The temperate rainforests of Western Washington are known for their old-growth forests and unique ecosystem processes, including seasonal rainfall regimes and epiphytic mats. These forests are particularly vulnerable to drought conditions. Adventitious roots have been found to grow into these mats, and that has enhanced the diversity of fungal associates relative to that found on the ground. With increasing weather extremes, it is valuable to understand the role of any mechanistic relationships that could contribute to the drought resilience of these forests. However, the functional roles of these rooting networks and their fungal associates are not known. Therefore, the question driving this research is whether the diversity of fungal associates found on adventitious roots in canopy soils buffer these forests from drought impacts because of their pulsed acquisition of nutrients and water. A pilot study was conducted to test whether the community diversity of fungi found on adventitious roots differed between canopy and forest floor soils. Root-tips from the canopy and forest floor of nine old-growth Acer macrophyllumtrees were sampled to determine fungal diversity using DNA techniques, results were complemented by examining stained hyphal structures. Also, pulses of available phosphorus (P) were compared between canopy and forest floor soils.
Preliminary DNA and morphological analyses of fungi associated with canopy roots suggests they have adapted to form a diversity of fungal associates that differs from those associated with forest floor roots. DNA sequences were matched at 97% or higher, and inferences about diversity were based on a maximum likelihood phylogenetic analysis (bootstrap = 1000). Sequences that were identified to genus or species level suggest that the diversity of fungi associated with adventitious canopy roots may be dependent on the canopy soil environment. A majority of sequences identified Basidiomycetes and Ascomycetes as potential mutualists. Preliminary available and total phosphorus analyses also show that canopy soils have significantly higher available P levels, but forest floor organic soil environments have significantly higher total P. These preliminary results support the need for additional research on the functional roles of canopy soils, especially the role of canopy adventitious roots and their fungal associates relative to the forest floor soils. This would elucidate any potential role of canopy soils on adaptation of temperate rainforests to drought resilience. It further suggests the need to test seasonal differences in available and total P in both soils and their relationship to fungal diversity, which is currently being implemented.