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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 CaCI2. Decomposition may be hindered by the low soil pH and by relatively low soil moisture contents in surface horizons during the dry season. [ABSTRACT FROM AUTHOR]

Organic soils up to 1 m thick cover tree surfaces within the canopies of old-growth redwood [Sequoia sempervirens (D. Don) Endl.] forests 50 m or more above the ground. Very little is known about litter quality and litter decomposition processes within these canopies. A combination of solid-state cross-polarization magic-angle spinning 13C nuclear magnetic resonance (NMR) spectroscopy, natural 13C and 15N isotopic analyses, and proximate analyses were used to gain insight into soil organic matter accumulation and decay processes within two arboreal soils. Arboreal soil, redwood, and fern (Polypodium scouleri Hook. & Grey.) litter as well as epiphyte roots were analyzed. Proximate analyses showed that the arboreal soil parent materials contained a high concentration of the acid residue fraction (i.e., Klason lignin), from 379 g kg-1 for fern rhizomes up to 652 g k-1 for redwood bark litter. This high concentration of acid residue fraction in the soil parent materials probably promotes organic matter accumulation within the tree crown. Results from 13C NMR spectroscopy analyses indicated a preferential decomposition of carbohydrates and an accumulation of alkyl structures from the redwood and fern litter to the < 2-mm soil materials and from the Oi to the Oa horizons in both soils. On the other hand, there was no consistent variation in terms of C/N ratios and stable isotope composition along the depth gradient, suggesting that organic matter in these soils has undergone limited microbial processing but instead reflects the selective preservation of recalcitrant litter moieties. [ABSTRACT FROM AUTHOR]