Reconstruction of Seasonality of Precipitation and Paleoclimate from Hydrogen and Oxygen Isotopic Composition of Tree Rings,


(NOAA Grant #NA56GP0180 1 April 1995 to 31 March 1997)

Principal Investigators:

Steven W. Leavitt and Elise Pendall


            This project was initiated and completed over a period of about 3 years, representing two full growing seasons.  In the original proposal, two sites were to be sampled, but early on, we decided to add a third site (in Nevada) to the transect, thereby greatly increasing the range of local climate conditions that would be influencing the isotopic composition of the trees.  In part, because of the added expense for travel and analysis of additional samples from a third site, we also decided to forego oxygen isotope analysis of leaf and tree-ring cellulose.  We obtained dD and d18O analysis of all of the water samples, but felt dD alone (whose source, like d18O, is environmental water) would provide sufficient primary data from which to infer environmental effects on incorporation of isotopes into leaves and tree rings.  The success of the project was predicated on the assistance of the student personnel we hired to work in the field and in the lab, and the people who volunteered help in collecting precipitation samples and in providing space and assistance in sample processing and analysis.  As problems were encountered, we were usually able to find solutions, such as our acquisition of water samples from the NADP network to provide precipitation samples for the period prior to the dates at which we initiated precipitation collection at the sites.

    We assessed the paleoclimatic significance of dD values of piñon 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.  In this calibration study, 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. This climatic gradient allowed us to test the influence of precipitation seasonality and relative humidity on dD values in tree-ring and leaf cellulose.
    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. The shifting depth of water use has implications for the source water available for cellulose synthesis, and suggests that trees growing in monsoonal climates could record summer precipitation variations.

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. Our measurements have quantified this poorly understood component of the most widely used deterministic model for two climatically important species.

Empirical models based on linear regressions demonstrated significant correlations between dDcn values and precipitation seasonality. An El Niño-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.

   In an application of the empirical and deterministic models, we assessed paleoclimatic conditions over the past 40,000 years in central New Mexico. dDcn values of piñon needles in packrat middens from Sevilleta Long Term Ecological Research area 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.


Publications and Presentations:


Pendall, E., 1999. Precipitation seasonality recorded in D/H ratios of pinyon pine cellulose in the southwestern U.S. Ph.D. Dissertation, University of Arizona, Tucson, 263 pp.

Pendall E, 2000. Influence of precipitation seasonality on pinon pine cellulose delta D values. Global Change Biology 6: 287-301.

Pendall, E., Betancourt, J. and Leavitt, S.W., 1996.  Do D/H ratios in fossil cellulose reflect temperature variations?  American Quaternary Association, Annual Meeting, Flagstaff, AZ, 20-22 May.

Pendall, E., Betancourt, J.L. and Leavitt, S.W., 1999.  Paleoclimatic significance of dD and d13C values in piñon pine needles from packrat middens spanning the last 40,000 years. Palaeogeography, Palaeoclimatology, Palaeoecology 147: 53-72.

Pendall, E., Betancourt, J.L., Leavitt, S.W., and Toolin, L.J., 1994.  Pleistocene-Holocene moisture conditions inferred from H and C stable isotopes in plant matter.  American Geophysical Union Fall Meeting, San Francisco, December 5-9, EOS (Nov. 1, 1994, supplement): 349

Pendall, E. and Leavitt, S., 1995.  Seasonal dynamics of stable isotopes in soil water, pinyon sap, and precipitation across a climatic transect.  American Geophysical Union Fall Meeting, San Francisco, CA.

Pendall, E.G. and Leavitt, S., 1996.  Isotopic tracers of water movement through the soil-plant-atmosphere continuum.  Conference on Stable Isotopes and the Integration of Biological, Ecological and Geochemical Processes, Association of Applied Biologists, Society for Experimental Biology, and British Ecological Society, Newcastle upon Tyne, UK, 9-11 July.

Pendall, E. and Leavitt, S., 1996.  Stable isotopic tracers of water flux through pinyon pines across a climatic transect.  Ecological Society of American Annual Meeting, Providence, RI, 10-14 August.

Pendall, E. and Leavitt, S.W., 1997.  El Niño's winter precipitation recorded in cellulose deuterium/hydrogen ratios.  7th Annual V.M. Goldschmidt Conference, 2-6 June, Tucson.  LPI Contribution No. 921, Lunar and Planetary Institute, Houston, pp. 161-162.

Pendall, E. and Leavitt, S.W., 1997.  El Niño's winter precipitation recorded in cellulose D/H.  82nd Annual Meeting of the Ecological Society of America, 10-14 August. Albuquerque. Ecological Society of America Bulletin 78: 161.

Pendall, E. and Leavitt, S.W. 1997. Climate change deduced from isotopes in tree rings. Invited abstract for the 8th Annual Conference on Global Change, American Meteorological Society, Long Beach, CA.

Pendall, E., Williams, D.G. and Leavitt, S.W., 2005. Comparison of measured and modeled variations in piñon pine leaf water isotopic enrichment across a summer moisture gradient. Oecologia 145: 605-618.