Western root disease model simulation versus plot remeasurement: 11 years of change in stand structure and density induced by Armillaria root disease in Central Oregon

The purpose of this analysis was to use long term permanent plots to evaluate the short-term predictive capability of the Western Root Disease Model extension (WRDM) of the Forest Vegetation Simulator (FVS) in central Oregon mixed-conifer forests in project planning situations. Measured (1991�2002) structure and density changes on a 100-acre unmanaged area in south-central Oregon were compared to those predicted by the Southern-Oregon Northern-California variant (SORNEC) of FVS and the WRDM. Within the study area there were 149 variable-radius plots within 12 stands. Predictions were assessed using five variables that were collectively chosen to represent changes in stand density, stand structure, and are commonly used in project planning. For each indicator variable, projections were made using SORNEC alone and then with the WRDM. Projections were made at both the stand and plot level.Where Armillaria root disease was present, the WRDM better predicted root disease impact than projections using SORNEC alone. Plot projections with the WRDM reduced the unexplained variation an average of 35% over projections made with SORNEC alone. Root disease impacts were generally overestimated using the WRDM as compared to measured changes. The correlations between the predictions and what was measured were much higher and always significant (p < .0.05) using plots and usually not significant using stands. The level of effort needed to parameterize and troubleshoot the WRDM creates significant barriers to its use as a project planning tool. Improvements that could reduce these barriers and thus, make the WRDM more attractive to project planners would be to provide users with a set of key parameters that are calibrated, offer full automation of the post-run data summaries, and make the root disease distribution and spread more transparent.