How resource availabilities affect the competitiveness of Canada bluejoint grass [Calamagrostis canadensis (Michx.) P. Beauv., hereafter referred to as bluejoint] is poorly understood. Bluejoint is a widespread grass species in boreal forests and competes with tree species such as white spruce [Picea glauca (Moench) Voss] for belowground resources (e.g., soil N and water) when their supply is limited. In this green house-based study, we tested the following hypotheses: (i) bluejoint competition reduces white spruce growth when belowground resource availabilities are limited; (ii) greater N and water availabilities may increase bluejoint competition and its adverse effects on white spruce growth; and (iii) white spruce foliar delta C-13 and delta N-15 are affected by soil N and water availabilities and bluejoint competition. A 2 x 2 x 2 (competition x N availability x water availability) factorial experiment was conducted using pots of planted white spruce seedlings with or without bluejoint. Bluejoint competition reduced the volume index (diameter(2) x height) of white spruce by 50%. The competitiveness of bluejoint appeared to be independent of resource availabilities, but bluejoint had greater growth response to increased N availability than white spruce. Bluejoint competition depleted white spruce foliar delta C-13 and delta N-15 by 1.2 and 1.2%o, respectively, even under adequate water supply, indicating that N deficiency caused by bluejoint competition had a C-13 discrimination during photosynthesis) compared with the potential effect of drought stress on foliar delta C-13, and that strong NH4 uptake by bluejoint may have prevented significant soil N losses and N-15 enrichment through nitrification and subsequent denitrification.