Acknowledgements
This research has been supported by grants of Academy of Finland (project #259072), Maj and Tor Nessling Foundation, and University of Oulu Arctic Interactions funding. We thank Matt Ayres for useful comments on the manuscript. We also thank Helena Hakkarainen for assistance in the growth measurements in 2004, and the Metsähallitus and Kilpisjärvi Biological Station for providing logistic support during field work.