Changes in water flow through Hawaiian forests due to invasive species and changing rainfall patterns

A large spreading tree with white trunks and branches fills the view.
Ecosystem conditions for native trees, like this Koa, are changed as more invasive plants encroach into native forests, particularly soil chemistry and water. (Photo: JJacobs/USGS)

Precipitation in Hawaiʻi’s higher elevation upland areas provides needed water to both people and ecosystems. Once it reaches the ground, rain can either run off and contribute to water flow in streams, or it can infiltrate into the ground and provide water for plants and recharge aquifers and groundwater. The exact route that water takes is controlled by many factors, including the duration and intensity of rainfall, the topography of the land, soil properties, and vegetation.

The introduction and spread of invasive plants and animals in Hawaiian forests, which alters the water-use and soil characteristics of ecosystems, can have large impacts on downstream water users. Increased demand and competition for limited water resources in Hawaiʻi has underscored the need for a better understanding of this relationship; however, relevant statewide data is currently lacking.

The objective of this project is to understand how expanding invasive plant populations and changing rainfall patterns will impact water resources in the future. Project researchers are exploring the differences in key soil characteristics that control runoff and groundwater recharge in managed and relatively intact native mesic (moderate amounts of water) and wet forests and in similar invasive-dominated forests. Primary soil characteristics of interest are the hydraulic conductivity of soils (ability of water to move through soils) and soil hydrophobicity (a property that causes water to collect on the surface rather than infiltrating into the ground). Data will also be collected on the canopy and understory composition in the forests and on disturbances by invasive animals, like feral pigs. Results from the study will be presented in reports and maps of vegetation and soil characteristics across the Hawaiian Islands under current and future conditions.





Lucas Fortini
Research Ecologist, USGS Pacific Islands Ecosystem Research Center