Vertical distribution of roots along a soil toposequence in the New Jersey Pinelands

Abstract

Root biomass was determined for four sites on each of two toposequences in the New Jersey Pinelands. The sites have similar plant species composition, disturbance histories, and soil parent materials; the soils at the different topographic positions vary principally in the thickness of the organic horizon. Each toposequence consisted of lowland, transition, and upland sites, plus a site in the pygmy pine stands found on exposed topographic rises within the region. Root biomass was determined for three depths (organic horizon, 0–10 cm and 10–20 cm mineral soil) and three size classes (<1, 1–5, and >5 mm). Total, fine, and intermediate root biomass was greatest at lowland sites and least at sites in the pygmy pine stands. Total and fine root biomass in both the forest floor and also the upper mineral layer was linearly related to thickness of the organic horizon. The percent allocation of roots among soil layers, and the percentage of roots in different size classes within each soil depth interval, were similar for lowland, transition, and upland sites. The pygmy pine stands, in contrast, had higher relative quantities of total and fine roots in the mineral soil than in the organic horizon. In the organic horizons at all topographic positions, 50–100% of the roots are fine roots, whereas only 10–20% of the roots in the mineral layers are fine roots. Although root biomass per unit area varies >2-fold from lowland to pygmy sites, biomass per unit volume of organic material does not vary among topographic positions. Volumes of organic matter are more densely filled with roots than are comparable volumes of mineral soil. Implications for the role of roots in nutrient cycling in different soil horizons are discussed.