| The forest-alpine tundra ecotone (FTE) is the transition area between alpine tundra and subalpine forest ecosystems. FTEs regulate the flow of water and materials between the adjacent ecosystems and may be responsive to climate changes. This study examines the relationship, in terms of structure and composition, between the FTE and the subalpine forest at Niwot Ridge, Colorado. First, I conducted a landscape-scale characterization of heterogeneity in the FTE as a result of differences in landscape features, environmental attributes, and disturbances. Three FTE classes were distinguished. FTEs 1 and 2 are characterized by different disturbance regimes and they are not appropriate FTEs for monitoring global changes. In contrast, FTE 3, with few disturbances and permanent features, was predominantly climate controlled, and therefore may be more suitable for observing changes in vegetation in response to global changes. Second, I described the spatial patterns of trees and investigated the relationship between tree abundance and environmental variables. The three FTEs exhibited different tree spatial patterns due to variations in disturbance type, environmental conditions, and endogenous processes occurring in established vegetation. Macro- and micro-terrain features as well as geologic substrate were associated with tree abundance in patch forests, which in turn was related to tree abundance in the upper subalpine forest. Third, I determined the relationships among vegetation diversity patterns, environmental factors, and canopy structures in the three FTEs. Species richness decreased from those plots in FTEs 1 and 2 underlain by colluvium to plots on geologic substrates other than colluvium, including FTE 3 plots. Within transects, species richness did not differ between patch forest and subalpine forest plots, but was inversely related to tree density. Both FTEs 1 and 2 contained more species from forest and alpine ecosystems than FTE 3. Mesic plots in FTE 2 were rich in forest species, while alpine species occurred mainly in FTE 1, characterized by avalanches and debris flows. Endemic species were present in all FTEs, but FTE 1 contained the highest number. Exotic species were found in FTEs 1 and 3. This has important management implications for monitoring the response of vegetation to changes in climate. |
