Influence of geopedological conditions on the root system and height growth of Douglas fir in the monts du Beaujolais area (France).

Abstract

This paper deals with a study of mature Douglas-fir root systems in 74 pure and even-aged forest stands of middle mountain areas (mounts of Beaujolais). Root systems were studied in situ using trenches open in soils at 1 meter distance from the stem of dominant trees. The results stress that geopedological constraints due to substratum and soil structures have a strong influence on root systems morphology, and on height growth of forest stands. Multivariate analyses (CA and HCA) were carried out on root systems variables and geopedological variables. Seven homogeneous groups were identified. They explain 52,6 % of variance of dominant height. Douglas fir height growth is correct or strong for all stands but distinctly reduced when root system is superficial and jammed in surface soil horizons. Highest growth corresponds to deep and regular root system profiles. A multiple linear regression was carried out between dominant height and major root system variables (R² = 0,57 %): estimated maximal rooting depth, maximal root density per horizon, fine roots surface (diameter < 5 mm). Height growth is strongly correlated (R² = 0,61) to surface of finest roots (diameter < 1 mm), which provide the main part of water and nutrients. These observations aim at choosing better sites and soils for settling Douglas fir, and at stressing risks of failure, pests or diseases.