Considering the temporal responses of carbon isotope discrimination (13 C) to local water availability in the spatial analysis of 13 C is essential for evaluating the contribution of environmental and genetic facets of plant 13 C. Using tree-ring 13 C from years with contrasting water availability at 76 locations across the natural range of loblolly pine, we decomposed site-level 13 C signals to maximum 13 C in well-watered conditions (13 C max) and isotopic drought sensitivity (m) as a change in 13 C per unit change of Palmer's Drought Severity Index (PDSI). Site water status, especially the tree lifetime average PDSI, was the primary factor affecting 13 C max. The strong spatial correlation exhibited by m was related to both genetic and environmental factors. The long-term average water availability during the period relevant to trees as indicated by lifetime average PDSI correlated with 13 C max , suggesting acclimation in tree gas-exchange traits, independent of incident water availability. The positive correlation between lifetime average PDSI and m indicated that loblolly pines were more sensitive to drought at mesic than xeric sites. The m was found to relate to a plant's stomatal control and may be employed as a genetic indicator of efficient water use strategies. Partitioning 13 C to 13 C max and m provided a new angle for understanding sources of variation in plant 13 C, with several fundamental and applied implications.< Leer menos