Characterizing and Modeling Arctic Shrub Expansion on the North Slope of Alaska, USA
Naito, Adam Takashi
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Shrub expansion is one of the most recognized components of terrestrial Arctic change and has been documented in studies involving fine-scale experimental manipulations or broad-scale satellite remote sensing. The characteristics and drivers of this phenomenon at the landscape scale, however, are understudied. The motivation of this dissertation was to develop an improved understanding of the historic spatial characteristics of shrub expansion on the North Slope of Alaska and its environmental drivers at this landscape scale. This work has three objectives, which include: 1) mapping and quantifying historic shrub expansion patterns; 2) examining a relationship between shrub expansion and its hydrological controls; and 3) designing and implementing a spatially-explicit simulation model to develop hypotheses regarding the landscape-scale drivers of shrub expansion (i.e., modes of reproduction, hydrological constraints, and their interactions). Shrubs maps were generated from semi-automated classification of historic vertical aerial photographs and contemporary high-resolution satellite imagery within a GIS. The spatial patterns of historic shrub expansion were quantified using FRAGSTATS and the multi-scale information fractal dimension. Relationships between shrub expansion and local hydrology was determined statistically through associations between areas that gained shrub cover and topographic wetness index values derived from a digital elevation model. The contribution of shrub reproductive characteristics was determined by developing a C#-based spatially-explicit simulation model that simulates clonal and sexual reproduction of shrubs. The reproductive mode(s) producing spatial patterns most similar to the observed patterns was determined through principal components analyses. Results from this work suggest that: 1) the shrub-tundra ecotone within river valleys on the North Slope is has either initiated or completed a phase transition from tundra to shrubland; 2) shrub development is promoted in areas where the potential for water accumulation or throughflow is higher; and 3) vegetative reproduction appears to have been dominant mode of reproduction . Considering our current understanding of the fine-scale relationships between shrub expansion and hydrology, surface energy balances, and C and nutrient cycling, continued expansion may have considerable implications for circumpolar tundra ecosystems. These findings will facilitate the development of improved projections of the structure and function of these ecosystems and their feedbacks to climate change.