Previous work has suggested that conifers could be more effective proxies for climatic changes than other plant groups, though taxonomic differences in isotopic C discrimination and leaf economics have not been studied in depth. Modern conifer shoot and dicot samples were collected from Hidden Lake Gardens (HLG; Michigan) and analyzed for isotopic and elemental composition; isotopic results are expressed in terms of Δleaf to allow for direct analysis of the consequences of biological processes. Significantly lower (p < 0.05) mean Δleaf values for conifer groups relative to the dicot outgroup are found, which indicates a difference in water use efficiency of the two groups. The deciduous conifer genus Larix more closely resembled the mean dicot Δleaf, which suggests that deciduous C3 plants use water less efficiently than the evergreen conifers. Of the conifer genera in the data set, Thuja had the smallest range of Δleaf, which suggests that its isotopic composition is more heavily influenced by climatic conditions than by physiology. Given the important role of hydraulic architecture in C fractionation and the small range of Δleaf, Thuja is likely the best candidate for a climate proxy for future study. Global relationships of MAP and Δleaf compiled in meta-analyses of C3 plants were not predictive of the mean Δleaf in HLG conifer and dicot samples, suggesting that climate inferences will require taxon-specific calibrations.
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