Improved Sample Selection Techniques to Generate a Modern High Resolution Carbon Isotopic Drought Record from the Missouri American Long Oak Chronology

Carbon isotopic measurements from latewood of recent and subfossil white oaks from northern Missouri appear to reflect growing season drought.  Given drought’s importance in agricultural productivity and the central position of food costs in current discussions of anthropogenic climate change, knowledge of Midwest climate history is critical.  It is also little known.  Reasons include that there are few good climate archives and that data collection can be inefficient.  The Missouri Tree Ring Laboratory Long Oak Chronology project helps address both shortcomings.  Its collections include a continuous chronology with more than 10 samples/year dated back 1,100 years and floating, century-long chronologies dated back to approximately 14,000 yr BP.  Most importantly, the samples within the database have had their ring widths compared to the pattern of pooled, normalized ring widths (i.e., to the dendrochronology) with the similarity expressed as a correlation coefficient (R).  R has a high value in trees whose ring widths most closely follow average trends and presumably have growth patterns which were more strongly influenced by regional than local conditions.  By selecting “high R” trees for isotopic analyses, efficiency in data collection can be greatly improved.  This idea was tested by measuring d¹³C in samples with R ≥ 0.6 and those with R ≤ 0.4 for the last century.  We focused on d¹³C as it should vary with water stress.  Results demonstrate the expected positive carbon isotopic excursions (up to 3‰) for known droughts in trees with high R values, and the d¹³C record of these trees correlated well with PDSI.  Records from trees with low R values, on the other hand, correlate poorly with any climate record.  Our focus is now on extending the isotopic record beyond the historical period, with the anticipation of examining potential drought mechanisms, cycles and global climate connections.