Space-time modelling of lightning-caused ignitions in the Blue Mountains, Oregon

Generalized linear mixed models (GLMM) were used to study the effect of veg etation cover, elevation, slope, and precipitation on the probability of ig nition in the Blue Mountains, Oregon, and to estimate the probability of ig nition occurrence at different locations in space and in time. Data on star ting location of lightning-caused ignitions in the Blue Mountains between A pril 1986 and September 1993 constituted the base for the analysis. The stu dy area was divided into a pixel-time array. For each pixel-time location w e associated a value of 1 if at least one ignition occurred and 0 otherwise . Covariate information for each pixel was obtained using a geographic info rmation system. The GLMMs were fitted in a Bayesian framework. Higher ignit ion probabilities were associated with the following cover types: subalpine herbaceous, alpine tundra, lodgepole pine (Pinus contorta Dougl. ex Loud.) , whitebark pine (Pinus albicaulis Engelm.), Engelmann spruce (Picea engelm annii Parry ex Engelm.), subalpine fir (Abies lasiocarpa (Hook.) Nutt.), an d grand fir (Abies grandis (Dougl.) Lindl.). Within each vegetation type, h igher ignition probabilities occurred at lower elevations. Additionally, ig nition probabilities are lower in the northern and southern extremes of the Blue Mountains. The GLMM procedure used here is suitable for analysing ign ition occurrence in other forested regions where probabilities of ignition are highly variable because of a spatially complex biophysical environment.