How does a Sierran Forest Grow? Fire, Thinning, and Regenerating Trees

Excluding fi re over the last century has allowed canopy cover to burgeon, to thicken, along with litter depth (dead needles and leaves accumulated on the forest fl oor), and tree density in western forests. These changes have altered the small scale (microsite) conditions that affect the ability of tree seedlings to establish. This study in a mixed-conifer forest in the Sierra Nevada revealed relationships between established understory trees and microsite quality, and examined the effect of fi re, thinning, and shrub cover on seedling establishment. Most of the conifer species grew on microsites with relatively high soil moisture and relatively low direct sunlight. Planted seedlings died in high numbers, particularly in exposed areas. Thinning and burning did not substantially increase the natural abundance of pine seedlings, which have become scarce in these forests. Although shrub cover may initially help regenerating trees survive, few conifer saplings were growing in shrub-dominated patches, possibly because shrubs can be aggressive competitors for soil moisture. The lack of new growth, logs, or snags in many openings suggests that large gaps are hostile environments for tree seedlings