for diverse natural regeneration is the desired management goal, then further management interventions could amplify the mechanisms of grass suppression we identified by introducing species that cast deeper shade and/or change the litter environment.

Citation

Esch, Clarice M.; Yelenik, Stephanie G. 2025. Restoring with forest monocultures: A mechanistic look at invasive species suppression. Restoration Ecology. 2025: e70162.