Nitrogen and Water Availabilities and Competitiveness of Bluejoint: Spruce Growth and Foliar Carbon‐13 and Nitrogen‐15 Abundance

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 greenhouse‐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 δ 13 C and δ 15 N are affected by soil N and water availabilities and bluejoint competition. A 2 × 2 × 2 (competition × N availability × 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 × 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 δ 13 C and δ 15 N by 1.2 and 1.2‰, respectively, even under adequate water supply, indicating that N deficiency caused by bluejoint competition had a dominant effect (increasing 13 C discrimination during photosynthesis) compared with the potential effect of drought stress on foliar δ 13 C, and that strong NH 4 uptake by bluejoint may have prevented significant soil N losses and 15 N enrichment through nitrification and subsequent denitrification.