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Dryad

Suspended sections within downed dead wood are drier, have altered decomposer communities, and slower decomposition

Abstract

The decomposition of dead wood plays a key role in forest carbon emissions. Most pieces of downed dead wood are partially suspended above the forest floor, but how this suspension affects decomposition rates is typically ignored and remains largely unexplored. Here we combine field observations and experimental manipulations to explore how partial suspension of downed wood (i.e., wood debris in contact with the ground) influences decomposer communities and patterns of decomposition in a lowland tropical forest. Experimental manipulations of wood sticks showed that small-scale suspension above the forest floor (ca. 5 cm) slowed decomposition and altered microbial community assembly, regardless of whether the suspended section was connected to a piece of downed wood. Across a 41-year chronosequence of dead trees, the average percent of wood volume suspended above the forest floor decreased during the initial 10 years post-death, but this trend reversed after 10 years, with the oldest logs being the forest floor had less moisture, fewer macrofungi, and more photosynthetic growth (e.g., moss, algae, etc.) than downed sections of the same bole. Surprisingly, wood density, termite presence, and mass-specific respiration did not differ with ground contact. Combined, these data suggest that suspension within downed wood reduces moisture content, influences decomposer community assembly, and contributes to the strong variability in decomposition rates. The strong effect of partial suspension within downed wood pieces heightens concerns about the accuracy and applicability of experiments and surveys focused on down dead wood, which are the foundation of our understanding of wood decomposition and associated carbon losses most suspended. Among downed woody pieces sampled in situ, sections suspended above the forest floor had less moisture, fewer macrofungi, and more photosynthetic growth (e.g., moss, algae, etc.) than downed sections of the same bole. Surprisingly, wood density, termite presence, and mass-specific respiration did not differ with ground contact. Combined, these data suggest that suspension within downed wood reduces moisture content, influences decomposer community assembly, and contributes to the strong variability in decomposition rates.