Carbon and oxygen isotope analysis of leaf biomass reveals contrasting photosynthetic responses to elevated CO₂ near geologic vents in Yellowstone National Park

Abstract. In this study we explore the use of natural CO₂ emissions in Yellowstone National Park (YNP) in Wyoming, USA to study responses of natural vegetation to elevated CO₂ levels. Radiocarbon (¹⁴C) analysis of leaf biomass from a conifer (Pinus contortus; lodgepole pine) and an invasive, non-native herb (Linaria dalmatica; Dalmation toadflax) was used to trace the inputs of vent CO₂ and quantify assimilation-weighted CO₂ concentrations experienced by individual plants near vents and in comparable locations with no geologic CO₂ exposure. The carbon and oxygen isotopic composition and nitrogen percent of leaf biomass from the same plants was used to investigate photosynthetic responses of these plants to naturally elevated atmospheric CO₂ concentrations. The coupled shifts in carbon and oxygen isotope values suggest that dalmation toadflax responded to elevated CO₂ exposure by increasing stomatal conductance with no change in photosynthetic capacity and lodgepole pine apparently responded by decreasing stomatal conductance and photosynthetic capacity. Lodgepole pine saplings exposed to elevated levels of CO₂ likewise had reduced leaf nitrogen concentrations compared to plants with no enhanced CO₂ exposure, further suggesting widespread and dominant conifer down-regulated photosynthetic capacity under elevated CO₂ levels near geologic vents.