Forest Service scientists examined 80 years of streamflow and vegetation data in an experimental watershed that underwent forest-to-grass-to-forest conversion (i.e., old-field succession treatment). They hypothesized that changes in forest species composition and water use would largely explain long-term changes in water yield. Aboveground biomass was comparable among watersheds before the treatment and again after 45 years of forest regeneration. However, management practices in the treatment watershed altered the resulting species composition compared to the reference watershed. Evapotranspiration and water yield in the treatment watershed recovered to pretreatment levels after nine years of abandonment, then water yield became less (averaging 5.4 percent less) than expected. In addition, evapotranspiration increased more (averaging 4.5 percent more) than expected. The changes became apparent after the tenth year and up to the present day. The scientsits demonstrate that the decline in water yield and corresponding increase in evapotranspiration could be explained by the shift in major forest species. Before treatment, the site was dominated by oak (Quercus) and hickory (Carya). After treatment and old-field succession, poplar (Liriodendron) and maple (Acer) dominated. Furthermore, the annual change in water yield can be attributed to changes in seasonal water yield. The greatest treatment effect on monthly water yield occurred during the wettest growing-season months, when water yield was significantly lower than expected. In the dormant season, monthly water yield was higher than expected during the wettest months. Even small shifts in species composition can affect water yield, and the effects may be especially noticeable in dry years. Droughts are expected to become more frequent and severe, so understanding how forests, tree species, and climate change interact to affect water resources will become increasingly important.