Trophic interactions and feedbacks maintain intact and degraded states of Hawaiian tropical forests

Abstract

Feedbacks within ecosystems can lead to internal reinforcement of the current state providing ecosystem resilience. Often, multiple biotic interactions across trophic levels play a role in such feedbacks, yet these are generally studied independently, obscuring the relative importance of interactions among different factors. We look at various potential feedbacks in intact and degraded mesic forests on Hawaiʻi Island where managers have planted native Acacia koa (koa) trees in an attempt to jumpstart succession in former cattle pastures. These restoration forests, however, have not undergone secondary succession, instead maintaining a koa overstory with an exotic pasture grass understory. We contrasted different trophic level processes that influence the capacity for natural understory regeneration: feedbacks between bird-mediated seed rain and fruiting understory ("top-down"), as well as links between understory composition and microhabitats for native seed germination ("bottom-up"). We quantified bird-mediated seed rain under canopy trees along transects spanning intact, fragmented, and restoration forests. Along these transects, we established plots around focal overstory trees to measure abundance of fruiting understory species, ground cover (e.g., exotic grass, bryophyte), and obtained estimates of bird density to evaluate the contribution of each of these factors to seedling abundance. We also used a factorial seed addition/grass removal experiment to directly compare the influence of seed rain and germination substrate. We found evidence of both top-down and bottom-up feedbacks that reinforced the current state of each forest type. In the intact and fragmented forests, the combination of comparatively high seed rain and ample germination substrate is likely critical for maintaining a diverse forest system. In contrast, exotic grasses exhibit priority effects in restoration forests, inhibiting seed germination and effectively negating any benefits that could be derived from bird-mediated seed rain. Such internal reinforcement suggests that active, rather than passive, restoration would be beneficial to increase forest diversity in restoration areas.

Crossing the threshold: Invasive grasses inhibit forest restoration on Hawaiian islands

“…Often, this process includes the introduction of grazing tolerant grasses (D'Antonio & Vitousek, 1992), and, depending on the size of the converted area, a large decline in seed rain of forest species may occur (Holl, 1999). Together these factors can create priority effects that inhibit succession back to forest once livestock grazing has ceased (Holl, 1999; Yelenik, Rose, & Paxton, 2022).…”

“…Past work suggested that multiple feedbacks worked to promote two alternative states: resilient intact forest or a persistent, restoration‐koa/grass state (Yelenik, Rehm, D'Antonio, & Caldwell, 2021; Yelenik, Rose, & Paxton, 2022). The majority of woody understory species are bird‐dispersed (although koa is not), so there is potentially a positive feedback between fruiting understory and bird density: Birds benefit from fruiting resources, and understory development benefits from bird‐mediated seed dispersal.…”

“…The majority of woody understory species are bird‐dispersed (although koa is not), so there is potentially a positive feedback between fruiting understory and bird density: Birds benefit from fruiting resources, and understory development benefits from bird‐mediated seed dispersal. While this feedback appears to have been broken by forest clearing (Chimera & Drake, 2010; Pejchar, 2015), seed dispersal has begun to recover as birds use the koa restoration areas (Paxton et al, 2018; Rose et al, 2017; Yelenik, Rose, & Paxton, 2022). Yet initial work comparing bird density and seed rain between koa restoration sites lacking understory and intact forest provides evidence that seed rain is substantially lower in koa restoration areas (Yelenik, Rose, & Paxton, 2022).…”

“…While this feedback appears to have been broken by forest clearing (Chimera & Drake, 2010; Pejchar, 2015), seed dispersal has begun to recover as birds use the koa restoration areas (Paxton et al, 2018; Rose et al, 2017; Yelenik, Rose, & Paxton, 2022). Yet initial work comparing bird density and seed rain between koa restoration sites lacking understory and intact forest provides evidence that seed rain is substantially lower in koa restoration areas (Yelenik, Rose, & Paxton, 2022).…”

The role of microtopography and resident species in post‐disturbance recovery of arid habitats in Hawaiʻi

“…Dodonaea viscosa ‐dominated plant communities tend to be more patchy (more open spaces), suggesting that competitive interactions are less likely to shape community dynamics than C. clandestinus ‐dominated plant communities, which tend to have continuous cover. Cenchrus clandestinus and other invasive grasses have also been shown to reduce recruitment, survival, and growth of native Hawaiian woody species, including D. viscosa (Denslow et al, 2006; McDaniel & Ostertag, 2010; Yelenik et al, 2022), leading us to hypothesize that biotic interactions would be important to consider with the HSI to predict restoration outcomes in this habitat. In addition, we evaluated the ability of the Quested et al (2014) HSI to predict post‐disturbance plant regrowth and native outplant success within habitats dominated by invasive and native plants.…”