Evan M. Gora and Jane M. Lucas
When trees are damaged or die, their biomass (leaves, trunks, and branches) decomposes with major implications for global carbon cycling. However, our understanding of decomposition focuses on the forest floor, whereas approximately half of dead wood mass is elevated or suspended above the ground. Decomposition rates are slower above the forest floor, and previous work had indicated that contact with the soil was the most important factor influencing vertical differences in decomposition. We conducted two experiments in a lowland tropical forest of central Panama to test how resources provided by soil, specifically nutrients and microbial colonizers, influence decomposition rates above the forest floor. Our first experiment manipulated epiphytes because they are a natural source of soil resources in the canopy. Specifically, we placed wood sticks and cellulose disks in each of three canopy-level treatments: 1) an intact epiphyte mat, 2) a control trunk section, and 3) a “removal” treatment in which an epiphyte and underlying organic matter were removed. After 14 months, we recorded mass loss of all substrates and the elemental content of wood sticks. We used DNA-based identification to characterize the fungal and prokaryotic communities of the underlying trunk or epiphyte sections at the beginning of the experiment and the wood sticks at the time they were collected. Our second experiment tested which nutrients limit decomposition of cellulose disks in the understory via a factorial nitrogen and phosphorus fertilization experiment.
Our findings indicated that fungal dispersal limitation and reduced nutrient availability slow decomposition above the forest floor. Mass loss was fastest on the epiphyte mat, intermediate in the removal treatment, and slowest in the control. Fungi exhibited dispersal limitation in the canopy, but prokaryotes did not. Moreover, local colonization by fungal saprotrophs was associated with faster decomposition rates in the epiphyte treatment. In the fertilization experiment, only the phosphorus treatment increased decomposition rates relative to the controls, indicating that decomposition above the forest floor was limited by phosphorus availability. These findings are important because they indicate that our ground-biased information is missing important decomposition processes that occur between the understory and the canopy.