Stable isotope variations from cultivated Metasequoia trees in the United States: A statistical approach to assess isotope signatures as climate signals

We measured bulk δC and δN values and carbon and nitrogen elemental concentrations of leaves collected from Metasequoia glyptostroboides Hu et Cheng trees cultivated at 39 sites across the United States under different latitudes and climatic regions. δD values from south-facing leaf n-alkanes of 27 trees were also determined. Climate data over the past 50 years (1950–2009) were compiled from stations near each site. Isotope data were cross plotted against each geographic and climatic parameter, including latitude, annual mean temperature (AMT), spring (February–May) mean temperature (SMT), annual mean precipitation (AMP), and spring mean precipitation (SMP). Statistical analyses revealed the following significant correlations: 1) a strong negative correlation between n-alkane δD and latitude; 2) statistically significant correlations between δD and both AMT and SMT; 3) a weaker but still significant correlation between δD and SMP; 4) statistically significant relationships between carbon concentration and both temperature and precipitation parameters, especially AMP; 5) an unexpected correlation between nitrogen concentration and SMP. These results bear strong implications for using δC and δD values obtained from fossil Metasequoia as paleoclimatic and paleoenvironmental proxies.