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Carbon isotope values of leaves (delta C-13(leaf)) from meta-analyses and growth chamber studies of C-3 plants have been used to propose generalized relationships between delta C-13(leaf) and climate variables such as mean annual precipitation (MAP), atmospheric concentration of carbon dioxide ([CO2]), and other climate variables. These generalized relationships are frequently applied to the fossil record to create paleoclimate reconstructions. Although plant evolution influences biochemistry and response to environmental stress, few studies have assessed species-specific carbon assimilation as it relates to climate outside of a laboratory. We measured delta C-13(leaf) values and C:N ratios of a wide-ranging evergreen conifer with a long fossil record, Thuja occidentalis (Cupressaceae) collected 1804-2017, in order to maximize potential paleo-applications of our focal species. This high-resolution record represents a natural experiment from pre-Industrial to Industrial times, which spans a range of geologically meaningful [CO2] and delta C-13(atm) values. Delta(leaf) values (carbon isotope discrimination between delta C-13(atm) and delta C-13(leaf)) remain constant across climate conditions, indicating limited response to environmental stress. Only delta C-13(leaf) and delta C-13(atm) values showed a strong relationship (linear), thus, delta C-13(leaf) is an excellent record of carbon isotopic changes in the atmosphere during Industrialization. In contrast with previous free-air concentration enrichment experiments, no relationship was found between C:N ratios and increasing [CO2]. Simultaneously static C:N ratios and Delta(leaf) in light of increasing CO2 highlights plants' inability to match rapid climate change with increased carbon assimilation as previously expected; Delta(leaf) values are not reliable tools to reconstruct MAP and [CO2], and delta C-13(leaf) values only decrease with [CO2] in line with atmospheric carbon isotope changes.