Pinyon Pine Tree-Ring d13C Across the U.S. Southwest

(continuing research from 1982 to present by

Steven W. Leavitt)


Pinyon-Juniper “(PJ”) woodland occupies vast areas around the semi-arid western states.  Tree rings were sampled from pinyon (Pinus edulis, Pinus monophylla) at 14 sites (§ symbol in map below) in 6 western states (photos of representative PJ sites below map) to develop d13C chronologies to investigate atmospheric chemistry, environmental influences and water-use efficiency.  Four trees were sampled at each site and four cores were taken from each tree.  Pentads from all 16 cores at each site were then pooled to provide samples for each pentad, then d13C was analyzed on cellulose.


This tree-ring isotope project was particularly noteworthy compared to other contemporary studies because of the exact dendro-dating, and its accounting for intra-tree and inter-tree variability through extended sampling (Leavitt and Long, 1984) and for climate effects using ring widths.  The pinyon core collections and isotope analysis were done in the 1980s, and were used to infer atmospheric chemistry (Leavitt and Long, 1988, 1989b), spatial and temporal drought conditions (Leavitt and Long, 1989a), and most recently the results were used by X. Feng and colleagues at Dartmouth to infer intrinsic water-use efficiency (Feng, 1998, 1999).

            The chronologies virtually all exhibit a downward d13C trend post-1800 (raw d13C chronologies plotted below [top]; mean chronology after correction for climate effects on Ci/Ca [lower left]; comparison of raw d13C results from pinyon and juniper in Arizona [lower right]), consistent with the rising inputs of fossil-fuel 13C-depleted CO2 to the atmosphere from the beginning of the Industrial Revolution (link to data).  The record from Arizona pinyon trees also matched that for Arizona juniper trees previously derived in another study (Leavitt and Long, 1983). A plot of intrinsic water-use efficiency of 7 representative pinyon chronologies versus atmospheric CO2 concentration (below) shows increasing A/g over the full concentration range since pre-Industrial times.



The influence of reduced stomatal conductance during drought acting to increase 13C/12C ratios (higher d13C) was used to identify drought pentads in the records (lower 13C/12C ratios/higher d13C were used to infer moister pentads):  the greater the departure of d13C from the long-term trend, the stronger the inferred drought or moisture excess.  This allowed not only development of moisture records at each site, but the ensemble permitted spatial characterization of moisture conditions.  For example, one of the strongest historical droughts appears in the 1950-54 pentad, and the d13C record reveals a pattern of pronounced reduced moisture centered in New Mexico and southern Colorado (figure below).  This pattern is confirmed in the instrumental records from which Palmer Drought Indices are calculated for climate subdivisions in these southwestern states. Maps of southwestern isotope drought are available at NOAA-NCDC.




            Beginning in 1999, an effort was initiated to resample the complete network of pinyon sites and bring the records up to date with single-year analysis after 1985.  The last of the 14 sites was collected in August 2003.  Efforts are now ongoing to date and analyze these rings, and interpret high-resolution d13C variation with respect to climate and intrinsic water-use efficiency.




Feng, X. 1998.  Long-term ci/ca response of trees in western North America to atmospheric CO2 concentration derived from carbon isotope chronologies.  Oecologia 117: 19-25.

Feng, X. 1999. Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric CO2 concentration. Geochimica et Cosmochimica Acta 63: 1891-1903.

Leavitt, S.W., Chase, T.N., Rajagopalan, B., Lee, E., Lawrence, P.J., Woodhouse, C.A., 2007. Southwestern U.S. drought maps from pinyon tree-ring carbon isotopes. Eos Tran. Am. Geophys. Union 88(4): 39-40 (Jan. 23, 2006).

Leavitt, S.W., Chase, T.N., Rajagopalan, B., Lee, E., Lawrence, P.J., Woodhouse, C.A., 2007. Tree-ring carbon isotope data and drought maps for the U.S. Southwest.  NOAA National Climate Data Center.

Leavitt, S.W. and Long, A., 1983.  An atmospheric 13C/12C reconstruction generated through removal of climate effects from tree-ring 13C/12C measurements.  Tellus 35B:92-102.

Leavitt, S.W. and Long, A., 1984.  Sampling strategy for stable carbon isotope analysis of tree rings in pine.  Nature 301:145-147.

Leavitt, S.W. and Long, A., 1988.  Stable carbon isotope chronologies from trees in the southwestern United States.  Global Biogeochemical Cycles 2:189-198. (Link to data)

Leavitt, S.W. and Long, A., 1989a.  Drought indicated in carbon-13/carbon-12 ratios of southwestern tree rings.  Water Resources Bulletin 25:341-347. (PDF)

Leavitt, S.W. and Long, A., 1989b.  The atmospheric d13C record as derived from 56 pinyon trees at 14 sites in the southwestern U.S.  Radiocarbon 31:469-474.