Background and aims
Canada bluejoint grass is a strong competitor for water and N against young white spruce trees in regenerating boreal forests in moist habitats. We investigated foliar δ13C and δ15N responses of white spruce to understory removal and N fertilization to infer treatment effects on soil water and N dynamics.
Methods
An experiment with understory removal and N fertilization as the main and split-plot treatments was conducted in a 13-year-old white spruce plantation in the boreal region for 2 years. Soil N availability, gravimetric soil moisture content, tree growth, foliar C and N concentrations, and foliar δ13C and δ15N values were determined.
Results
Foliar δ13C was increased by understory removal in the second year and by N fertilization in the first year. There was a positive (P < 0.01) relationship between foliar δ13C and N concentration, suggesting that understory removal and N fertilization improved tree growth via increased N availability and thus increased carboxylation rate of photosynthesis. The positive correlation (P < 0.01) of foliar δ15N with foliar N concentration and with available soil N indicated that understory removal and N fertilization facilitated N isotope fractionation accompanying increased soil N turnover.
Conclusions
Foliar δ13C and its relationship with other variables suggested that understory removal and N fertilization improved white spruce growth mainly through enhanced carboxylation rate due to greater N availability rather than increased stomatal conductance. Specifically, understory removal and N fertilization increased soil N availability and subsequently induced N loss from the system, leading to a higher N concentration and δ15N in the foliage. Control of competing vegetation and N fertilization could be used as management tools to reduce N limitation.