The relationship between tree establishment patterns and positive feedback at alpine treeline.

摘要

The spatial pattern of trees found at alpine treeline results from the interaction of many variables. Environmental stresses, competition with herbaceous and woody tundra species, the impact of disturbance, the effects of changing climate and the ability of species to respond to climate change all influence the patterns found at alpine treeline. The alpine treeline ecotone may be particularly responsive to climatic changes as species found at this ecotone are often at some limit to their existence. Determining the significance of specific processes in determining pattern is important in order to be able to predict vegetation changes under potential climate change, as well as to determine which patterns are particularly susceptible to the invasion of vegetation as a consequence of global climate change. The aim of this research is to determine the significance of positive feedback, how trees improve local conditions and allow for the establishment of more trees, in structuring tree establishment patterns at alpine treeline. To accomplish this, a cellular automata based model was developed that captures the primary influence of a few important environmental variables. Through iterations of simple rules, the model generates complex patterns that are similar to those patterns found at alpine treeline. It is the clear logic and simplicity of this model that allow for generalizations to be made concerning the importance of underlying local processes in determining patterns at alpine treeline. Using spatial metrics, the patterns generated by different permutations of the model parameters are compared with data collected on tree establishment pattern at Lee Ridge in Glacier National Park, Montana. Results, including Multivariate Analysis of Variance and Principle Components Analysis, show that the model realizations that include a directional positive feedback produce patterns that are more similar to those found on Lee Ridge than those produced by model realizations that do not include a directional positive feedback. Further research is necessary to determine the strength of feedback necessary to produce the complex patterns of tree establishment found at Lee Ridge, and in many other treeline environments.