STABLE ISOTOPE ECOLOGY AND PALEOCLIMATE IMPLICATIONS OF MODERN THUJA PLICATA IN THE PACIFIC NORTHWEST

The global concentration of atmospheric carbon dioxide (pCO₂ in ppm) and localized precipitation regimes have both changed since the start of the Industrial Revolution as a result of human influence. In the Pacific Northwest, precipitation has been substantially increasing over the past 100 years, and is projected to continue to do so, but precipitation in the form of snow specifically has been decreasing. How these changes have influenced ecosystems or individual organisms is not well understood. Here we use a combination of elemental analysis and stable isotope geochemistry to track those impacts on the Pacific red cedar (Thuja plicata), which also has a rich fossil record dating back into the Late Cretaceous. Seventy-eight Pacific red cedar specimens collected from 1876 through 2012 were sampled from several herbaria (n = 5). The elemental (C:N) and stable carbon isotope (δ¹³C_leaf) compositions were measured for leaf samples using an Elemental Analyzer and Cavity Ring-Down Spectrometer at the University of Michigan. A second parameter (Δ_leaf) was calculated by the subtracting δ¹³C_leaf from δ¹³C_atmosphere values derived from direct and interpolated measurements. Δ_leaf reflects the deviation of a tree’s ¹³C fractionation from the moving atmospheric baseline — a result of the burning of fossil fuels (i.e., the Suess Effect). δ¹³C_leaf values ranged from -22.78 to –28.48‰ and showed more negative values with time. This demonstrates that δ¹³C_leaf values of T. plicata correlates with δ¹³C_atmosphere, making Thuja a potentially useful recorder of the paleo-atmosphere at both historic and potentially longer time scales. Δ_leaf values were nearly constant with a mean of 18.31‰ and a standard deviation of 1.14‰, indicating that the water use efficiency of T. plicata is not affected by changes in pCO₂ or precipitation over the period of Industrialization. Thuja plicata’s Eastern North American relative, Thuja occidentalis, exhibits a similar relationship (mean Δ_leaf value of 17.98‰, standard deviation of 1.11‰) in spite of growing in a significantly different climatic regime, with a. The consistency within this genus supports its potential use as a paleoclimate indicator across a broad range of (paleo-)environments.