Carbon isotope ecology of gymnosperms and implications for paleoclimatic and paleoecological studies

Abstract Plants are direct recorders of the environmental conditions in which they were growing through their carbon assimilation. Both δ13Cleaf (measured C isotopic composition of the leaf) and Δleaf (isotopic discrimination from δ13Catm by the leaf) values have been proposed to reflect a variety of environmental conditions including MAT, MAP, latitude, elevation, [CO2], and δ13Catm. Herein, we examine three sets of data from gymnosperms with long fossil records as well as selected co-occurring angiosperms to compare responses to potential environmental controls. The first dataset comprises species growing across a wide array of climatic and environmental conditions to examine MAT, MAP, latitude, and elevation. Among those potential environmental factors, none exhibits a strong control on either δ13Cleaf or Δleaf at the family, genus, or species level for either the focal gymnosperms or most of the co-occurring angiosperms. This result holds whether temperature or precipitation are considered annually or seasonally. Plant meta-analyses that have suggested a strong relationship between MAP and δ13Cleaf or Δleaf can instead be interpreted as reflecting constant Δleaf for individual species over their environmental occurrence ranges where individual ecosystems are made up of various species at different points along their individual MAP tolerance. The second dataset comes from an array of species all growing under the same environmental conditions. While differences in δ13Cleaf or Δleaf are small at the family level, at the genus and species level variation is much larger. This suggests that for any paleo-proxy reconstruction, accurate plant identification is critical to selecting the appropriate Δleaf. It also may indicate that there is an evolutionary component to measured Δleaf values, with large differences potentially between early-divergent and derived lineages. The third dataset is derived from herbarium collections of species that span the period of Industrialization (1850-present). During this time period CO2 levels have increased by ~50% and δ13Catm has shifted by >2‰ due to the burning of isotopically more negative fossil fuels. All of the historical records show that plants shifted their δ13Cleaf value, tracking the δ13Catm changes, but they did not change their Δleaf values. This contrasts strongly with some experimental results based on fast-growing herbaceous angiosperms and suggests either that there are fundamental differences between woody and herbaceous plants and/or that gymnosperms respond differently to increased CO2 than angiosperms. Finally, a brief case study is presented that examines how water use efficiency (represented by ci/ca) changed in response to elevated atmospheric CO2 levels in the Eocene to Miocene. Both Thuja and Metasequoia substantially increase their ci/ca ratios during the highest CO2 levels in the Eocene and decreased them in response to falling atmospheric CO2 during the Miocene. This suggests that the compiled Δleaf values described herein can be used to improve a variety of paleo-proxies for atmospheric CO2 levels and for plant responses to changing atmospheric stressors.