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Coastal temperate old-growth forests of the Olympic peninsula contain large accumulations of epiphytic material and decomposed organic matter known as canopy soils.   Canopy soils provide substrate and habitat for a broad community of vascular and non-vascular epiphytes, insects, and other arboreal species, and could be a subsidiary source of nutrients for temperate forests. In this study we characterize the canopy soils of Sitka spruce (Picea sitchensis) and big leaf maple (Acer macrophyllum) trees in an old-growth temperate rainforest in Olympic National Park.  Soil samples were obtained at a mean height of 45 m and 15 m above the ground for spruce and maple respectively. To access the forest canopy we used rope-based tree-climbing techniques.  Soil characterizations include bulk density (BD), pH, CEC, mineral content, fiber, C and N content, and N-availability.  

Sitka spruce soil depth ranged between 15 cm and 20 cm, whereas Maple soil depth ranged from 13 cm to 30cm.  Two explanations for the variation in these results are, (1) maple trees drop their leaves during the winter and some of this organic material remains in the canopy environment, and (2) by dropping their leaves, maples increases the light available for the moss development and growth.

Both canopy soil types exhibit acidic pH (4.77 spruce, 4.05 maple; p-value < 0.001) with very low BD values (0.12 spruce; 0.06 maple, p-value<0.018).  Furthermore, we found an increase of BD with depth for both soil types.  Fiber content is high in both soil types reaching values above 60% in some cases caused mainly by the abundance of rhizomes from the cryptogams that form this organic suspended soil.  

Results from this study indicate that Sitka spruce soils have higher levels of decomposition, overall higher BD, and higher hemic and sapric materials compared with Maple canopy soils.  Sitka spruce canopy soils are formed in large spruce trees of an approximate age of 250 -300 years, whereas maples in the study area do not reach  ages >200 yrs.  

This study characterizes for the first time the properties of canopy soils in Sitka Spruce and big leaf Maple in the area where epiphytic mats and canopy soils can comprise more than 10 Mg of biomass ·ha-1.  Understanding the functions and characteristics of canopy soils will provide a framework for recognizing canopy ecosystems, as well as providing an