Potter K.M., Giardina C., Hughes R.F., Cordell S., Kuegler O., Koch A., & Yuen E. (2023). How invaded are Hawaiian forests? Non-native understory tree dominance signals potential canopy replacement. Landscape Ecology, 38, 3903–3923. https://doi.org/10.1007/s10980-023-01662-6

Summary

The Hawaiian Islands are a global hotspot of non-native species richness with around half of the flora being made up of introduced and naturalized plant species. Approximately half of all Hawaiian land cover is either dominated by invasive plants, or consists of a mix of native and non-native species. However, forest land cover only reflects tree species that make up the forest canopy. Because non-native trees in the understory can limit the regeneration of native trees and respond rapidly following canopy disturbance, their presence can serve as an indicator of potential future forest composition and structure.

This study used a network of 238 standardized vegetation monitoring plots established by the USDA Forest Service Forest Inventory and Analysis (FIA) program across Hawaiʻi to create the first comprehensive assessment of plant invasions in Hawaiian forests. They also assessed how invasion related to environmental and management related factors including ownership/management (e.g., public vs. private), forest type (e.g., wet vs. dry), and fencing status (fenced to exclude feral ungulates). Analyses were based on data collected beween 2010-2015 on five of the major Hawaiian Islands including Hawaiʻi, Maui, Oʻahu, Lānaʻi, and Kauaʻi. Molokaʻi and Niʻihau were not sampled. The authors also assessed the presence and cover of 40 species of non-native plants of particular concern that were determined by local experts as likely to cause economic or environmental harm. Plots were considered dominated by non-native trees if a non-native tree species represented the most canopy cover for the plot (i.e., canopy trees), regardless of the number of stems.

Results

Fifty-six percent of Hawaiʻi’s 533,184 hectares of forest land contained non-native trees, with 39% of these forests being dominated by non-native trees. Only 44% of forest remains entirely native. In all forest types, non-native trees made up a larger proportion of the understory than the overstory tree component (66% of the saplings and 63% of the seedlings were non-native). Non-native plants of particular concern due to their invasiveness were identified in 27% of the plots surveyed. Strawberry guava was the most abundant invasive tree species in Hawaiʻi and were found on 37% of the plots. The endemic ʻōhiʻa (Metrosideros polymorpha) was the second most abundant tree species in Hawaiʻi and was found on ~68% of the plots. The sapling abundance of strawberry guava was nearly twice that of the native ʻōhiʻa. Ninety-six percent of forests in the lower-elevation areas of all islands contained non-native trees, with 75% dominated by non-native trees, whereas in higher elevation forests, only 46% of forests contained non-native trees with 31% dominated by non-natives. The forest type with the highest invaded area was lowland tropical rainforest, followed by mesic forest. All forest types were significantly more impacted by invasive trees on the lower elevation islands (Oʻahu/Kauaʻi/Lānaʻi), than on the higher elevation islands (Hawaiʻi/Maui) across all three size classes (seedlings, saplings, and large trees).

Management Implications

• The high prevalence of non-native seedlings and saplings in forest understories points towards an eventual shift from native to non-native dominance without management intervention. If all Hawaiian forests are following this invasion trajectory, non-native species could eventually constitute 75% or more of the forest tree stems and basal area on all islands and across forest types and elevations.
• The dominance of non-native plants in seedbanks, combined with their ability to germinate at higher percentages than native species, makes many invasive trees well-positioned to increase rapidly following disturbances, such as after disease outbreaks (e.g., rapid ʻōhiʻa death), or extreme climatic events (e.g., storms, floods, drought, heatwaves).
• Higher elevation forests, which are relatively less impacted by large or sapling-sized non-native trees, are prime candidates for early detection and rapid response efforts, particularly those that focus on invasive plants in the understory that are effective at suppressing native tree regeneration, like Kahili ginger (Hedychium gardnerianum), Koster’s curse (Clidemia hirta), or invasive grasses.
• Most forests in Hawaiʻi are now hybrid communites composed of both native and non-native trees with a large fraction being novel forests dominated by non-native species. Restoration of these forests to a purely native condition, such as lowland wet forest, is not feasible without massive and sustained investment.
• Non-native trees appear to pose less of a threat to mesic forests on Hawaiʻi Island as compared to the same forest type on other islands. This indicates that per-unit-area costs for invasive tree removal for Hawaiʻi Island may not be as high as elsewhere.

Take Home Points

• Forests on public lands, in conservation reserves, or in fenced areas were less impacted by non-native trees and shrubs, indicating possible benefits of conservation management.
• There is a need to develop disturbance-ready restoration techniques that include rapid response for invasive seedling removal following disturbances, especially in the remaining native-dominated forests.
• Management costs for removal will likely increase over time as non-native seedlings and saplings grow and mature. Early detection and rapid response (EDRR) is the most cost-efficient approach for removal.
• With limited resources and a daunting challenge revealed by this study, managers will need to decide whether to Resist changes to forests by removing invasive trees, Accept the changes that are occurring in some places to focus efforts elsewhere, or try to Direct  ecosystem trajectories towards more desirable states by, for example, using non-invasive non-native or Polynesian introduced plants.