Stand growth model calibration for use in forest pest impact assessment

Quantitative assessment of forest pest impacts is an important element in design of forest and pest management programs. Such assessment requires forecasts of pest populations, definition of the nature and extent of damage inflicted on trees by those populations, and translation of the damage effects across scales, from the tree to the stand to the forest. Central to this process are stand development forecasts which embody tree-level impacts of damage and which provide input to forest-level models. We discuss the role of stand growth forecasting in this context and propose a method for calibrating stand growth models that can be used to incorporate the effects of pest damage on tree and stand development. This calibration methodology is demonstrated for spruce budworm (Choristoneura fumiferana Clem.) effects on spruce (Picea sp.) and balsam fir (Abies balsamea (L.) Mill.) stands using permanent sample plot data acquired in New Brunswick over the past 20 years. The results showed linear relationships between tree diameter growth loss and cumulative defoliation, and non-linear relationships between reduced survival and cumulative defoliation. Growth loss relationships were similar for the species considered, while reduced survival relationships varied between species and age classes. Using these relationships as input to the STAMAN stand growth model, forecasts were made and compared against empirical studies of stand development under defoliation during spruce budworm outbreaks. The results suggest that reasonable approximations of stand response to pest incidence can be generated with relatively simple models and data sets. Judicious use of stand growth forecasts generated by such methods can help serve the needs of forest and pest management strategy design.