Projecting Nonnative Douglas Fir Plantations in Southern Europe with the Forest Vegetation Simulator

In Italy, Douglas-fir has great potential in terms of wood production and drought tolerance. However, a growth reference for mature stands is lacking. We calibrated and validated the Pacific Northwest variant of the Forest Vegetation Simulator (FVS) for Douglas-fir plantations in Italy and then ran the calibrated model to test management alternatives. We calibrated the height-diameter, crown width, crown ratio, and diameter increment submodels of the FVS using multipliers fitted against tree measurements (n = 704) and increment cores (n = 180) from 20 plots. Validation was carried out on tree-level variables sampled in 1996 and 2015 in two independent permanent plots (275 trees). Multiplier calibration improved the error of crown submodels by 7-19%; self-calibration of the diameter growth submodel produced scale factors of 1.0-5.2 for each site. Validation of 20-year simulations was more satisfactory for tree diameter (-6% to +1% mean percent error) than for height (-10% to +8%). Calibration reduced the error, relative to that of yield tables, of the predicted basal area and yield after 50 years. Simulated responses to thinning diverged, depending on site index and competition intensity. The FVS is a viable option for modeling the yield of Douglas-fir plantations in Italy, reflecting the current understanding of forest ecosystem dynamics and how they respond to management interventions.