In forested watersheds, forest changes and climatic variability have been commonly recognized as two major drivers for streamflow variations. Previous research has separated their relative contributions but mainly focused on either deforestation and climate or reforestation and climate, but rarely with single studies on both. This study used the Meijiang watershed (6983·2 km2), situated in the upper reach of the Poyang Lake basin, as an example to quantify how climate and forest changes (both deforestation and reforestation) consecutively affect streamflow dynamics. Two methods, namely modified double-mass curves and sensitivity-based approach, were used in this study. Two breakpoints (years 1968 and 1985) with significant annual streamflow changes were detected, and together with the control period, they were then used to define three distinct periods: the control (1957–1967), deforestation (1968–1984) and reforestation (1985–2006) periods. Our results show that in the deforestation period, the average annual streamflow increment attributed to deforestation was 112·78 mm year−1, while the annual streamflow variation attributed to climate variability was −111·39 mm year−1. In the reforestation period, the average annual streamflow decrease caused by reforestation was −51·04 mm year−1, while the annual streamflow variation attributed to climate variability was 52·52 mm year−1. The sensitivity-based approach also provided similar results. The positive and negative values in the streamflow changes suggest offsetting effects between forest changes and climate variability in both deforestation and reforestation periods. The similar magnitudes of streamflow changes demonstrate that the hydrological effects of forest changes can be as great as those caused by climate change.