Growth model for Italian Douglas fir plantations

The fundamental elements of a growth model for Douglas fir plantations in central and southern Italy have been developed and tested. Growth and yield of Douglas fir in Italy is still estimated by yield tables. No variable density growth function has yet been published. This work concentrates on diameter and basal area growth as functions of site and stand characteristics.
The model can be used both as stand (average) model or as size-class model. The main function estimates annual basal area growth of the stand through age, current basal area and site characteristics. If current diameter distribution is also known, a second function estimates size-class allotment of overall stand growth enabling distribution projection.
The research is based on a set of 55 plots distributed over 3 Italian regions: Toscana, Puglia and Basilicata. In Toscana, where more than half of the plots are located, each plot has been remeasured 3 to 5 times during the last 15 years(la Marca, Scotti, 1986; Corona et al., 1990). In the other regions the plots have been established more recently (Scotti et al., 1995).
The main factors influencing current stands status are: initial plantation density (ranging from 800 to over 3000 trees/ha) and thinning regime (no thinning, selective or combined, removing from 10 to over 35% of the basal area). The model is expected to become a basic tool for developing decision support systems for silvicultural planning, optimizing plantation density, thinnings and rotation for specific forest sites and timber management objectives.
Results include a basic evaluation of model performance at stand and size-class level. Stand basal area is accurately estimated over a wide range of conditions. Diameter distribution projections compare quite well with correspondent observed distributions: even the worst cases do not appear to be significantly biased.