Biomass and production of understory vegetation in seral sitka spruce-western hemlock forests of southeast Alaska Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/s7526f607

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  • Several understory conununities display successional stages during the first two hundred years following logging or fire disturbance in the coastal Picea-Tsua forests of southeast Alaska. Residual shrubs and tree seedlings increase their growth exponentially after overstory removal. Understory biomass peaks at about 5000 kghayr fifteen to twenty-five years after logging. Vascular plant understories are virtually eliminated (0-70 kgha^-1yr^-1) after forest canopies close at stand ages of twenty-five to thirty-five years. Bryophytes and ferns dominate understory biomass during the following century. A vascular understory of deciduous shrubs and herbs is reestablished after 140 to 160 years. Subsequent to this successional stage vascular understory biomass continues to increase, while bryophyte biotnass and tree productivity decline. Departures from this developmental sequence are related to unusual conditions of stand establishment, soil, microclimate, or disturbance. During the earliest phases of ecosystem development following forest canopy closure, the decline in understory development is associated with increases in tree basal area and higher percent tree canopy cover, In the oldgrowth forests increases in mean stand diameter, age, and biomass are correlated with increases in understory biomass, The pattern of understory development over the chronosequence is hypothesized as responding primarily to changes in the light environment wrought by developments in forest canopy structure. The development and duration of the depauperate understory stage that follows canopy closure in southeast Alaska was hypothesized as being related to the canopy characteristics of shade tolerant, high-leaf-area Tsuga forests. Maintenance of these forests in the aggradation stages of development (0-100 years) by forest management activites would minimize vascular understory vegetation development. Even including the pulse of vegetation growth during the first 30 years, understory vegetation productivity on normal soils would be less over a 100-year forest tree rotation than the annual productivity in old-growth forests. Food for herbivores and nutrient cycling processes associated with understory vegetation would be concomitantly minimized under this forest management policy.
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