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Landscape level effects of invasive plants and animals on water infiltration through Hawaiian tropical forests


Watershed degradation due to invasion threatens downstream water flows and associated ecosystem services. While this topic has been studied across landscapes that have undergone invasive-driven state changes (e.g., native forest to invaded grassland), it is less well understood in ecosystems experiencing within-system invasion (e.g. native forest to invaded forest). To address this subject, we conducted an integrated ecological and ecohydrological study in tropical forests impacted by invasive plants and animals. We measured soil infiltration capacity in multiple fenced (i.e., ungulate-free)/unfenced and native/invaded forest site pairs along moisture and substrate age gradients across Hawaii to explore the effects of invasion on hydrological processes within tropical forests. We also characterized forest composition, structure and soil characteristics at these sites to assess the direct and vegetation-mediated impacts of invasive species on infiltration capacity. Our models show that invasive ungulates negatively affect soil infiltration capacity consistently across the wide moisture and substrate age gradients considered. Additionally, several soil characteristics known to be affected by invasive ungulates were associated with local infiltration rates, indicating that the long-term secondary effects of high ungulate densities in tropical forests may be stronger than effects observed in this study. The effect of invasive plants on infiltration was complex and likely to depend on their physiognomy within existing forest community structure. These results provide clear evidence for managers that invasive ungulate control efforts can improve ecohydrological function of mesic and wet forest systems critical to protecting downstream and nearshore resources and maintaining groundwater recharge.

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We are grateful for the support of the Pacific Island Climate Adaptation Science Center (PICASC) that made this project possible. We are also grateful for many of the individuals that contributed their time and expertise to this project: Cody Dwight, Colleen Cole, Shalan Crysdale, Chris Mottley, Adam Williams, Lucas Behnke, Melissa Fisher, Kira Rowan, Alan Mair, Delwyn Oki, Lauren Kaiser, Karen Courtot, Nick Agorastos, and Sierra McDaniel. We thank Aurora Kagawa-Viviani, Alan Mair, Helen Sofaer and two anonymous reviewers for thoughtful, thorough manuscript reviews. Several management organizations were instrumental in this large effort, from expertise, staff, field support, field site selection, land access, to study design feedback: The Nature Conservancy, Hawaii State Commission on Water Resource Management, Hawaii Cooperative Studies Unit, Kauai Watershed Alliance, Kohala Watershed Partnership, Parker Ranch, Three Mountain Alliance, U.S. Fish & Wildlife Service Big Island National Wildlife Refuge Complex, Hawaii Department of Forestry and Wildlife, Hawaii Volcanoes National Park, Hawaii Natural Areas Reserve System, Kamehameha Schools, U.S. Department of Agriculture, Forest Service Hawaii Experimental Tropical Forest, Kokee State Park. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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Correspondence to Lucas Berio Fortini.

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Berio Fortini, L., Leopold, C.R., Perkins, K.S. et al. Landscape level effects of invasive plants and animals on water infiltration through Hawaiian tropical forests. Biol Invasions 23, 2155–2172 (2021).

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