Stable carbon isotope measurements in tree rings have been explored as potential indicators of past climates and as proxy data for reconstructions of the history of ¹³C/¹²C variations of atmospheric carbon dioxide as related to the global carbon cycle. Results of these studies often conflict, perhaps partly due to the limited style of sampling natural systems with large inherent spatial variability. Stable carbon isotopic trends in tree rings among several radii of the same tree <i>(Pinus edulis</i> Engelm.) and among several trees from the same site are examined to determine the extent of this variability. Tests to determine how well various combinations of cores fit the trend of an individual tree or of the site as a whole indicate that pooling four cores from four trees accurately represent site ¹³C trends and absolute values. -- AATA