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

Abstract

Forest removal for livestock grazing is a striking example of human-caused state change leading to a stable, undesirable invasive grass system that is resistant to restoration efforts. Understanding which factors lead to resilience to the alternative grass state can greatly benefit managers when planning forest restoration.We address how thresholds of grass cover and seed rain might influence forest recovery in a restoration project on Hawaiʻi Island, USA. Since the 1980s, over 400,000 Acacia koa (koa) trees have been planted across degraded pasture, and invasive grasses still dominate the understory with no native woody-plant recruitment. Between this koa/grass matrix are remnant native Metrosideros polymorpha (ʻ ohiʻa) trees beneath which native woody plants naturally recruit.We tested whether there were threshold levels of native woody understory that accelerate recruitment under both tree species by monitoring seed rain at 40 trees (20 koa and ʻ ohiʻa) with a range of native woody understory basal area (BA).We found a positive relationship between total seed rain (but not bird-dispersed seed rain) and native woody BA and a negative relationship between native woody BA and grass cover, with no indication of threshold dynamics. We also experimentally combined grass removal levels with seed rain density (six levels) of two common understory species in plots under koa (n = 9) and remnant ʻ ohiʻa (n = 9). Few seedlings emerged when no grass was removed despite adding seeds at densities two to 75 times higher than naturally occurring. However, seedling recruitment increased two to three times once at least 50% of grass was removed. Existing survey data of naturally occurring seedlings also supported a threshold of grass cover below which seedlings were able to establish. Thus, removal of all grasses is not necessary to achieve system responses: Even moderate reductions (~50%) can increase rates of native woody recruitment. The nonlinear thresholds found here highlight how incremental changes to an inhibitory factor lead to limited restoration success until a threshold is crossed. The resources needed to fully eradicate an invasive species may be unwarranted for state change, making understanding where thresholds lie of the utmost importance to prioritize resources.

Overcoming ecological feedbacks in seagrass restoration

“…not false or structural zeros), and therefore used a hurdle model approach that accounts for and includes these excess zeros (Cragg 1971). Such an approach is widely used for ecological count data that has excess zeros, including for terrestrial forest restoration (Rehm et al 2023) and bird observations (Balderama et al 2016).…”

Tradeoffs in access to light and root networks of established host trees for restoration planting of the root hemiparasite Santalum paniculatum

“…Despite casting relatively low shade through its canopy, in part due to the orientation of mature phyllodes, koa is a light‐demanding species (Baker et al 2009; Rose et al 2019); therefore, the ability of underplanted seedlings to capture available light is of interest. Due to pressures from invasive grasses, without intervening practices such as grass management or underplanting, planted koa in abandoned pastures will likely remain koa and grass with little other diversity (Scowcroft et al 2008; Rehm et al 2023). When other limiting factors are accounted for (e.g.…”