Abstract
Carbon isotope values of leaves (δ13Cleaf) from meta-analyses and growth chamber studies of C3 plants have been used to propose generalized relationships between δ13Cleaf 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 δ13Cleaf 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 δ13Catm values. Δleaf values (carbon isotope discrimination between δ13Catm and δ13Cleaf) remain constant across climate conditions, indicating limited response to environmental stress. Only δ13Cleaf and δ13Catm values showed a strong relationship (linear), thus, δ13Cleaf 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 Δleaf in light of increasing CO2 highlights plants’ inability to match rapid climate change with increased carbon assimilation as previously expected; Δleaf values are not reliable tools to reconstruct MAP and [CO2], and δ13Cleaf values only decrease with [CO2] in line with atmospheric carbon isotope changes.
Highlights
The concentration ([CO2]) and isotopic value (d13Catm) of atmospheric CO2 are changing at a pace unprecedented in geologic time (Keeling et al, 2005; Zhang et al, 2013).How to cite this article Stein RA, Sheldon ND, Smith S. 2019
Temperature, latitude and seasonality There were a number of climate variables that we did not expect to have a correlative relationship with Dleaf, but we addressed to ensure that they were not confounding variables (MAT, maximum summer temperature, latitude, seasonality)
Though d13Cleaf and Dleaf values have been proposed as a proxy for [CO2] and mean annual precipitation (MAP) based on previous research, this natural-world, species-controlled study shows no indication of such relationships
Summary
The concentration ([CO2]) and isotopic value (d13Catm) of atmospheric CO2 are changing at a pace unprecedented in geologic time (Keeling et al, 2005; Zhang et al, 2013).How to cite this article Stein RA, Sheldon ND, Smith S. 2019. The concentration ([CO2]) and isotopic value (d13Catm) of atmospheric CO2 are changing at a pace unprecedented in geologic time (Keeling et al, 2005; Zhang et al, 2013). The rapid decline in the carbon isotopic composition of CO2 (d13Catm) due to fossil fuel combustion, deforestation, and other human inputs, is known as the Suess Effect, and is a chemical representation of anthropogenic changes to the atmosphere—and more broadly, the environment. D13Catm values provide a useful way to see changes in CO2 sources, sinks, and fluxes in the modern environment (Keeling, 1979; Boutton, 1991; Deines, 1992). D13Catm values are useful because they are parameters in models that reconstruct past changes to atmospheric [CO2] using paleosol carbonates (Cerling et al, 1991, 1992) or atmospheric [CO2] using plant stomatal parameters (Franks et al, 2014). The biosphere provides an excellent system that directly interacts with the atmosphere and fills the gap to provide high-resolution recent and long-term records, potentially extending into geologic time
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