We explored the ecological impact of a nitrogen-fixing shrub (Ulex europaeus, common gorse) cultivated as an intercrop species in the interlines of a young stand of maritime pine (Pinus pinaster). Nitrogen fixing species are well-known for their advantage of increasing the soil fertility in agriculture. Nevertheless, in forest ecosystems, their use is uncommon except in some associations in the tropics. The Landes de Gascogne forest range (SW France) has soil fertility issues and Ulex europaeus is a vigorous plant, native to this region. Therefore, an investigation of the association of this N-fixing species with trees is worthwhile. To this end, we compared a standard pine plantation management (control) with a treatment where gorse seeds were sown in the interlines at tree plantation (gorse sowing). This factor was combined with a phosphorus supply (control versus P-fertilisation), resulting in four treatments. The forestry practices evaluated here distinguish two main periods: a first period of six years after tree plantation when gorse individuals grew as an intercrop and formed dense vegetation, and a second period of three years after crushing this vegetation in the interlines. In the first period, pine mortality in the gorse sowing treatment was higher during the dry season than in the control treatment, suggesting competition for water. Pine diameter growth was reduced, but not height growth, indicating avoidance of light competition with gorse. In the presence of gorse, pine foliar-N was appreciably enhanced relative to the control, and natural pine pruning occurred, while pine browsing by animals was limited due to the protection of the thorny gorse thickets surrounding the trees. In the second period, soon after crushing the gorse vegetation in the interlines, enrichment of nitrogen in needles continued, suggesting a fertilising effect of the gorse remnants. Crushing also freed the pines from shrub competition and a “catch-up growth” was identified regarding diameter and stem volume growth. Two years after the gorse crushing, tree size and biomass were similar in the experimental treatments regardless of gorse sowing or not. Overall, during the first nine years after tree plantation, gorse introduction in the pine tree forests had various effects. On the one hand, gorse induced competition for light and water. On the other hand, it resulted in high inputs of nitrogen and organic matter into the ecosystem which could lead to sustained soil fertility and this might improve tree nutritional status. We recommend continuing the assessment of such pine-gorse forestry associations for longer observation periods and in very nutrient-poor sites in this forest range.