Arboreal histosols in old-growth redwood forest canopies, Northern California

Within the canopy of old-growth redwood (Sequoia sempervirens [D. Don] Endl.) forests, accumulations of plant debris in crotches and on massive limbs serve as parent materials for "arboreal soils" up to 1 m thick and over 50 m above the forest floor. These soils are important habitats and water sources for desiccation-sensitive organisms in the forest canopy ecosystem. We investigated two of these arboreal soils to better understand how they form and function. The primary vascular epiphyte growing in these soils is leather-leaf fern (Polypodium scouleri). Fern biomass and redwood leaves and bark are the main parent materials of the soils, which are entirely organic. The soils have distinct horizons and soil structure, and have been changed from their initial parent materials as a result of additions, losses, and transformations of organic matter. They are classified as Typic Udifolists. The soils become more decomposed with depth as fibrous components transition to sapric materials. They have low bulk densities (0.07-0.16 g cm(-3)) and lose most of their water at relatively high matric potentials. Field water contents and potentials of the arboreal soil materials taken near the end of the dry season indicate that water remains available for uptake by ferns. High base saturation values (>55%) suggest that nutrients are available for plant uptake. The soils are extremely acidic, with pH values around 3.0 in 0.01 M CaCl2. Decomposition may be hindered by the low soil pH and by relatively low soil moisture contents in surface horizons during the dry season.