Carbon isotopic variations in 7 southwestern United States plants from herbarium collections of the last 150 years

Abstract

Since industrialization atmospheric CO₂ concentrations have increased from 280 to 365 ppmv and δ¹³Cₐᵢᵣ has decreased from -6.5 to -8.2‰. These two trends have consequences for plant physiology. I examine δ¹³C plant and physiological parameters in herbarium specimens of Atriplex confertifolia, Atriplex canescens, Ephedra viridis, Pinus edulis, Pinus flexilis , Juniperus scopulorum, and Quercus turbinella. For all species, I found relatively high and unsystematic variability. δ¹³C values for A. confertifolia and A. canescens varied by up to 7.9 and 9.5‰ respectively; δ¹³C values of these C₄ shrubs are unsuitable for reconstructing δ¹³Cₐᵢᵣ, as previously claimed. δ¹³C(plant) generally becomes more depleted except in P. edulis. Other calculated parameters such as Δ, Cᵢ/Cₐ, Cᵢ, and A/g have varying responses even among similar functional groups. Because much of the isotopic variability caused by interplant, intertree, intersite, and interannual differences is implicit, herbarium specimens are inadequate for precise detection of direct CO₂ effects on plant physiology.