White spruce foliar δC and δN indicate changed soil N availability by understory removal and N fertilization in a 13-year-old boreal plantation.

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 δC and δN 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 δC and δN values were determined. Results: Foliar δC 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 δC 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 δN 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 δC 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 δN in the foliage. Control of competing vegetation and N fertilization could be used as management tools to reduce N limitation. [ABSTRACT FROM AUTHOR]