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Vector analysis of understory competition, N fertilization, and litter layer removal effects on white spruce growth and nutrition in a 13-year-old plantation

Matsushima, M. and S. X. Chang . 2006. Forest Ecology and Management 236: 332-341


Canada bluejoint grass (Calamagrostis canadensis (Michx.) Beauv.) is a very competitive understory species and can create a thick litter layer that may decrease tree growth in white spruce (Picea glauca (Moench.) Voss) stands in the boreal region. We examined the effects of Canada bluejoint grass dominated understory competition, N fertilization, and litter layer removal on the nutrition and growth of white spruce, in relation to soil N availability in a 13-year-old plantation near Whitecourt, Alberta, Canada, in 2003 and 2004. We hypothesized that: (1) understory competition reduces white spruce growth as a result of competition for available N; (2) litter layer removal increases white spruce growth by increasing soil temperature and N availability; (3) N fertilization improves white spruce growth by increasing N availability. Soil gravimetric moisture content in LFH was reduced by understory competition and litter layer removal in 2003, which had a very dry summer. Understory removal increased white spruce diameter growth, 100-needle biomass, and needle N concentration and content. In 2003, N fertilization consistently increased 100-needle weight and needle N content when litter layer was removed, but decreased or did not affect those values without litter layer removal. Nitrogen fertilization increased needle N concentration in 2004. The inconsistent effects of N fertilization may be due to the short-term nature of fertilization effects and immobilization of N by organic matter. Rates of white spruce diameter growth and soil mineral N supply in 2004 were related (R2 = 0.65, P < 0.001). Vector analysis showed that N fertilization or litter layer removal with intact understory vegetation either increased or did not affect needle N concentration but decreased tree biomass. We conclude that understory control will benefit tree growth in young white spruce stands in the boreal region that have passed the free-to-grow stage. However, N fertilization or litter layer removal alone in bluejoint infested sites may not benefit the trees.

Key Words

Belowground competition, Calamagrostis canadensis, N supply rate, Vector analysis, Boreal forest