The long-term change in the hydroclimatic environment could be a risk factor affecting native vegetation, especially on oceanic islands. To consider the implications of climate change on the native vegetation of the Ogasawara Islands, we examined the responses of the dominant endemic species (Distylium lepidotum) to severe drought conditions, measured in terms of leaf water potentials, daily sap flow rate, and stem growth. During summer dry season, the reduction in soil water availability resulted in leaf water potentials that were significantly lower than those in other seasons and the daily rates of sap flow were also suppressed by water deficit. The stem growth in D. lepidotum occurred mostly in summer just after the rainy season, but it was adversely affected by summer drought. In particular, the severe drought conditions, when the soil water content dropped below 20%, caused pronounced stem shrinkage, and stem growth was abruptly terminated. Consequently, summer drought shortened growth period and reduced the productivity of the native plant species. The summer drought is predicted to be longer and drier on the Ogasawara Islands and, therefore, will have negative effects on the native vegetation.