Modeling complex dynamics at Alpine treeline ecotones

Alpine treeline ecotones (ATE) are the transition zones between contiguous subalpine forest and open alpine tundra. Because of their transitional natures between different ecosystems in high mountain areas there are a variety of interactions between different species, between vegetation and environmental factors, and between ecological pattern and process. These interactions, or feedbacks, are often nonlinear and potentially make alpine treeline ecotones sensitive to environmental change, especially climate change. Feedbacks between pattern and process create a variety of distinctive yet sometime surprising alpine treeline patterns in three dimensions. These nonlinear interactions and resultant patterns are considered to be an example of spatial complexity, and in this study the research framework of complexity theory was adopted. Dynamic simulation of alpine treeline ecotones is used as the basic research method, and local nonlinear interactions, or more specifically positive feedbacks, are considered the key mechanism driving alpine treeline dynamics. A cellular simulation was created with tree/no-tree states that change as a function of probabilities of tree establishment and mortality, which are functions of the neighborhood and an underlying gradient; the former changes in space and time endogenously; the latter can change in space and time exogenously. Three research projects were conducted for this dissertation; they explore the endogenous and exogenous aspects of alpine treeline dynamics. First, the endogenous dynamics of alpine treeline ecotones were examined. Findings indicate that local positive feedbacks originated from interactions between trees can create fractal spatial dynamics in space and time and self-organization constrains the range of pattern-process relations. Second, the impacts of geomorphologic factors, which impose an exogenous spatial structure on alpine treeline dynamics, were investigated. The results show that there is a geomorphic limit to the endogenous fractal alpine treeline dynamics and self organization. Third, the impacts of climate change, which imposes an exogenous temporal structure on alpine treeline dynamics, were examined. In this case, the self-organized nature of alpine treeline dynamics are not significantly affected by external climate change and the use of alpine treeline ecotones as potential indicator of climate change is called into question. Results of this study suggest that further research using complexity theory is needed to improve our understanding of alpine treeline dynamics and their interactions with exogenous environmental factors.