Seed tree density, variable retention, and stand composition influence recruitment of white spruce in boreal mixedwood forests

Abstract

In an operational-scale experiment, we examined natural regeneration of white spruce ( Picea glauca (Moench) Voss) under various degrees of variable retention harvesting across different overstory canopy compositions. We sampled four types of overstory canopy compositions (ranging from deciduous dominated to conifer dominated) and six rates of retention (2%, 10%, 20%, 50%, 75% and 100%). Spruce seedling densities, maximum height, and stocking rates were assessed in 432 transects and were found to be lowest in the deciduous stands and with 100% canopy retention (control). Regression tree analysis indicated that densities, maximum height, and stocking of spruce seedlings were greatest with higher availability of seed trees (>30·ha–1) and on machine corridors; in these circumstances stocking reached 74%. By contrast, stocking was less than 14% on retention strips with no machine traffic, when seed tree density was less than 11 seed trees·ha–1. However, stocking also declined with higher density of residual trees. Stocking in relation to density of seed trees was highest within clearcuts and lowest within the 75% retention intensity where low light transmission may have limited regeneration success. Passive soil disturbance from skidding appeared to be more important for the establishment of spruce seedlings than was the abundance of competing vegetation.