An individual-based model of alpine plant distributions

Abstract

Our objective was to evaluate the relationships between alpine plant distributions and environmental conditions in order to predict the spatial distribution of major alpine plant communities found along the Front Range of the Rocky Mountains in the western U.S. Our approach was to modify the individual plant-based steppe model to simulate plant responses for three alpine communities found on Niwot Ridge, Colorado, USA: fellfield, dry meadow, and wet meadow. We used the alpine model to evaluate the relationship between alpine plants and their environment both for conditions typical of each community and for conditions along a gradient that includes transitions between communities. We also analyzed model behavior using sensitivity analysis, and tested the ability of the model to simulate alpine plant community dynamics using verification and validation procedures. The individual plant-based alpine model tracks the recruitment, growth, and mortality of individual plants on a small plot (0.09 m 2) through time at an annual time step. Eighteen species with the greatest abundance in fellfield, dry meadow, and moist meadow communities, and representing a range of life history characteristics and environmental responses are included. Environmental factors in the model are average daily maximum temperature during the growing season, growing season soil water availability, snow depth, and disturbances. Recruitment and mortality by species have elements of stochasticity whereas growth of an individual plant is determined by resource availability and abundance of plants. Patterns of species composition and aboveground biomass simulated by the model were in general agreement with observations from the three alpine communities. Simulated fellfield communities were dominated by cushion plants, dry meadow communities were dominated by Kobresia myosuroides and Acomastylis rossii, and moist meadow communities were dominated by Acomastylis rossii and Deschampsia caespitosa. Shifts in species composition and dominance were observed along the two environmental gradients investigated (soil water availability and snow depth). Cushion plants dominated areas of low soil water and snow depth representative of fellfield conditions. A shift to graminoid-dominated vegetation, in particular K. myosuroides, occurred over a narrow range of soil water conditions and snow depth. At high soil water and snow depth, this species decreased in biomass as a result of snow-induced mortality, and a forb and graminoid species ( A. rossii and D. caespitosa, respectively) dominated the vegetation. The transition from dry meadow to moist meadow was more gradual than the transition from fellfield to dry meadow. Our results suggest that an individual-based approach is useful in understanding alpine plant community dynamics, and in synthesizing information on processes operating across a range of spatial and temporal scales in alpine communities.