Historically, the survival and growth of trees at northern or altitudinal treelines

Treelines are areas where the potential of upright woody vegetation to occur ends and only the survival of prostrate vegetation is possible. Northern and alpine treeline areas are transition zones between boreal forest and alpine or arctic tundra. While sometimes conceived of as a broad transition zone, treelines in our study areas are relatively sharply defined borders between forested sites and tundra. In this paper we apply the term treeline to an imaginary line connecting the highest patches of forests on a given slope (KOERNER 1998). Historically, the survival and growth of trees at northern or altitudinal treelines was seen as limited by only one environmental factor. The lack of available warmth has generally been interpreted as the limiting factor for tree growth in cold regions. This apparent simplicity of only one controlling factor made treeline environments the focus of sustained research interest (GRIGGS 1934; KREBS a. BARRY 1970; ELLIOT-FISK 1983; GRUDD et al. 2002). Especially in the last decade treeline ecotones have been investigated as potential “early warning stations” for Climate Change (LESCOP-SINCLAIR a. PAYETTE 1995; HOGG a. SCHWARZ 1997; KULLMAN 1996, 2000, 2001, 2002). Warming in the high latitudes is of a greater magnitude than elsewhere (OVERPECK et al. 1997; SERREZE et al. 2000) and during the last century Alaska experienced one of the strongest warming trends on the globe (CHAPMAN a. WALSH 1993). M. Wilmking et al.: Modeling spatial variability of white spruce (Picea glauca) in Alaska 113