COS 159-4 - Testing planting strategies to restore native Hawaiian forests after land-use change

Abstract: Tropical forests cover ~12% of Earthʻs land surface and play outsized roles in global and local scale processes and services, yet they have been drastically transformed over the last 200 years due to deforestation. Large portions of native forest have been lost throughout Hawaiʻi, in part due to historic land-use change, such as the conversion of forest to pastureland. Loss of native forests, and especially conversion to grass, has cascading impacts on ecosystem functions and creates more fire prone landscapes. These pastures were often planted with aggressive exotic pasture grasses that suppress native woody seedlings, and thus do not easily return to a forested condition, making active restoration necessary. Targeted efforts to restore historically deforested areas have the potential to create multilayered native forests that allow for the return of key ecosystem functions, maintain habitat for endangered species, and decrease wildfire risk. Yet, information is lacking about the efficacy of different strategies to successfully reforest degraded areas. In areas converted to pastureland, native seedling out-planting is often unsuccessful due to intense grass competition, and even when successful, natural regeneration is rarely seen. Here, we test how the density and diversity of plantings affects restoration success to increase our understanding of tradeoffs between these strategies. We hypothesized that greater diversity and higher density plantings would better exclude invasive grasses over time.

We established 55 experimental out-planting plots (6m2) in the Upper Honohina forest tract in areas under mature, out-planted Acacia koa canopy, with an understory of Cenchrus clandestinum (kikuyu grass) and scattered Rubus argutus. A total of five different diversity treatments (planting combinations) were used in single and double densities (16 and 32 plants), along with one control plot. Planted species included: ʻōhiʻa (Metrosideros polymorpha), ʻōlapa (Cheirodendron trigynum), kāwaʻu (Ilex anomala), laukahi (Dryopteris wallichiana), pilo (Coprosma rhynchocarpa), ʻākala (Rubus hawaiensis), pūkiawe (Leptocophylla tameiameiae), and māʻohiʻohi (Stenogyne calaminthoides). Plants were out-planted at the field sites in April 2021 and surveyed three times for survival and size. Preliminary results showed high surivorship (92% + 19%) in the first year across our treatments and sites. Pilo and ʻākala, an understory subtree and shrub, respectively, had the highest survivorship after 12 months, while, across species, density did not affect survivorship.These results suggest that double density plantings may not lead to higher mortality during establishment, and thus have potential to exclude grasses more expediently than single density plantings.