Dynamics of Understory Biomass in Sitka Spruce-Western Hemlock Forests of Southeast Alaska

Understory vegetation undergoes successional stages during the 1st 300 yr after logging or fire disturbance in the coastal Picea—Tsuga forests of southeast Alaska. Residual shrubs and tree seedlings increase their growth within 5 yr after overstory removal. Understory biomass peaks at 5 Mg°ha — 1 °yr — 1 °15—25 yr after logging. Shrubs and herbs are virtually eliminated (<0.1 Mg/ha) from the understory after forest canopies close at stand ages of 25—35 yr. Bryophytes and ferns dominate understory biomass during the following century. An understory of deciduous shrubs and herbs is reestablished after 140—160 yr. Thereafter, biomass of the shrubs, herbs, and ferns continues to increase, while bryophyte biomass and tree productivity decline. Departures from this developmental sequence are related to unusual types of stand establishment, soil, microclimate, or disturbance. The development and duration of the depauperate understory that succeeds canopy closure in southeast Alaska is closely related to the canopy structure of shade—tolerant Tsuga forests with their high foliar biomass. In young—growth forests (<100 yr), the decline in understory development immediately after canopy closure is significantly associated with tree basal area and percentage of tree canopy cover. In old—growth forests, in contrast, understory biomass is correlated with mean tree diameter, age, and volume. It is hypothesized that understory development over the chronosequence responds primarily to changes induced in the light environment by developments in the forest canopy. Maintenance of the most productive forests in the aggradation stages of development (0—100 yr) through forest management will minimize the development of a productive vascular understory and thus deprive herbivores of forage during 70—80% of the forest rotation.