Ecosystem-level significance of acid forest soils

Abstract

The distribution of plant species is generally thought to be determined by an interaction of soil factors functioning within the constraints of climatic factors. The objectives of this paper are to focus on the significance of soil factors, particularly extreme soil acidity, on ecosystem processes of a southeastern U.S. pine forest, and to address the hypothesis that these acid soils represent the primary component of the ecosystem, controlling such processes as nutrient cycling and plant species dynamics. Soils at this Coastal Plain forest site were derived from acidic, clayey marine-deposited sediments, creating the potential for low-nutrient, acidic soil conditions. Field data support this observation. Mean pH for the top 5-cm of soil throughout the watershed was 4.3. Extractable Na, K, Ca, and Mg were 0.48, 0.50, 5.07, and 3.21 meq kg⁻¹, respectively, suggesting that base saturation could be <10%. ‘Available’ N (extractable NH ⁺₄ NO ⁻₃ ) was 0.51 meq kg⁻¹. Such conditions are well-suited for the loblolly and longleaf pines that dominate this system. These pines produce acid, nutrient-poor litter, which accumulates from resultant slow decomposition. Fire thus has become important in this ecosystem, selecting for plant communities which are both fire-adapted and acid-tolerant. Coastal Plain pine flatwoods soils appear to be sensitive to further acidification and other related changes in soil and soil solution chemistry from long-term acid deposition.