Density, biomass, productivity, and nutrient-cycling changes during stand development in wave-regenerated balsam fir forests

Regeneration waves in the high-altitude fir forests of the northeastern United States create a gradient of stands of different ages that are ideal for research on forest developmental processes. Changes in density, biomass, productivity, litterfall, and nutrient content of the vegetation over the course of stand development were documented in a series of these stands on Whiteface Mountain, New York. After a regeneration wave passes through an area and the overstory dies, formerly suppressed seedlings are released, and stand density increases to 10-13 trees/M2 at about age 10 (counting only stems over 25 cm tall). Stand density then begins to decrease, at first slowly but then more rapidly, with a maximum mortality rate of 20-25%/yr between years 20 and 30. Thereafter mortality continues, but the rate decreases over time to t4%/yr at age 55. After about age 20 the relationship between stand age and stand density is p = 0.04 - exp( 115/AGE). For stands that have begun to self-thin, the relationship between stand density and mean aboveground tree mass is log10 mn = 3.94 - 1.24 log1op, with a slope significantly different from - 1.5. Basal area (measured at 25 cm) increases for the first 15-20 yr of stand development, then remains constant at 60-70 m2/ha for the remainder of the life of the stand. Total aboveground biomass of a mature fir stand is z 11.8 kg/M2 (1.5 kg/M2 foliage, 1.7 kg/M2 branches, 0.9 kg/M2 bole bark, and 7.7 kg/M2 bole wood). The biomass accumulation rate is highest during the first decade of stand development (320 g m-2 yr- 1), and declines to z 115 g m-2 yr- I for a 55-yr-old stand. Foliage biomass increases rapidly during the first 10-15 yr of stand development, then remains constant after crown closure. Branch biomass increases until about age 15, declines for about a decade, then increases again for the rest of the life of the stand. Bole bark increases until about age 30, then stabilizes. Bole wood continues to increase throughout the life of the stand. Aboveground net primary productivity for a mature stand is 960 g m-2 yr-I (foliage, 380 g m-2 yr-1; twigs, 150 g-m-2 yr-1; branch wood and bark, 170 g.m-2 yr-1; bole bark, 25 g-m-2-yr-'; and bole wood, 235 g.m-2 yr-1). Productivity remains generally constant as the stand develops, but in the early stages almost 45% of the total aboveground production is devoted to producing bole wood and bark, while in a mature stand this proportion drops to z 25%, with the remainder going to foliage and branch wood production. There is thus a shift from production of boles to branches as the stand ages. This may reflect a shift in the primary stresses on the trees, from intraspecific competition in the younger stands to physical environmental factors (e.g., wind abrasion and rime formation) in the older stands. After the initial increase, woody tissue respiration remains approximately constant throughout the life of the stand, as an increase in the size and height of boles is balanced by death of small trees and a decrease in total bole surface area. A mature fir stand contains nitrogen, potassium, magnesium, and calcium in the amounts 47.7, 18.1, 4.6, and 30.4 g/m2, respectively. All these nutrients accumulate rapidly in the early stages of stand development as the stand accumulates nutrient-rich foliage. This is particularly true for nitrogen; a 9-yr-old stand contains half as much nitrogen as a 60-yr-old one. However, the difference between nutrient uptake by the trees and nutrient release in decomposition is greatest somewhat later in stand development, when the pulse of litter accompanying stand die-off has decomposed but the young stand is still taking up significantly greater quantities of nutrients than it is returning in litterfall. Thus if ecosystem nutrient losses are controlled by nutrient accumulation in living biomass, the period of maximum nutrient conservation should occur in the years immediately after disturbance, when the stand is accumulating nutrients most rapidly. If the critical factor is the balance between uptake and release, then minimum losses should occur later. Increased meteorologic inputs of nitrogen and acid in the high-altitude fir forests may have altered the natural nutrient cycles of the preindustrial era.