Condo or cuisine? The function of fine woody debris in driving decomposition, detritivores, and their predators

Community structure and ecosystem function may be driven by the size or the energy within a given habitat, but these metrics (space and energy) are difficult to separate, especially in systems where the habitat itself is also food such as detritus. Only a handful of studies have attempted isolate potential mechanisms experimentally, which has left a notable knowledge gap in understanding the drivers of community structure and function. Here we tested whether fine woody debris (FWD) affects leaf litter communities primarily as a source of space or energy. We used a crossed factor design to isolate the effects of fine woody debris as space and energy, with four treatments: (1) no fine woody debris, (2) only energy-providing fine woody debris (sawdust), (3) only space-providing synthetic wood debris, and (4) a combination of both space and energy. We hypothesized that the highest levels of diversity, carnivore-to-detritivore ratio, and decomposition rate would occur on plots supplied with sawdust (representing energy), synthetic woody debris (representing space), or a combination of both, depending on the relative significance of fine woody debris as a source of either energy and space. After 7 months, FWD as a source of energy but not space led to decreased decomposer abundance and richness. Conversely, increased proportion of carnivores and labile substrate decomposition was primarily driven by FWD as a source of space. However, the fastest decomposition of more recalcitrant substrates required both space and energy (additive), and the synergy of space and energy supported the greatest proportion of carnivores. These results suggest that the presence of FWD in forest ecosystems supports increased diversity and decomposition through a synergistic interaction of space and energy and the maintenance of deadwood like FWD in forest ecosystems can thus significantly contribute to forest ecosystem function.

Methods
We measured site characteristics such as standing stocks of fine woody debris, leaf litter and litterfall. We then set up an experiment to test whether fine woody debris primarily serves as space or energy for leaf litter invertebrates and their function (decomposition rate). Litterbags were placed on plots, with standard substrates collected monthly for 7 months and natural substrates collected at the end of the experiment. Fine woody debris was either added as just space (synthetic fine woody debris) or just energy (ground fine woody debris). Leaf litter invertebrates were collected and identified at the end of the experiment. Chemistry measurements were analyzed for soils and initial chemistries of natural substrates. Data were collected from the field and laboratory components of the project.