The amputated limb study: Effects of movement from canopy to forest floor on temperate rainforest epiphyte and arboreal soil communities
In wet temperate forests, canopy-held live epiphytes and canopy soils (epiphytic material, EM) enhances nitrogen (N) cycles through fixation, capture, and storage of atmospheric N. In old-growth rainforests on the Olympic Peninsula, accumulations of epiphytes can be significant, up to 80 kg per tree, with arboreal histosols up to 30 cm thick. When EM falls to the forest floor, it declines and disappears, potentially releasing nutrients impounded from atmospheric sources to the ecosystem. This shift in N dynamics – from accumulating organic matter in the canopy environment to decomposition and N release on the forest floor - presents a unique opportunity to manipulate substrates to understand N dynamics, from the standpoints of soil ecological processes and microbial community composition. We investigated elements that might affect the shift from accumulation to decomposition of fallen EM: physical disturbance, microclimate differences; and/ changes in microbial communities. Our experimental study involved placing replicate segments of cut limbs and their EM on the forest floor and suspended within the canopy, and monitoring the vitality of the mats over 24 months, comparing these to undisturbed controls. We also compared the microclimatic regimes and the microbial communities of canopy vs. forest floor soils.
Canopy soils were more acidic, sustained a lower moisture content in the dry season than terrestrial soils beneath them. After 24 months, we found no significant differences in vitality between EM on canopy-suspended and canopy-attached canopy branch segments. However, samples on the forest floor all had an overall lower vitality index (i.e., higher rate of decline) than canopy counterparts. EM attached to branches maintained a higher vitality index than EM placed directly on the forest floor. Environmental DNA was extracted from canopy and forest floor soil samples, with ca. 50,000 sequences of 16S rDNA amplicons (Illumina MiSeq platform). We found a striking difference between the canopy and ground soils in the proportion of bacterial families associated with nitrification (Nitrospiraceae, Nitrosomondaceae). Canopy soil has much smaller amounts (and in 80% of the samples, none) of the nitrifying bacteria that are common in ground, which suggests that certain processes of decomposition might be suppressed in the arboreal microhabitat. EM decomposition was much faster on the forest floor environment, and even faster when EM has direct contact with the forest floor, but we do not know whether these differences are due to changes in the environment or the microbial decomposer community.