Seed origin trials of Douglas fir (Pseudotsuga menziesii) in Britain have provided important information on growth differences between origins (Lines, 1987), but the variation in wood properties between origins has not been reported. The present paper describes a study of the variation in natural durability against two brown rot fungi (Coniophora puteana and Postia (Poria) placenta) and in a fungal cellar test between four seed origins of Douglas fir. In addition to natural durability, tree diameter, heart-, sap- and total cross-sectional areas, density, bending properties, extractive content and composition were determined to examine their effect on natural durability. The effects of extractive content and composition on durability are discussed in a second paper (Part II: Effect of extractive content and composition). The proportion of the total variation in natural durability explained by differences between seed origins and differences between trees within origins were estimated to indicate the potential of tree breeding programmes for modifying this wood property. (Poria) placenta) and in a fungal cellar test between four seed origins of Douglas fir. In addition to natural durability, tree diameter, heart-, sap- and total cross-sectional areas, density, bending properties, extractive content and composition were determined to examine their effect on natural durability. The effects of extractive content and composition on durability are discussed in a second paper (Part II: Effect of extractive content and composition).

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