Relationships between landscape patterns and fire occurrence within a successional gradient in sagebrush steppe–juniper woodland

Abstract

Expansion of western juniper (Juniperus occidentalis Hook. var. occidentalis) has altered vegetation composition, fire behaviour and fire potential throughout south-western Idaho and eastern Oregon. Utilising GIS-derived products and fire-simulation software, the influence of the spatial arrangement of different woodland developmental stages on simulated surface fire occurrence was evaluated. Custom fuel models and a recent vegetation map processed in FARSITE under moderate fire conditions were used to create a fire-occurrence grid in three sixth-order watersheds on the Owyhee Plateau of south-western Idaho. Landscape pattern metrics were selected to quantify the spatial arrangement of plant communities within a neighbourhood around points from each successional stage randomly placed within each watershed. Linear regression analysis of fire occurrence and each of the selected landscape metrics was compared for four successional stages of western juniper encroachment to assess the effect of landscape-scale vegetation arrangement on fire occurrence. The landscape structure had little influence on whether an early-successional area burns in a surface fire, whereas the surrounding landscape structure influenced whether a late-successional or mature woodland area burned. Landscape metrics that showed significance in late-successional and mature woodland stages include patch density, mean area and Simpson’s diversity.