Rapid response to anthropogenic climate change byThuja occidentalis: implications for past climate reconstructions and future climate predictions

Carbon isotope values of leaves (δ 13 C leaf ) from meta-analyses and growth chamber studies of C 3 plants have been used to propose generalized relationships between δ 13 C leaf and climate variables such as mean annual precipitation (MAP), atmospheric concentration of carbon dioxide ([CO 2 ]), 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 δ 13 C 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 [CO 2 ] and δ 13 C atm values. Δ leaf values (carbon isotope discrimination between δ 13 C atm and δ 13 C leaf ) remain constant across climate conditions, indicating limited response to environmental stress. Only δ 13 C leaf and δ 13 C atm values showed a strong relationship (linear), thus, δ 13 C 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 [CO 2 ]. Simultaneously static C:N ratios and Δ leaf in light of increasing CO 2 highlights plants’ inability to match rapid climate change with increased carbon assimilation as previously expected; Δ leaf values are not reliable tools to reconstruct MAP and [CO 2 ], and δ 13 C leaf values only decrease with [CO 2 ] in line with atmospheric carbon isotope changes.