Glasshouse trials were conducted for nine provenances of monkey puzzle (Araucaria araucarza), a vulnerable tree native to southern South America that occurs over a wide ecological range (annual precipitation ranges from the western to the eastern side of the Andean Range from more than 4000 to less than 200 mm, and elevation ranges from 600 to almost 2000 m above sea level). Samples of needle tissues were used to determine stable carbon isotope 6I3C values, a time-averaged indicator of water use efficiency. A significant relationship was found between 6I3C and mean annual rainfall with higher (less negative) 6I3C ratios found for populations within the drier Argentinian region. Root mass ratio was also found to be significantly correlated with mean annual rainfall, providing further evidence for genetic adaptation to drought.

This study demonstrates the utility of carbon isotope discrimination in describing genetic adaptation to arid environments, although it is probably most useful in detecting differentiation when the strategy of the species under investigation is to increase water use efficiency, rather than drought-avoidance. The results suggest that populations on the eastern and western sides of the Andes should be treated as separate management units for the purposes of conserving the genetic resource of this species.

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