Branch whorls of juvenile Araucaria araucana (Molina) Koch: are they formed annually?
The growth of Araucaria araucana, like that of many other conifers, is characterised by the production of a monopodial stem with branch whorls. However, the periodicity of whorl formation in this species has not been subjected to systematic study. We used growth ring counts to determine the frequency of whorl formation in sun- and shade-grown juvenile A. araucana in a montane forest in the Chilean coast range. Whorls were not annual in any of the 35 individuals studied, growth rings outnumbering whorls by a factor of 1.8 to 3.9. The mean interval between successive whorls was significantly shorter in sun-grown (2.1 yrs whorl -1) than in shade-grown trees (2.6 yrs whorl-1), suggesting a relationship with plant carbon balance. Mean distance between whorls was also sensitive to light environment. The overall result of this effect of light environment on both whorl spacing and production rate was that mean height growth rates were > 50% higher in sun-grown trees (169 mm yr-1) than in shade-grown individuals (111 mm yr-1). Despite the non-annual nature of whorls in the study population, whorl counts may have some potential as a non-invasive method of age estimation in juvenile A. araucana. In both shade- and sun-grown subpopulations, whorl number was a better predictor of stem age than were stem diameter or height