1. a Division of Forest Protection , Norwegian Forest Research Institute , P.O. Box 61, Ås‐NLH, 1432, Norway
2. b Division of Forest Yield Studies , Norwegian Forest Research Institute‐Bergen , Fana, 5047, Norway
3. c Department of Forest Economy , Agricultural University of Norway , P.O. Box 44, Ås‐NLH, 1432, Norway