Spatial variability in tree-ring carbon isotope discrimination in response to local drought across the entire loblolly pine natural range

Considering the temporal responses of carbon isotope discrimination (Delta C-13) to local water availability in the spatial analysis of Delta C-13 is essential for evaluating the contribution of environmental and genetic facets of plant Delta C-13. Using tree-ring Delta C-13 from years with contrasting water availability at 76 locations across the natural range of loblolly pine, we decomposed site-level Delta C-13 signals to maximum Delta C-13 in well-watered conditions (Delta C-13(max)) and isotopic drought sensitivity (m) as a change in Delta C-13 per unit change of Palmer's Drought Severity Index (PDSI). Site water status, especially the tree lifetime average PDSI, was the primary factor affecting Delta C-13(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 Delta C-13(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 Delta C-13 to Delta C-13(max) and m provided a new angle for understanding sources of variation in plant Delta C-13, with several fundamental and applied implications.