Biomass, net primary production, and nutrient distribution for an age sequence of Populustremuloides ecosystems

Abstract

The amounts and proportions of dry matter and macronutrients were measured in overstory trembling aspen (by tissue), understory vegetation, woody detritus, the forest floor, and the mineral soil in duplicate stands of 8, 14, 18, and 32 years and in a single stand of 63 years on an Entic Haplorthod in northern Wisconsin. The following changes occurred during secondary succession: (i) above- and below-ground aspen biomass increased, (ii) aboveground aspen production peaked between the ages of 18 and 32 years, (iii) the root to shoot ratio (R/S) of overstory aspen declined, (iv) the amounts and ecosystem proportions of macronutrients (N, P, K, Ca, and Mg) in the aspen overstory increased, and (v) the ecosystem proportions of nutrients in the understory declined. As the relative abundance of nutrients in overstory aspen increased, the labile nutrient pool in the mineral soil, particularly Mg and Ca, declined. The study suggests that understory roots play an important role in nutrient retention of successionary aspen stands in the upper Great Lakes region. Understory vegetation had an average R/S of 4.5 (compared with 0.18 to 0.58 for overstory aspen), with their small-root (< 0.3 cm) biomass comprising about 76% of the total ecosystem small-root pool. The proportion of nutrients in the living vegetation accounted for by the understory declined with stand age. However, from 13 to 24% of the total plant-available ecosystem amounts of nutrients resided in the understory of the 63-year-old stand. Relative to the ecosystem development model of Bormann and Likens, the reorganization phase in successionary aspen stands of the upper Great Lakes region may last 5 years or less. The aggradation phase lasts until about 70 years, after which time aspen is replaced by northern hardwoods.