Reintroducing tree cover to degraded pastureland can facilitate understory recruitment by shading out sun-loving pasture grasses while providing structure to encourage seed dispersal by animals. Nitrogen (N) fixing trees, common to many tropical forests, are popular for use in restoration. Yet they may elevate soil N beneath their canopies, and elevated soil N availability could promote invasive species in the understory. This talk will focus on the question: Is there a tree canopy density of N fixer, that best promotes a native understory composition? Specifically, we ask whether N fixing trees can achieve a canopy density that will reduce grass competition via shading in a subtropical ecosystem degraded by invasive grasses and fire. To address this question, we investigated planted stands of two Hawaiian native N-fixing trees (Acacia koa and Sophora chrysophylla), and compared these to one another and to intact woodland and unrestored open patches in burned seasonal submontane woodlands in Hawaii Volcanoes National Park. We measured light availability, soil depth, and N availability and described stand architecture in terms of basal area, canopy closure, and tree height. We characterized understory composition in terms of species cover, species richness, and total biomass of the dominant exotic grass in this system. For one of the N fixing trees, Sophora chrysophylla, we also compared the understory composition of stands across a density gradient to understand whether there might be a low density (reduced N input) or high density (reduced light availability) threshold beyond which this species supports more native woody understory species.
We found that the understory of all stands of both planted native N fixers was dominated by exotic grasses, particularly Melinis minutiflora, regardless of tree canopy cover or basal area. N-fixer plots were always weedier than intact forest and many native species were lacking in them. N availability was high under the N fixers compared to the open and to intact woodland (lacking fixers), particularly under S. chrysophylla. Biomass of invasive grasses was not reduced in low light, at least across the canopy densities measured here. For Sophora, we did not find a low or a high density where it was associated with lower exotic grass cover and higher native shrub cover. Instead it was consistently associated with high exotic cover regardless of canopy density. Our data therefore suggest that in the N and light environment created under these trees there is no low light, or high light refuge from grass competition.