A defining characteristic of Washington’s old-growth rainforests is the presence of epiphytic mats on tree branches. In old-growth stands, the epiphytic standing crop was estimated to weigh 6,800 kg/ha, and organic canopy soils develop underneath reaching depths >40 cm. Old-growth trees, such as Acer macrophyllum, have adapted to the presence of canopy soils by growing extensive adventitious rooting systems that have unique fungal associates. Yet, there remain many unknowns regarding nutrient availability for adventitious root allocation and acquisition, and how these processes may be impacted as these forests face seasonal extremes. The objective of this study was to compare net nitrogen and phosphorus mineralization between the canopy and forest floor soils under ambient conditions, and determine if wetter and drier conditions impacted the fluxes of available nitrogen and phosphorus in canopy soils. To do so, 12 trees in the Hoh and Queets rainforests were rigged for access to the canopy. In each plot, two trees were manipulated with wetter or drier conditions at the canopy soil level. Triplicate soil samples were collected seasonally for a year, and mineralization rates were determined using the tube method. Available nitrogen and phosphorus was extracted using the KCL and Bray-P method, respectively.
Results/Conclusions
Both the Hoh and Queets rainforests had higher amounts of available phosphorus in the forest floor soils than available nitrogen. The Queets had more phosphorus available for plant uptake than the Hoh forest floor, with net phosphorus mineralization of 346.62 kg/ha-1/yr-1 compared to 272.52 kg/ha-1/yr-1. In contrast, the Hoh had higher amounts of available nitrogen, with a net mineralization of 73.63 kg/ha-1/yr-1 compared to 39.68 kg/ha-1/yr-1. Canopy soils notably contributed to the available nitrogen and phosphorus pools. In the Hoh, net mineralization for nitrogen and phosphorus was 7.37 kg/ha-1/yr-1 and 22.07 kg/ha-1/yr-1, respectively. In the Queets, net mineralization for nitrogen and phosphorus was 14.24 kg/ha-1/yr-1 and 35.63 kg/ha-1/yr-1, respectively. There were seasonal differences in available nitrogen and phosphorus. Immobilization was observed, but was dependent on the sampling season, treatment, and tree. This study shows that canopy soils provide available nutrients for the host trees’ adventitious rooting system and other epiphytic organisms. Each adventitious rooting network has a suite of fungal mutualists that are likely accessing these nutrients for the host tree. Overall, this study demonstrates that the canopy soil environment acts as a nutrient reserve and may be a critical component to forest resiliency, as these ecosystems face climatic extremes.