Planting old pastures with the native tree Acacia koa is a common forest restoration strategy in Hawaii. The goals of this strategy are to shade out alien grasses, attract birds that disperse native seeds, and lead to secondary succession with little further management input. Often, however, there is no secondary succession, leading to the question of whether restoration forests are resilient to change.

This resilience could be caused by multiple processes: feedbacks between birds, seeds, and understory (seed limitation), understory and germination habitats (establishment limitation), or a combination of the two. A lack of food (i.e. fruit) and suitable habitat (i.e. structure) often discourages birds from visiting degraded sites. Because birds disperse seeds that are critical for tree and understory plant recruitment, this lack of bird visitation can lead to reinforcement of a degraded state. In addition, in degraded tropical forests, suitable germination sites are often limited by the presence of highly competitive exotic pasture grasses, which can stall natural forest regeneration even when seed dispersal occurs.

We explored the influence of both of these processes on the capacity for natural understory regeneration in Hawaiian forests. We quantified bird-mediated seed rain (deposition) under canopy trees along transects spanning intact, fragmented, and restoration forests. We also established plots around focal overstory trees to measure abundance of fruiting understory species and ground cover (e.g., exotic grass, moss), and we obtained estimates of bird density to evaluate the contribution of each of these factors to seedling abundance. We also used a seed addition/grass removal experiment to directly compare the influence of seed rain and ground cover. We found evidence of multiple feedbacks that reinforced the current state of each forest type.  

Key Findings

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  The direction, nature, and intensity (line thickness) of trophic relationships are shown in this diagram, as well as the importance of each factor (box grayscale).
  Acacia koa restoration forests with an alien, invasive grass understory are resilient to change due to multiple feedbacks.
• Alien grasses benefit from Acacia koa’s high light and nitrogen understory habitat, which then keep other native woody species from recruiting in, reinforcing the low diversity state of restoration areas.
• The lack of native fruiting understory in Acacia koa restoration forests leads to lower density of fruit-eating bird species that would otherwise disperse seeds into these areas, reinforcing a lack of seed rain and secondary succession.
• Intact forests with diverse species assemblages have both the seed rain of native species and abundance of hospitable germination habitats to maintain native regeneration and remain resilient to change.