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The N-fixing shrub Alnus sinuata (Sitka alder) dominates recently deglaciated sites along Glacier Bay but is replaced by Picea sitchensis (Sitka spruce) after c80 yr. While Alnus dominated, soil organic matter and N increased very rapidly. Above-ground biomass was accompanied by a decline in soil N. In maturing Picea stands (100-160 yr old), N in above-ground biomass, especially current-year foliage, declined, perhaps because N began to accumulate in poorly decomposable organic (O) and podzolic (Bh) soil horizons. The O horizon of the Alnus site contained greater concentrations of most extractable macro- and micro-nutrients than that of Picea sites, suggesting rapid soil weathering, vegetative uptake, and cycling back to the O horizon. Soils under Picea experienced rapid podzolization. Associated with soil changes, above-ground net primary productivity of Picea decreased 50% over the 160-yr Picea-dominated portion of the chronosequence. Decreased net primary productivity could not be accounted for by increased respiratory cost of sapwood or foliage, suggesting a nutrient limitation to growth. Increase or decrease in productivity of these ecosystems appeared related to the effects of individual species on soil characteristics. -from Authors
Bormann et al. (Sat,) studied this question.