Predicting yield of Norway spruce and Douglas-fir using a morphopedological approach in the granitic landscapes of French Massif Central

To investigate the productivity of two coniferous species in granitic uplands of the French Massif Central, relationships between soil–landscape units (= morphopedological units) and forest yield were examined. An alveola landscape model composed of seven main morphopedological units (MPU) characterizes these two granitic areas. In total, 460 forest stands were studied and joined to the MPU. Each type of MPU was correlated to the yield of Norway spruce and Douglas-fir, evaluated by the site index (dominant height of stand at a reference age). Results stress that MPU explain a large part of site index variations: 68.7% in Cevennes uplands and 50% in Limousin for Norway spruce, 42% in Limousin for Douglas-fir. The hierarchy of the productivity among MPU is comparable for both species and study areas. Highest site index values correspond to MPU with deep Brunisols in vales and bottom of alveolas, with a high soil water storage capacity but without waterlogging. Ridges and summits with coarse parent materials topped by shallow and nutrient-depleted soils experience the lowest site index values. The strong relationship between MPU and forest yield can be explained by a combination of ecological factors affecting forest yield: topoclimatic conditions, parent materials and soils. These results stress that inheritances from Tertiary, and from Quaternary periglacial environments still influence the present-day potentials of ecosystems. In conclusion, the advantages and drawbacks of morphopedological surveys for forest applications are discussed. Key words: Soil–landscape units, forest yield, Picea abies (L.) Karst., Pseudotsuga menziesii (Mirb) Franco