The impact of biodiversity loss on the functioning of forest ecosystems has become a central issue in ecology. Most reports of the positive effects of tree mixture on the biodiversity–productivity relationship focus on mixtures that combine tree species with contrasting traits. Nevertheless, little is known about how coniferous mixtures of the same genus affect forest productivity, what mechanisms are involved, and how the understory is affected. Here, we assessed the effect of mixed versus monospecific stands of Pinus sylvestris L. and P. pinaster Ait. on productivity, its impact on the understory, and its relationship with soil water and fertility, based on research with six triplets (6 triplets × 3 forest stands × 1 plot = 18 plots) in North-Central Spain. Each triplet consisted of two plots dominated either by P. sylvestris or P. pinaster and of one mixed plot that contained both species. Productivity, at the stand and neighborhood levels, and the understory richness, and soil water and fertility at the stand level were analyzed. A positive effect of pine mixture on productivity was observed at the smaller spatial scale, and it had no negative effect on the understory richness. The greater space-use efficiency (higher tree density and basal area) of both Pinus species in the admixtures was related to soil water and fertility niche complementarity. The fundamental role of scale in determining the relationship between species richness and ecosystem functioning in forests is highlighted.