Decomposition rates in the canopy and forest floor in an old-growth coastal temperate forest in Washington

Background/Question/Methods

Ecological roles of epiphytes and their associated canopy soils include capture and retention of fog and water, provide habitat for organisms and increase inputs of nutrient from the canopy to the ground.  Epiphytes capture water and nutrients from mist, rain and particulates that settle within the canopy, which then provide moisture and nutrients to canopy-dwelling organisms.  Nutrients accumulated in epiphytic mats can be released to terrestrially rooted plants when epiphytes fall to the forest floor, die, and decompose; or when epiphytic mats are leached by precipitation.  The goal of this study was to compare decomposition rates of epiphytic (plants and canopy soil) and forest floor material (O-horizon and tree leaves) under the canopy of Acer macrophyllum in a coastal old-growth forest in Washington.  The study was conducted in an >250 yrs. old-growth forest at the Queets river watershed, Olympic National Park, Washington.  Samples of epiphytic material were taken at an average height of 15 m, whereas samples from the ground were taken under the canopy of A. macrophyllum.  To access the forest canopy we used rope-based tree-climbing techniques.  Annual decomposition rates (k) were estimated using the nylon mesh bag technique or calculated from the mean residence time.

Results/Conclusions

After one year of incubation in the field, decomposition rate of green leaves of A. macrophyllum showed the highest values (k= 0.95 and 0.86 for the canopy and forest floor environment respectively), but we did not find significant differences between the canopy and forest floor environment (p < 0.1).  In the forest floor, we found significant differences in decomposition rates between A. macrophyllum and canopy soil; brown epiphytes and senescent A. macrophyllum leaves.  In the canopy, we found significant differences between A. macrophyllum and canopy soil.  These results suggest that differences in k are explained by the composition of the substrate more than differences between the canopy and forest floor environment.  Decomposition of materials in the canopy is important for understanding the development of canopy soils and their conservation. 

Keywords: Litter decomposition, canopy ecology, temperate forests.

Acknowledgments: Olympic National Park, Decagon Inc.