Demonstration and verification of a model that generates defoliation patterns in forested landscapes

Research into the sensitivity of a forest health monitoring network to outbreaks of defoliating insects spurred development of a new model that simulates outbreaks and generates patterns of defoliation in forested landscapes. Key features of the model included (1) input of a few data layers that were readily obtainable, (2) user control over the pattern generation process, (3) strong linkage between model parameters and a readily available source of parameter values, (4) simulation of large areas in a reasonable amount of time, and (5) detailed spatial output. A verification of the model was made by comparing model results with the results from actual outbreaks. Spruce budworm incidence maps for the period 1997–2003 in Minnesota were obtained and 12 different outbreaks identified. Model runs were developed, one for each of the outbreaks, and the numbers of plots defoliated in the actual outbreaks were compared to simulation frequencies. The number of plots falling in the actual outbreak matched the number of plots with the highest and second highest simulation frequency in nine and three of the verification runs, respectively. Results suggested that the computer model was an accurate implementation of the conceptual model and therefore suited to the intended application.