Cactus needles hold tales of past climates
Info from spines compared with weather records
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The sharp spines found on a saguaro cactus are a painful way the stately sentinel of the desert protects itself from threats from animals and man.
But those needle-like spines have been tapped as a source of climate variation information by University of Arizona researchers.
By studying isotopes of carbon and oxygen found in the spines, researchers can determine if the saguaro was full or dry, the relative evaporation and how the plant ate - or converted light energy into chemical energy - at different times during its life, said Nathan B. English, a graduate student in UA's department of geosciences.
The percentage of "heavy" oxygen isotopes - atoms with extra neutrons - can tell how much water the plant had to drink and how much evaporated, he said.
The carbon isotope content of the spines tell what type of photosynthesis was used to covert sunlight, carbon dioxide and water into glucose and plant tissue.
Saguaro growth emerges from the top, and the oldest spines are found at ground level, acting like vertical tree rings.
By analyzing spines from various areas on the cactus, researchers can determine when and what type of climate changes took place, he said.
Researchers collected data from Saguaro 162, a cactus found on Tumamoc Hill west of downtown whose height has been tracked since 1964, as well as from a dozen potted cactuses.
Comparing Tucson climate records with information derived from the spines allowed the research team to verify its results, English said.
English is lead author of a paper that will appear in the November issue of Oecologia, an ecology journal.
Co-authors are David Dettman, research scientist with UA's department of geosciences and head of the school's environmental isotope lab, former UA professor David G. Williams and Darren R. Sandquist.
While Tucson has daily weather records dating back to 1894, many desert areas lack such precise - or even any - monitoring and tree ring data to check the climate history, he said.
Using the methods developed with saguaro spines, a weather history can be determined for any area where columnar cactuses or plants with woody, durable tissue that grows seasonally are found, he said.
Researchers will be able to see how plants reacted to past climate changes, and may be able to forecast how they will react to future droughts and temperature changes, he said.
Transplanted saguaros have a low survival rate, and information derived from their spines can help determine if they are good candidates for moving, Dettman said.
The isotope levels can act like an electrocardiogram to determine a plant's health before outward symptoms appear, Dettman said.
With an additional $150,000 grant from the National Science Foundation, the team will study cactuses in the Tucson, Phoenix and Quartzsite areas, English said.
The results were: 1) this method works for identifying past climate anomolies, like very wet El Nino Winters; 2) and it provides a possible new tool for monitoring saguaro health and photosynthesic pathway. We also describe a new technique for rapidly determining the growth rate of columnar cacti species, very useful if you're trying to develop plans for conserving an endangered or threatened cactus. These methods may also have use in cactus agriculture (think Prickly Pear, a major crop).
You don't have to pay for the answers, you already did. If you'd like a copy of the article, find Nathan English's email at the UofA geosciences website (I'm reluctant to post it here) and he will email a pdf of the paper to you ASAP. Your gov't dollars at work. Cheers!