The eradication of dominant, nonnative trees can alter soil conditions in forest ecosystems by changing the forest's water balance. To test this idea, we measured water in surface soil in forests dominated by an invasive nonnative tree (Casuarina equisetifolia Forst.) on Nishijima Island, a subtropical island in the Ogasawara Islands group, northwestern Pacific Ocean. The volumetric water content of surface soils at sites where all trees of C. equisetifolia were killed by herbicide was compared with adjacent invaded control sites, and effects of time since tree removal (up to 3 yr) were assessed. Dry weights of accumulated litter, total canopy openness of the forest canopy, maximum heights, and total plant cover of herbaceous vegetation were also compared between treatment and control plots. Volumetric soil water content in quadrats where C. equisetifolia trees were removed (removal area) was significantly higher than at control quadrats. We found that the effect of C. equisetifolia removal on soil water content slightly decreased 3 yr after removal. In addition, the dry weights of accumulated litter in removal quadrats decreased with time after removal, and maximum heights and total plant cover increased. We observed higher values of total canopy openness at removal sites regardless of time after removal. These results suggested that the death of these dominant, nonnative trees could increase the available water in soils of invaded forests, and that increases may be related to changes in the water balance of the ecosystem, which can in turn affect restoration of forest ecosystems.