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.