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Carbon isotope values of leaves (δC) from meta-analyses and growth chamber studies of C plants have been used to propose generalized relationships between δC and climate variables such as mean annual precipitation (MAP), atmospheric concentration of carbon dioxide ([CO]), 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 δC values and C:N ratios of a wide-ranging evergreen conifer with a long fossil record, (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 [CO] and δC values. Δ values (carbon isotope discrimination between δC and δC) remain constant across climate conditions, indicating limited response to environmental stress. Only δC and δC values showed a strong relationship (linear), thus, δC 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 [CO]. Simultaneously static C:N ratios and Δ in light of increasing CO highlights plants' inability to match rapid climate change with increased carbon assimilation as previously expected; Δ values are not reliable tools to reconstruct MAP and [CO], and δC values only decrease with [CO] in line with atmospheric carbon isotope changes.