How invaded are Hawaiian forests? Non-native understory tree dominance signals potential canopy replacement

Context Non-native species invasions are altering the composition, structure, function, and dynamics of forests globally. The Hawaiian Islands are a global biodiversity hotspot for non-native invasive plant species. New spatial inventory data for forests of Hawaiʻi can provide insights into invasive species presence and dominance across complex landscapes. Objectives We employed a network of 238 standardized plots spanning climate and soil gradients to conduct the first comprehensive assessment of non-native plant invasions in forests of Hawaiʻi. We examined non-native plant dominance from the forest floor to canopy to understand how invasion related to environmental and management-related factors. Methods We tested whether significant differences in non-native dominance across forest strata existed based on ownership/management, fenced status, island group, and forest type. These analyses were conducted separately for each of six plot-level non-native dominance metrics, to assess the abundance and importance of non-native plants across forest strata. Biomass estimates for dominance were translated into carbon (C) units to assess invasive species impacts on C budgets. Results Across forest types, non-native tree species accounted for 30% of large tree stems, 65% of sapling stems, and 67% of seedling stems. Distribution of C was very similar. Low-elevation forests were particularly degraded, but even montane forests were widely impacted and may become more so following forest disturbance. 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. Conclusions In all forest types, non-native trees constituted a larger proportion of the understory than the overstory tree component, which points to the potential eventual replacement of native canopy trees by non-native trees. The patterns and processes of plant invasion in Hawaiian forests provide data for the conservation of Hawai‘i’s unique flora and insights into how invasion trajectories may play out in other forests.