Paper No. 9
Presentation Time: 10:35 AM


SCHERER, Jacquelyn, Geology, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, SUAREZ, Marina B., Dept. of Geological Sciences, U. of Texas, San Antonio, 1 UTSA Circle, San Antonio, TX 78249, BUSH, Janis, Environmental Science, U. of Texas, San Antonio, 1 UTSA Circle, San Antonio, TX 78249 and UPCHURCH Jr, Garland R., Biology, Texas State University, 601 University Drive, San Marcos, TX 78666,

Stable carbon isotopes of C3 leaves have been shown to correlate with precipitation, although a universal model continues to be elusive. In this study, we test the model published by Kohn (2011) with modern Texas floras across a gradient in mean annual precipitation ranging from 250 mm to 1500 mm. Leaf litter, bulk leaf matter, and wood were collected from thirteen relatively undisturbed sites and analyzed for δ13C, taking care to exclude grasses and C4 dicots. Values averaged across species and sample type ranges from an average of -26.34‰ (VPBD) in the Davis Mountains to an average of -30.06‰ (VPBD) from near Beaumont, Texas. Regression of latitude, altitude, and log10 of mean annual precipitation + 300 mm against δ13C averages produces an R2 of 0.92 (p=0.0007). Precipitation predicted from the Kohn equation ranged from 272 mm to 2720 mm. However, compared to actual precipitation, the Kohn model produced error generally increasing from west (3% under-prediction) to east (75% over-prediction. Stepwise regression (R2 = 0.53, p = 0.005) of individual sample δ13C indicates that elevation differences in this sample set are not significant and produces the equation δ13C = -33.4773 –8.199log10(MAP +300) + 1.0031 „ Latitude, °„ . Data averaged for each site closely agrees with the Texas model, showing a small improvement over the Kohn (2011) model, and agrees better with actual mean annual precipitation. Based on these results, the Kohn (2011) model (and other similar equations) overestimates MAP in areas where MAP exceeds 500 mm/year. Use of d13Cplant material as a proxy for MAP is best utilized in arid to sub-humid settings.