Soil available water capacity (SAWC) is becoming a crucial issue for forest management in the context of climate change. However, the SAWC indicator, which was created for agricultural soils, needs specific adaptations when it is estimated for forest soils. This study aims to determine the best way to apply the SAWC indicator with regard to its significance for explaining variation in tree fertility across a given region. An extensive investigation with spatial sampling of 100 Douglas fir stand plots was conducted in the Haut Languedoc Regional Nature Parc (Southern France). It involved stand fertility assessments, morphological observations of deep soil pits including the saprolite, quantitative estimations of coarse fragments, texture and organic matter laboratory analysis and water retention and density measurements on a limited sample of horizons (35). Different approaches were used to compute SAWC from the collected data. These approaches differed in the methods i) for estimating the fine earth water retention - national vs local pedotransfer function -, ii) for incorporating coarse fragments - assumed to contribute or not to the water retention of soils- and iii) for estimating soil volumes - variable thicknesses and considerations of rooting. The results showed that the estimate of SAWC that explained the greatest variation in tree fertility over the study area (23% of the variance) was obtained by estimating fine earth water retention from a local pedotransfer function, by considering the water retention of the coarse fraction and by exploring the thickest soil volumes (thicknesses of 200 cm, 300 cm and equal to the depth to bedrock) without explicit consideration of rooting observations. The ranking of SAWC component impact on SAWC values was i) soil volume, ii) coarse fraction water retention and iii) fine earth retention. These results indicate that improvements in estimating coarse fraction water retention and soil volume and, to a lesser extent, fine-earth water retention estimated by a forestsoil-adapted pedotransfer function would greatly benefit the application of the SAWC indicator to forest soils.