Abstract
Abstract The predictive ability of the Central States growth and yield system TWIGS 3.0 was evaluated for Illinois forests. The data used to validate the system were collected from permanent plots that had been established throughout the state and maintained for up to 30 years. TWIGS growth predictions were analyzed through the differences between observed and predicted stand and species characteristics. TWIGS consistently overpredicted mean stand diameter and underpredicted tree survival over a 30-year growth projection. The combined result of prediction errors in diameter growth and tree survival was a consistent underprediction of basal area per acre. Percent error at the twentieth year of projection was -6% for mean stand diameter, 18% for tree survival, and 6% for basal area. TWIGS, with its simulation features and small relative prediction errors for some major timber species, is an adequate growth and yield system for Illinois' mixed hardwood forests. North. J. Appl. For. 6:154-156, December 1989.
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