The pattern of invasion and establishment of Piceamariana (Mill.) BSP. into the subcanopy layers of Pinusbanksiana Lamb. dominated stands

Abstract

Stand age-structure analysis is used to determine whether establishment of P. mariana (Mill.) BSP. in the subcanopy of P. banksiana Lamb. dominated stands takes the form of a gradual influx or a sudden event. Twenty-six P. banksiana dominated stands were investigated of which 15 contained P. mariana trees at moderate to high densities. Postfire ages ranged from 46-132 yr for the oldest P. banksiana cohort in each stand. The 26 stands were subjectively assigned to 1 of 7 groups on the criterion of age-structure histogram appearance. A multivariate test of the hypothesis that such age-structure differences could be due to soil differences was not significant. Three patterns of P. mariana influx were indicated: gradual influx over a long time span in the oldest stands; contemporaneous postfire reestablishment of both P. banksiana and P. mariana on a site; invasion of P. mariana cohorts following surface fire activity as evidenced by fire scar dates. Results from a P. mariana seedling survival experiment indicated that depth of burn is of critical importance for black spruce seedling establishment in these stands. However, circumstantial evidence suggests that P. mariana seed supply may also be limiting to establishment success. These observations are discussed briefly in relation to models of forest succession.