Stand growth model calibration for use in forest pest impact assessment

Quantitative assessment of forest pest impacts is an important element in d esign of forest and pest management programs. Such assessment requires fore casts of pest populations, definition of the nature and extent of damage in flicted on trees by those populations, and translation of the damage effect s 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 calibr ating stand growth models that can be used to incorporate the effects of pe st damage on tree and stand development. This calibration methodology is de monstrated for spruce budworm (Choristoneura fumiferana Clem.) effects on s pruce (Picea sp.) and balsam fir (Abies balsamea (L.) Mill.) stands using p ermanent 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 su rvival and cumulative defoliation. Growth loss relationships were similar f or the species considered, while reduced survival relationships varied betw een species and age classes. Using these relationships as input to the STAM AN stand growth model, forecasts were made and compared against empirical s tudies of stand development under defoliation during spruce budworm outbrea ks. The results suggest that reasonable approximations of stand response to pest incidence can be generated with relatively simple models and data set s. Judicious use of stand growth forecasts generated by such methods can he lp serve the needs of forest and pest management strategy design.