Composition and development of conifer regeneration in thinned and unthinned natural stands of western hemlock and Sitka spruce in southeast Alaska

Abstract

Natural regeneration of understory conifers was studied in 1988, 9–14 years after thinning of even-aged stands of western hemlock (Tsugaheterophylla (Raf.) Sarg.) and Sitka spruce (Piceasitchensis (Bong.) Carr.) on upland sites in southeast Alaska. Two age-classes of stands were compared: young stands, <30 years old that before thinning had open forest canopies and understories of conifer regeneration, shrubs, herbs, and forbs; and older stands, 31–98 years old, that before thinning had closed forest canopies and little or no understory vegetation. In 1988, dominant understory conifers in young stands were 2–4 m tall, and about the same age as trees in the overstory. In the older stands there was dense new regeneration that germinated 2–3 years after thinning and averaged 0.6–1.5 m tall. Seventy-two to 100% of all regeneration was hemlock. The rest was Sitka spruce. The amount of regeneration increased and the percentage of hemlock decreased with increasing thinning intensity. Thinning of young stands on upland sites appears to benefit understory conifers, which rapidly expand to fill in the available growing space. Heavy thinning in older stands promotes dense germination of understory conifers making it difficult for other understory plants to become established.