Changes in productivity and distribution of nutrients in a chronosequence at Glacier Bay National Park, Alaska

(1) By assessing productivity and ecosystem nutrient distribution, this work extends the early classic studies of the influence of N2 fixers and conifers on soil development and nitrogen accumulation in a primary succession chronosequence at Glacier Bay, Alaska, U.S.A. (2) The nitrogen-fixing shrub Alnus sinuata (Sitka alder) dominates recently deglaciated sites along Glacier Bay but is replaced by Picea sitchensis (Sitka spruce) after about 80 years. (3) While Alnus dominated, soil organic matter and nitrogen (N) increased very rapidly. Above-ground biomass accumulated much more rapidly as Picea gained dominance over Alnus. Increase in biomass was accompanied by a decline in soil N. In maturing Picea stands (100-160 years 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. (4) 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. (5) Associated with soil changes, above-ground net primary productivity of Picea decreased 50% over the 160-year 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. (6) Increase or decrease in productivity of these ecosystems appeared related to the effects of individual species on soil characteristics. The amount and type of organic matter and its location within the ecosystem appeared highly correlated to the past, present and future productivity of these stands. The importance of future disturbance regimes in altering organic matter and nutrient distribution is emphasized.