Canopy soils, litterfall and litter decomposition in a coastal old-growth temperate rainforest, Washington

Abstract

The canopy of trees has been an unreachable portion for scientific exploration until few decades ago, and the wonders and enigmas that the treetops enclose are increasingly unfolding. This study investigated the canopy environment and its epiphytes on the formation of canopy soil, transfers of biomass and N to the ground and decomposition processes in relation to climatic variables in a coastal temperate rainforest in the Queets River watershed, Olympic National Park, Washington. Canopy soils developed in bigleaf maple (<italic>Acer macrophyllum</italic> Pursh) and Sitka spruce (<italic>Picea sitchensis</italic> Bong. Carriere) canopies were studied. Canopy soils developed in bigleaf maple were classified as Typic Haplohemist, whereas Sitka spruce canopy soil was classified as Typic Haplosaprist. Bigleaf maple canopy soils are dominated by hemic horizons, have lower bulk density and C/N ratio relative to Sitka spruce canopy soils, which have lower pH, N content, CEC and extractable N. Relative to the forest floor, canopy soils have lower total C and N content, however canopy soils in this ecosystem are enhancing the pool of C and N by 20% and 25%, respectively, relative to the C and N pools of the forest floor. Annual litter inputs under bigleaf maple were 4700 kg ha<super>-1</super> with the biggest biomass contribution from maple leaves. Epiphytic litterfall contributed with 550 kg ha<super>-1</super> of the total litter inputs. This is nearly 12% of total annual litter input, the highest contribution of epiphytic litterfall registered. Carbon and nitrogen returns in litter were dominated by maple; epiphytic litterfall contributed 240 kg ha<super>-1</super> yr<super>-1</super> of C and 5.7 kg ha<super>-1</super> yr<super>-1</super> of N to the forest floor. In Pacific Northwest forest productivity is often limited by N availability, and high inputs of N from leaves and epiphytes under the canopy of big leaf maple could make this tree a hotspot of N in this old-growth ecosystem. Decomposition rates of green and senescent bigleaf maple leaves were determined in the bigleaf maple canopy and on the forest floor beneath maples. Decomposition rates of canopy and forest floor Oi horizons were also determined on the forest floor. Decomposition rates after two years ranged from 0.15 yr<super>-1</super> (for canopy Oi) to 0.58 yr<super>-1</super> (for green maples leaves). Soil temperatures had seasonal variation and were not significantly different between the canopy and forest floor. Moisture content was significantly lower in the canopy soils compared with soils on the ground. Despite significant differences on the moisture regime between the canopy and forest floor, there was no significant difference in decomposition rates between the canopy and forest floor. Wet/dry cycles of canopy soils and potential activity of soil organisms (that live in canopy soils) might have influenced decomposition aboveground. Decomposition rates for this study were strongly correlated to N concentration of the substrate. The results of these studies highlighted the importance of the canopy environment in general as hotspots of C and N to the whole forest. The canopy of big leaf maple in particular could be a subsidiary of biogeochemical activity in the area.