Epiphytic litterfall in an old-growth temperate forest at Olympic National Park, Washington

Background/Question/Methods

Fluxes of fallen litter provide a pathway for nutrient and energy transfer from plants to soil, providing  inputs to soil organic matter.  Litterfall inputs can be influenced by a variety of factors including climate, seasons, forest type and site productivity. In forests where epiphytic materials (EM) are a conspicuous component of the total biomass, epiphytic litterfall (such as moss, lichens, or vascular plants) can be an important contributor to total litterfall biomass. Our objective is to quantify biomass and seasonality of epiphytic litterfall relative to total aboveground litterfall. Inquiries such as the decomposition rate and contribution to litterfall from fallen branches containing EM will also be studied. The study site was located at the Queets river watershed, Olympic National Park, Washington, in a >250 yrs. old-growth forest dominated by Picea sitchensis.  Litter collection was associated with the canopy of Acer macrophyllum using two sizes of littertraps: small (0.25 m²) and large (27 or 54m²) traps for fine and coarse litterfall respectively.  Samples were collected monthly from July 2010 to November 2011. These samples were air dried and separated into woody debris, epiphytic material, and reproductive parts and leaves (according to plant species). The sorted EM was also weighed and recorded for further analysis.

Results/Conclusions

Fine litter deposition is higher during the fall and is correlated with the leaf abscission from A. macrophyllum.  Deposition of fine epiphytic litterfall was constant during the year, reaching its maximum during the winter.  Litterfall from P. sitchensis was highest during the winter time as well.  Highest values of coarse litterfall were registered during the spring 2011.  Overall, we concluded that contribution of coarse epiphytic materials to litter biomass followed seasonal patterns associated with inputs of woody material.  Nutrients accumulated on epiphytic mats have an allochthonous origin (mist, rain and particulates).  The pool of nutrients accumulated in epiphytic mats (EM) can be released to terrestrially rooted plants in three ways: a) epiphytic mats are leached by precipitation, b) nutrients can also be captured for the host tree via canopy roots, or c) EM falls to the forest floor, dies and decomposes.  Thus, inputs of epiphytic litterfall could be enhancing the nutrient cycling in this forest.

Keywords: Epiphytic litterfall, temperate forests, Pacific Northwest.

Acknowledgments: Olympic National Park, Decagon Inc.