COS 51-2 - Searching high and low for life: exploring the microbiome of suspended canopy soils

Abstract: Old-growth forests are experiencing global decline resulting in loss of the world’s largest and oldest trees. As trees are physically removed (e.g., during logging) or experience increased mortality rates (e.g., due to environmental stress), other plants and animals lose a critical source of shelter and food. Hence, the decline of these trees likely has cascading consequences for ecosystem-wide biodiversity. Much of this endangered biodiversity dwells in the canopy, where tree branches have become home to a diverse assemblage of plants, animals, and microbes. Abundant life in the canopy has resulted in the accumulation of thick soil mats on tree branches as plant and animal biomass senesces and decomposes. These soils serve as an important nutrient source for branch-dwelling plants and the host tree itself via adventitious tree roots. Still, little is known about microbial ecology and nutrient cycling in suspended canopy soils. Given this knowledge gap, we ascended bigleaf maple trees (Acer macrophyllum) in Olympic National Park (Washington, USA) in September 2022 to sample their canopy soil. We sampled a total of 21 branches in seven trees (three branches per tree), ranging from 5 to 20 meters above ground. Soil was also collected from the forest floor adjacent to each tree. We aim to understand how soil-dwelling organisms (namely, micro-invertebrates, bacteria, and fungi) influence carbon and nitrogen cycling in the canopy. While data analysis is still in progress, we have observed that nematodes are at least twice as abundant in canopy soil as compared to ground soil. Similarly, microbial respiration, soil ammonium, and soil nitrate levels are higher in the canopy. Microbial biomass carbon increased with branch heights up to 15 meters but decreased to near ground soil levels in higher branches. These data indicate that a trove of diversity and activity dwells high in the canopy of old growth forests, providing key ecosystem services. As such, understanding these dynamics holds critical implications for preserving biodiversity in the face of a changing climate.