Ecological Archives E092-043-A8

Kirk M. Stueve, Rachel E. Isaacs, Lucy E. Tyrrell, and Roseann V. Densmore. 2011. Spatial variability of biotic and abiotic tree establishment constraints across a treeline ecotone in the Alaska Range. Ecology 92:496–506.

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).

LITERATURE CITED

Calef, M. P., A. D. McGuire, H. E. Epstein, T. S. Rupp, and H. H. Shugart. 2005. Analysis of vegetation distribution in Interior Alaska and sensitivity to climate change using logistic regression. Journal of Biogeography 32:863–878.

Danby, R. K., and D. S. Hik. 2007b. Response of white spruce (Picea glauca) to experimental warming at a subarctic alpine treeline. Global Change Biology 13:437–451.

Geddes, C. A., D. G. Brown, and D. B. Fagre. 2005. Topography and vegetation as predictors of snow water equivalent across the Alpine treeline ecotone at Lee Ridge, Glacier National Park, Montana, USA. Arctic, Antarctic, and Alpine Research 37:197–205.

Holtmeier, F. K. 2003. Mountain timberlines:  Ecology, patchiness, and dynamics, Kluwer Academic Publishers, Boston, MA, USA.

Viereck, L. A. 1970. Forest succession and soil development adjacent to the Chena River in interior Alaska. Arctic and Alpine Research 2(1):1–26.

Viereck, L. A. 1979. Characteristics of treeline plant communities in Alaska. Holarctic Ecology 2:228–238.

Wardle, P. 1968. Engelmann spruce (Picea engelmannii engel.) at its upper limits on the Front Range, Colorado. Ecology 49:483–495.

Wilmking, M., G. P. Juday, V. A. Barber, and H. J. Zald. 2004. Recent climate warming forces contrasting growth responses of white spruce at treeline in Alaska through temperature thresholds. Global Change Biology 10:1–13.


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