Physiological Basis of Isotopic Variation in Giant Saguaro: Are Responses to Climate Recorded in Spines?

EPA Grant Number: F5F11611
Title: Physiological Basis of Isotopic Variation in Giant Saguaro: Are Responses to Climate Recorded in Spines?
Investigators: English, Nathan B.
Institution: University of Arizona
EPA Project Officer: Lee, Sonja
Project Period: August 1, 2005 through August 1, 2007
Project Amount: $111,344
RFA: STAR Graduate Fellowships (2005) RFA Text |  Recipients Lists
Research Category: Academic Fellowships


This research will develop a potential ecologic proxy for precipitation using analyses of light isotopes in the spines of columnar cacti. Of particular interest is the impact of ENSO on summer precipitation in the southwestern United States and the Bolivian Altiplano.


Saguaro and other columnar cacti produce spines at the apex of the cactus. Typically 4 to 8 long spines are produced in sequence each year, so that the oldest spines are at the base of the plant and the youngest at the apex. These spines grow and reach their full length in less than one year. Thus the isotopic analysis of spines in order yields a high-resolution (sub-annual) stable isotope record, analogous to tree-ring studies. Two major lines of research will be pursued,

  1. the calibration of the stable isotope ( δ18O, δ13C, δD) ratios in cacti spines to major environmental controls,
  2. and the construction of five century-long high-resolution records of δ18O in cactus spines from climatically sensitive locations.
  1. Calibration and modeling

    Twelve potted saguaro have purchased for stable isotope calibration and growth studies. The cacti will be used to study the growth of spines in saguaro, the water uptake and transport within the cactus and the isotope systematics of plant water and spines.

    Using the experimental and natural cacti, needle growth will be marked and photographed for more than one year to document the onset of needle production, its duration, their oxygen and carbon isotopic ratios, measure micrometeorological variables and the impact water availability has on the onset (or termination) of needle growth. Changes in the δ18O and δD of water in a saguaro will be studied by regular collection of stem tissue samples. These data will help develop a calibrated model of spine stable isotope ratios.

  2. Historical records

    Four saguaros from the Tucson, AZ area will be sampled for which there is excellent climate and rainfall records for comparison. Three composite records of δ18O will be constructed in spine series using a transect from the Phoenix area through Tucson and farther south into Sonora, Mexico. A previously collected spine series from the Bolivian Altiplano will also be analyzed.

Expected Results:

An understanding of the environmental controls on spine stable isotope ratios is sought. The length of time it takes to grow a spine will be determined, the impact of water availability on spine growth, and if secondary spine growth takes place. A calibrated model will allow reconstruction of environmental variables based on spine stable isotope ratios.

Patterns of drought and the climate response to ENSO from Arizona, northern Mexico and the Bolivian Altiplano will be documented in this data set. A comparison of recent data on ENSO events to our records will allow the extension of the ENSO record in these areas well into the past century.

Supplemental Keywords:

RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Monitoring/Modeling, Environmental Monitoring, Ecology and Ecosystems, Environmental History, cactus, Saguaro, climate, precipitation modeling

Progress and Final Reports:

  • 2006
  • Final