Post-fire Tree Establishment Patterns at the Subalpine Forest-Alpine Tundra Ecotone: A Case Study in Mount Rainier National Park

Abstract

Climatic changes have induced striking altitudinal and latitudinal vegetation shifts throughout history. These shifts will almost certainly recur in the future; threatening other flora and fauna, and influencing climate feedback loops. Changes in the spatial distribution of vegetation are most conspicuous at physiognomically distinct ecotones, particularly between the subalpine forest and alpine tundra. Traditionally, ecological research has linked abiotic variables with the position of this ecotone (e.g., cold temperatures inhibit tree survival at high elevations). Thus, the prevailing assumption states that this ecotone is in equilibrium or quasi-equilibrium with the surrounding physical environment and that any dynamic shifts express direct linkages with the physical environment. This dissertation employs a landscape ecology approach to examine the abiotic and biotic ecological mechanisms most important in controlling tree establishment at this ecotone. The study site is on the western slopes of Mount Rainier, which was severely burned by a slash fire in 1930. Therefore, a crucial underlying assumption is that the ecological mechanisms controlling tree establishment are similar at disturbed and undisturbed sites. I exploited the use of 1970 CORONA satellite imagery and 2003 aerial photography to map 33 years of changes in arboreal vegetation. I created detailed maps of abiotic variables from a LIDAR-based DEM and biotic variables from classified remotely sensed data. I linked tree establishment patterns with abiotic and biotic variables in a GIS, and analyzed the correlations with standard logistic regression and logistic regression in the hierarchical partitioning framework at multiple spatial resolutions. A biotic factor (proximity to previously existing trees) was found to exert a strong influence on tree establishment patterns; equaling and in most cases exceeding the significance of the abiotic factors. The abiotic setting was more important at restricted spatial extents near the extreme upper limits of the ecotone and when analyzing coarse resolution data, but even in these cases proximity to existing trees remained significant. The strong overall influence of proximity to existing trees on patterns of tree establishment is unequivocal. If the underlying assumption of this dissertation is true, it challenges the long-held ecological assumption that vegetation in mountainous terrain is in equilibrium with and most strongly influenced by the surrounding physical environment.