The vegetation of postlogged black spruce lowlands in central Canada. I. Trees and tall shrubs

Abstract

A total of 122 lowland, postlogged stands in northeastern Ontario, Canada, previously dominated by Piceamariana (Mill.) B.S.P., were surveyed for the abundance of woody species to determine the extent of natural P. mariana regeneration. Detailed information on soils, site characteristics, time since disturbance, and the type of logging was included in the survey. Stand ages ranged from 0 to 56 years since logging, and the carbon to nitrogen (C/N) ratio, used as an index of site nutrient regime, ranged from 16.2 to 60.6. The average density of trees in postlogged stands was far less than in unlogged stands on comparable sites (3050 vs. 42 000–342 000/ha). Among the logged stands, P. mariana density was highest in stands more than 20 years old and on sites with a C/N ratio greater than 40, whereas density was lowest in young stands on sites with a low C/N ratio. On sites with a C/N ratio between 25 and 35, Abiesbalsamea was the most important tree species, whereas broadleaved trees and shrubs dominated sites with a C/N ratio less than 25. In addition, broadleaved species, notably Populustremuloides, P. balsamifera, and Betulapapyrifera, achieved greater annual height growth on the sites with a low C/N ratio than did P. mariana. Twenty years before the survey marked the onset of large-scale mechanical logging. The absence of young stands with a high C/N ratio in the soil indicates that mechanical wheel skidding caused a more drastic site conversion, through disruption of the surface peat, than occurred with the earlier horse skidding practices. Mechanical logging, therefore, places fast-growing species at a competitive advantage over P. mariana by creating microsites for broadleaved seedling establishment and promoting a nutrient-enhanced substrate. These results are discussed in relation to the postfire regeneration of P. mariana and the growth strategy of the species.