COS 237-5 - Restoration to increase forest diversity and stall grass-fire cycles in Hawaiʻi

Abstract: A widespread challenge for land managers is restoring post-fire habitats to native-dominated states that are resilient to fire. Globally, invasive grasses exacerbate fires by increasing fine fuel loads and fire return intervals. When grasses invade the understory of a woodland or forest, they easily carry fires into forest areas, further degrading forests and leading to grass-fire cycles. Post fire seeding may help species return to degraded forests and help increase ecosystem resistance to invasion. In Hawaiʻi, a greater percentage of land burns each year, on average, than across continental states. Yet, research is lacking from Hawaiʻi on how fire affects plant communities and best practices for restoration. We tracked post-fire natural regeneration after a 2018 fire in Hawaiʻi Volcanoes National Park burned 2,978 acres across an elevation gradient and various vegetation types (Acacia koa (koa)-dominated forest and Sophora chrysophylla (māmane)-dominated woodland) to better understand how native woody plant regeneration relates to fire severity and invasive grasses. In addition, we implemented a post-fire seeding experiment in koa-dominated forests to explore how different densities of seed affect native plant recruitment, invasive grass cover, and whether higher seeding rates lead to greater resistance to invasive grasses.



Elevation, which was tied to pre-fire condition, affected post-fire dynamics. We found that lower elevation koa forests that had burned previously and had an invasive grass-dominated understory, quickly returned to a koa/invasive grass state. In these koa- grass forests, few other native woody seedlings successfully recruited without seeding. In contrast, higher elevation māmane-dominated woodlands, which had a pre-fire native woody shrub, fern and perennial bunchgrass understory, had high numbers of native woody seedlings and resprouting bunchgrasses. We focused our seeding experiments in koa forests due to their low diversity states even before fire. Of the seeded species, Dodonaea viscosa (ʻaʻaliʻi) and māmane showed the greatest recruitment levels. Even though invasive grass cover increased over time, these two native species maintained individuals in seeded plots, with plots seeded at high density retaining the greatest numbers. At the last sample date, invasive grass cover showed a negative relationship with the cover of both māmane and ʻaʻaliʻi, and this relationship was more negative in plots than had been seeded at the higher seeding rate. Longer term monitoring may show that high seeding rates lead to greater competition and thus confer resilience to grass invasion over time.