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PRECIPITATION SEASONALITY RECORDED IN D/H RATIOS OF
PINYON PINE
CELLULOSE IN THE SOUTHWESTERN UNITED STATES
Department of Geosciences
The University of Arizona
1997
ABSTRACT
I
assessed the paleoclimatic significance of dD values
of piņon 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 precipitaion,
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 dD and d18O values
of precipitation inputs to soil. Only after heavy monsoons did soil water
and sap isotopically resemble recent precipitation. Average precipitation dD values
set the baseline for dDcn values
at each site, but interannual variations in
relative humidity and precipitation amount altered wood and leaf dDcn values, via leaf water effects. Leaf
water (lw) was evaporatively
enriched by seasonal moisture stress. dDlw and d18Olw
values were strongly correlated with relative humidity on a seasonal basis,
but not on a diurnal basis. Measured d18Olw
values fit a steady-state model, with an offset attributable to relative
humidity. Measured dDlw values
were more depleted than predicted by the model, suggesting leaf
water-organic matter isotopic exchange. Biochemical fractionation (eB) of
hydrogen isotopes between leaf water and cellulose was inversely correlated
with relative humidity. Empirical models based on linear regressions
demonstrated significant correlations between dDcn values and precipitation seasonality.
An El Niņo-Southern Oscillation signal (wood dDcn values inversely related to winter precipitation
amount) was found in New Mexico and Arizona. A summer rain signal (leaf dDcn values inversely related to summer
humidity) was found at all sites. dDcn values of piņon 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.
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