Precipitation seasonality recorded in D/H ratios of Pinyon Pine Cellulose in the southwestern United States

Abstract

I assessed the paleoclimatic significance of SD values of pition pine (Pinus edulis and P. monophylla) cellulose nitrate (cn) by developing, testing and applying deterministic and empirical models, in the context of the soil-plant-atmosphere continuum. Stable isotope values of precipitation, soil water, xylem sap, leaf water, atmospheric vapor, annual and sub-annual samples of tree-ring and needle cellulose, and climatic parameters, were measured along a gradient of decreasing summer rain in the southwestern U.S. Stable isotope composition of sap indicated depth of moisture extraction. Over the growing season in New Mexico and Arizona, where monsoon rains are important, trees shifted their water use to shallower depths. In Nevada, where summer rain is scarce, trees shifted to deeper moisture late in the growing season. Evaporation altered δD and δ¹⁸O values of precipitation inputs to soil. Only after heavy monsoons did soil water and sap isotopically resemble recent precipitation. Average precipitation δD values set the baseline for δD(cn). values at each site, but interannual variations in relative humidity and precipitation amount altered wood and leaf δD(cn) values, via leaf water effects. Leaf water (1w) was evaporatively enriched by seasonal moisture stress. δD(1w). and δ¹⁸O(1w) values were strongly correlated with relative humidity on a seasonal basis, but not on a diurnal basis. Measured δ¹⁸O(1w) values fit a steady-state model, with an offset attributable to relative humidity. Measured δD(1w) values were more depleted than predicted by the model, suggesting leaf water - organic matter isotopic exchange. Biochemical fractionation (ε(B)) of hydrogen isotopes between leaf water and cellulose was inversely correlated with relative humidity. Empirical models based on linear regressions demostrated significant correlations between δD(en) values and precipitation seasonality. An El Nirio-Southern Oscillation signal (wood δD(en) values inversely related to winter precipitation amount) was found in New Mexico and Arizona. A summer rain signal (leaf δD(en) values inversely related to summer humidity) was found at all sites. δD(en) values of pirion needles in packrat middens from Sevilleta LTER, New Mexico, suggest that late Pleistocene summers were as wet as today's, and/or that storm tracks could have shifted, bringing in more tropical moisture than currently.