PREDICTING THE YIELD OF DOUGLAS-FIR FROM SITE FACTORS ON BETTER QUALITY SITES IN SCOTLAND

In Scotland, as a result of recent changes in agricultural policy and grant schemes, there is now greater potential for planting a wider ran ge of more productive forestry species on better quality land. In orde r to permit accurate production forecasting and financial appraisals f or any such afforestation, it is necessary to develop predictive yield models. This article describes the development of a multiple linear r egression model for the prediction of General Yield Class (GYC) of Dou glas fir using readily assessed, or derived, site factors. Climate sur faces developed by spatial analysis of weather data were used to predi ct temperature and rainfall for 87 sample sites to a resolution of 1 k m(2). Estimates of wind climate were derived from a regression model u sing geographic location, elevation and topographic exposure. Multivar iate analysis of these and other soil and topographic variables indica te that temperature and exposure are most important in determining the productivity of Douglas fir on better quality sites in Scotland. As c rop age increases, GYC declines and the possible reasons for this effe ct are discussed. Other factors are also discussed, such as the geneti c variability of Douglas fir, and problems associated with establishme nt and form.