Latitudinal gradients in tree ring stable carbon and oxygen isotopes reveal differential climate influences of the North American Monsoon System

Paul Szejner, William E. Wright, Flurin Babst, Soumaya Belmecheri, Valerie Trouet, Steven W. Leavitt, James R. Ehleringer, Russell K. Monson

Research output: Contribution to journalArticlepeer-review

42 Scopus citations


The arrival of the North American Monsoon System (NAMS) terminates a presummer hyperarid period in the southwestern United States (U.S.), providing summer moisture that is favorable for forest growth. Montane forests in this region rely on winter snowpack to drive much of their growth; the extent to which they use NAMS moisture is uncertain. We addressed this by studying stable carbon and oxygen isotopes in earlywood and latewood from 11 sites along a latitudinal gradient extending from Arizona and New Mexico to Utah. This study provides the first regional perspective on the relative roles of winter versus summer precipitation as an ecophysiological resource. Here we present evidence that Ponderosa pine uses NAMS moisture differentially across this gradient. 13C/12C ratios suggest that photosynthetic water use efficiency during latewood formation is more sensitive to summer precipitation at the northern than at the southern sites. This is likely due to the fact that NAMS moisture provides sufficiently favorable conditions for tree photosynthesis and growth during most years in the southern sites, whereas the northern sites experience larger summer moisture variability, which in some years is limiting growth. Cellulose δ18O and δ13C values revealed that photoassimilates in the southern sites were produced under higher vapor pressure deficit conditions during spring compared to summer, demonstrating a previously underappreciated effect of seasonal differences in atmospheric humidity on tree ring isotope ratios. Our findings suggest that future changes in NAMS will potentially alter productivity and photosynthetic water use dynamics differentially along latitudinal gradients in southwestern U.S. montane forests.

Original languageEnglish (US)
Pages (from-to)1978-1991
Number of pages14
JournalJournal of Geophysical Research: Biogeosciences
Issue number7
StatePublished - Jul 1 2016


  • cellulose
  • seasonality
  • stable isotopes
  • stomatal conductance
  • vapor pressure deficit
  • water use efficiency

ASJC Scopus subject areas

  • Soil Science
  • Forestry
  • Water Science and Technology
  • Palaeontology
  • Atmospheric Science
  • Aquatic Science
  • Ecology


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