The effects of weed control on the growth and survival of black spruce (Picea mariana (Mill.) B. S. P.) were examined up to 11 years after planting. The experiment was conducted in Kenogaming Township in northeastern Ontario, on an upland, mixed-wood, herb-rich site in the boreal forest region. A split-plot experimental design with a completely randomized arrangement of whole-plot treatments (2 herbicides x 3 replicates) was used. There were 6 split-plot treatments distinguished by stock type (0.4g, 0.6g, and 1.5g paperpots and 1.5 + 1.5 bareroot transplants) and planting season (spring [i.e., May] and summer [i.e., July] 1982). The 2 weed control treatments were an untreated control and glyphosate [N-(phosphonomethyl) glycine] formulated as the isopropylamine salt [Roundup® 356g a.e. L"1]. The herbicide was applied at 70L/ha with a spinning disc applicator at 2.14kg a.e. ha*1 in August 1984.
Reduction of weed competition almost always accelerated the growth of the black spruce outplants. Eight growing seasons after weeding the trees on the weeded plots were up to almost 3 growing seasons ahead of their counterparts on the non-weeded plots. By the end of the experiment, the growth advantage for trees in weeded plots relative to those on non-weeded plots was increasing with respect to volume at about 1.5 times its rate with respect to height. Tree survival was not significantly affected by weed control, planting season, or stock type.
Planting season and stock type both affected tree growth. The relative rates of volume growth of the spring-planted stock exceeded that of the corresponding summer-planted stock by 10-14%. The bareroot stock was initially taller and increased in volume at a relative rate which was 4-22% faster than that of the 0.4g paperpots in the same weed control and planting season regimes. This superiority of the bareroot stock over the paperpot stock was 3-5% greater when planted in the summer than in the spring.