Multiscale Relationships Between Alpine Treeline Elevation and Hypothesized Environmental Controls in the Western United States

Abstract

Multiple environmental factors contribute to the spatial and compositional character and elevational patterns of alpine treeline ecotones (ATEs), and the relative influence of these factors is scale dependent and spatially variable. Frameworks detailing the hierarchical structure of the ATE have been developed to characterize scale dependencies of the pattern and controls of treeline, but this topic has not been studied across a broad range of scales (e.g., from the hillslope to the region). This research directly examines scaling by comparing relationships among treeline elevations and a set of possible controls as geographic extent is varied. The data set used for this research consists of elevational data at the ATE and a set of hypothesized controls for 1,006 sites in twenty-six mountain ranges across the Western United States. The response and predictor variables are quantified from digital data sets using geographic information systems and remote sensing methodologies and then analyzed using a Mantel test framework. Results generally support, and add empirically derived detail to, existing theoretical frameworks, with climatic controls (i.e., variables characterizing temperature and precipitation) having higher correlations with ATE elevation at coarser scales and topographic variables having higher correlations at finer scales. These scale relations support the conceptual hierarchical frameworks that have been proposed, and they are useful guides of covariate selection for future ATE modeling endeavors.