The net effects of a warming planet are uncertain and highly dependent on local climate and vegetation. The Southern Appalachian Mountains of western North Carolina are one of the wettest biomes in North America and are home to highly productive forests. In these forests, warm temperatures in early 2012 caused leaf-out to occur two weeks earlier than in cooler years and led to higher seasonal productivity. However, these warmer temperatures also increased winter ecosystem respiration. This increase offset much of the springtime carbon gain. Years with warmer growing seasons had 10 percent higher respiration and sequestered about 40 percent less carbon than cooler years. In contrast, annual evapotranspiration was relatively consistent among years despite large differences in precipitation. The increasing frequency of high summer temperatures is expected to have a greater effect on respiration than growing season length, reducing forest carbon storage.