Predicting site index of Douglas-Fir plantations from ecological variables in the Massif Central area of France

Abstract

Douglas-Fir is the main species used in France for reforesting mid-elevation regions, mostly on former heathlands and coppices. In order to clarify its auto-ecological limits, and to provide forest managers with relevant information for planting, we studied its productivity in a wide range of site conditions. Based on data from 202 Douglas-Fir pure stands at mid-elevation regions, site productivity was assessed using variance analysis with site types, site groups, and multiple regression analysis with ecological variables. Site index, determined from stem analysis data, was used as a species specific measure of site quality. Results show that Douglas-Fir site index is correlated to soil nutrient status, and secondarily to soil moisture regime. Mean stand productivity was generally high, but considerable variation in site index was determined within a study area that was classified by forest managers as uniform. Correlation between site classification and site index was confirmed, because it was based on synoptic factors that are simple and robust indicators of site productivity. Other methods to assess site quality, such as multiple regression on ecological variables and multivariate site groups, appear to be less explicative and robust for predicting site index. The results allow forest managers to predict the site index at parcel scale. Further investigation should be carried out in order to explain the major causes of the unexplained variance of site index by ecological variables, especially genetic variation, and effect of age.