Carbon sequestration has focused renewed interest in understanding how forest management affects forest carbon gain over timescales of decades, and yet details of the physiological mechanisms over decades are often lacking for understanding long-term growth responses to management. Here, we examined tree-ring growth patterns and stable isotopes of cellulose (δ¹³C cell and δ¹⁸O cell ) in a thinning and fertilization controlled experiment where growth increased substantially in response to treatments to elucidate physiological data and to test the dual isotope approach for uses in other locations. δ¹³C cell and δ¹⁸O cell results indicated that fertilization caused an increase in intrinsic water-use efficiency through increases in photosynthesis (A) for the first 3 yr. The combination treatment caused a much larger increase in A and water-use efficiency. Only the thinning treatments showed consistent significant increases in δ¹⁸O cell above controls. Changes in canopy microclimate are the likely drivers for δ¹⁸O cell increases with decreases in relative humidity and increases in leaf temperature associated with thinning being the most probable causes. Tree-ring isotopic records, particularly δ¹³C cell , remain a viable way to reconstruct long-term physiological mechanisms affecting tree carbon gain in response to management and climate fluctuations.