Modeling patch-scale expansion of arctic shrubs

Studies of Arctic vegetation change have typically been done at either the plot level or at regional/ecosystem scales. The intermediate scales associated with patches of individual shrubs have largely been ignored. This research will explore patch scale dynamics of arctic shrub expansion by quantifying and modeling the changes in shrub cover previously documented by repeat photography for the Colville River basin. Additional historic aerial photographs of the same sites will be acquired and the shrubs mapped to determine in more detail the changes that have occurred over the latter half of the 20th century. The approach relies on the spatial characteristics of the expansion as the critical indicator of whether we understand the process. Cellular models will be developed that rely on a set of simple rules to determine how much shrub cover increases and if the pattern matches those patterns shown in the Colville photos. A similar approach has been effective for modeling forest expansion at the alpine treeline. Manipulating the strength of feedbacks between existing vegetation and the establishment probabilities of new shrubs will be used to develop plausible hypotheses to guide future experimental research. Although the controls on establishment and expansion in this version of the model are relatively simple and not explicitly based on the biogeochemical processes that are thought to be the key controls on shrub growth in the Arctic, the model produced here can easily be modified in the future to include these processes more explicitly once the patch level controls are known. The model that will be created is a very simple stochastic cellular model that is spatially explicit. The model will use a logistic growth model and place the new shrubs on the landscape in ways that test the importance of different types of reproduction and with different feedback strengths. This project will expand significantly what we are currently able to learn from the repeat photo pairs and serve as a crucial link between the plot and ecosystem scales that currently characterize research on this topic.