Rapid response to anthropogenic climate change by Thuja occidentalis: implications for past climate reconstructions...

Carbon isotope values of leaves (δ¹³C_leaf) from meta-analyses and growth chamber studies of C₃ plants have been used to propose generalized relationships between δ¹³C_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 δ¹³C_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 δ¹³C_atm values. Δ_leaf values (carbon isotope discrimination between δ¹³C_atm and δ¹³C_leaf) remain constant across climate conditions, indicating limited response to environmental stress. Only δ¹³C_leaf and δ¹³C_atm values showed a strong relationship (linear), thus, δ¹³C_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 Δ_leaf in light of increasing CO₂ 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 [CO₂], and δ¹³C_leaf values only decrease with [CO₂] in line with atmospheric carbon isotope changes.