Performance of the Forest Vegetation Simulator in Managed White Spruce Plantations Influenced by Eastern Spruce Budworm in Northern Minnesota

Silvicultural strategies such as thinning may minimize productivity losses from a variety of forest disturbances, including forest insects. This study analyzed the 10-year postthinning response of stands and individual trees in thinned white spruce (Picea glauca [Moench] Voss) plantations in northern Minnesota, USA, with light to moderate defoliation from eastern spruce budworm (Choristoneura fumiferana Clemens). Using the Forest Vegetation Simulator, model results suggested overprediction of stand basal area growth and tree diameter increment in these stands. Growth modifiers indicated that trees growing in unthinned stands and with greater defoliation levels (i.e., 20-32%) would need the largest adjustment for diameter increment. Modifiers for height were similarly specified to compensate for the underprediction of height increment in these stands. Thinned stands continued to maintain target live crown ratios in excess of 0.40, suggesting long-term productivity. Results highlight the need for simulation models that represent appropriate responses to stands and trees affected by forest insects and diseases. Ultimately, accurate representations of growth and development in these models that account for influences of biotic disturbance agents are essential under future global change scenarios, particularly as silvicultural strategies are implemented to reduce the impacts of forest health threats and other stressors.