Appendix H. Discussion of spatial patterns of tree establishment associated with the less important independent variables.
Explaining influences of local site conditions
Elevated tree establishment rates on moderate slopes are likely facilitated by less competition from herbaceous tundra cover (common on gentle slopes) (Wardle 1985, Holtmeier 2003) and less soil degradation (common on steep slopes) (Holtmeier 2003). The insulating effect of snow (seedling protection from wind exposure) and its ability to increase soil moisture and temperature likely enhance tree establishment rates in areas with moderately high snow potential (Viereck 1970, Geddes et al 2005). Yet, too much snow decreases the length of the growing season and photosynthesis, thereby inhibiting tree establishment (Geddes et al. 2005, Danby and Hik 2007b). As anticipated, we observed increased tree establishment rates associated with a high moisture potential because the study site is on the comparatively dry north slope of the Alaska Range (Calef et al. 2005). In terms of sun exposure, moderately exposed slopes probably favor tree establishment in the summer because of the expanded growing season length (Viereck 1979, Danby and Hik 2007b), although too much exposure may introduce stressors (e.g., drought stress and large diurnal to nocturnal temperature ranges) suppressing tree establishment (Wilmking et al. 2004).
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