Young individuals of a single clone of black cottonwood, in Iceland, were exposed for 3 years to elevated atmospheric CO2 concentrations [CO2] in whole-tree chambers at natural and high nutrient availability. No treatment effects were found at bud break or the start of shoot extension in spring. Autumn phenology was, however, affected both by elevated [CO2] and changes in nutrient status. The time of annual growth cessation was linearly related to leaf nitrogen concentration, irrespective of CO2 treatment. At low (natural) nutrient availability, elevated [CO2] accelerated growth cessation and bud set, which reduced the period of active growth. An earlier and more pronounced leaf senescence and corresponding loss of photosynthetic capacity further decreased carbon acquisition in elevated [CO2]. The negative [CO2] effect on duration of shoot extension and leaf senescence existed, but was not as pronounced, when trees grew at higher nutrient availability. Improved nutrient availability extended the shoot extension period and delayed leaf senescence. It is suggested that trees grown in elevated [CO2] altered their autumn phenology as an effect of a signal similar to that in trees growing at low nutrient availability, i.e. an imbalance between carbon and nitrogen sources. These alterations in autumn phenology may be important when predicting how trees will grow in a future CO2 environment.
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Acknowledgements The present study was part of a Nordic collaborative project ‘The Likely Impact of Rising CO2 and Temperature on Nordic Forests at Limiting and Optimal Nutrient Supply’, where the Icelandic part was co-ordinated by H. þorgeirsson at the Icelandic Agricultural Research Institute. The project was made possible by financial support from the Nordic Council of Ministers, Nordic Forest Research Co-operation Committee (SNS) and the Icelandic Research Council. Support from the European Community’s Marie Curie Research Training Grant (ENV4-CT98–5106) and the Nordic Research Academy (NorFa) is gratefully acknowledged. I would also like to thank: Á.L. Aradóttir, I.B. Strachan and T. þorsteinsson for help during the field-study; M. Freeman for deriving Eq. 2; J.G.K. Flower-Ellis, S. Linder and P. Roberntz for constructive comments on earlier versions of the manuscript. This work contributes to the Global Change and Terrestrial Ecosystems (GCTE) Core Project of the International Geosphere-Biosphere Programme (IGBP).
- Bud set
- Growth phenology
- Leaf senescence
- Shoot extension