Development of GIS-based models to predict plant community structure in relation to western juniper establishment

Abstract

During the past 130 years, western juniper ( Juniperus occidentalis Hook.) has rapidly expanded into sagebrush communities impacting plant structure and function. Predictions of intercanopy shrub density and cover with woodland development can provide insight into site condition and ecological resiliency in western juniper encroached areas. The purpose of this study is to model shrub density and cover in relation to juniper competition across a heterogeneous landscape in a southeast Oregon watershed. Independent variables included in model development were derived from high-resolution color imagery, a 10 m digital elevation model, and field-based vegetation and soil moisture measurements. Juniper cover and three intercanopy classes, representing different levels of juniper competition, were delineated from 1:5000 scale color (RGB) imagery. A competition index was produced from the classified image representing a gradient of potential competition with juniper. An integrated moisture index was generated representing variability in soil moisture due to hillslope solar radiation, curvature of the landscape and flow path direction and flow accumulation (flow path density). Plant density, plant cover, and percent soil moisture (gravimetric) were measured from 10 m × 10 m plots located throughout the watershed study area. Multiple stepwise regression produced the best-fit model for predicting plant density or cover for mountain big sagebrush ( Artemesia tridentata ssp. vaseyana Beetle), bluebunch wheatgrass ( Pseudoroegneria spicata (Pursh)), and all shrub species combined. The prediction of total shrub density was significantly greater than expected by chance ( p < 0.001), with a 0.865 coefficient of determination. Total shrub cover also had a high correlation between observed values and environmental gradients ( R 2 = 0.773). These data can be used by land managers to assess the change in plant structure with increasing juniper canopy cover, a measure important for monitoring impacts from juniper woodland establishment.