COS 275-6 - Local and regional drivers of litter decomposition across eleven temperate forests

Abstract: Regional patterns in climate have long been shown to control rates of carbon cycling in forests. However, recent work suggests that these broad-scale patterns may be highly dependent on local-scale variation. Indeed, the decomposition of leaf litter may respond to fine spatial variation in belowground communities and the controls on their activities. We test this idea using a litter decomposition study at eleven eastern US forests in the National Ecological Observatory Network (NEON). At each site, we collected leaf litter of three species and deployed litterbags at thirty six locations (twelve locations per species) that capture local scale variation in topography and soil variables including pH, soil moisture, and microbial activity. Alongside each litterbag we also included a wooden dowel as a common substrate. We returned after ten months and collected the litterbags and measured the mass loss of the leaf litter and the dowel.

We found wide variation in the decomposition rate of leaf litter among both species and sites. On average, leaf litter lost 32% of its original mass, but this varied from 16% in Pinus palustris in a southern pine forest to 56% for Liriodendron tulipifera in a mid-Atlantic hardwood forest. Within a single species that occurred across seven sites, Acer rubrum, mass loss varied from an average of 24% in a cold, dry forest to 54% in a warm, wet forest. Within each site, variation in Acer rubrum mass loss was of a similar magnitude: for example, at one forest Acer rubrum mass loss varied from 12% to 45%. Variation within a species at a site was often associated with local variation in soil moisture, which did not align with mean annual precipitation patterns. Mass loss from the common substrate was also highly variable both within and among sites: dowels lost on average only 12% of their original mass in a northern mixed hardwood mesic forest, but 45% in a southern hardwood bottomland forest with seasonal flooding. Within sites, soil pH was weakly positively associated with dowel mass loss. Across all sites, mass loss from dowels and leaf litter in the same bags were positively correlated (r = 0.227), but dowel mass loss did a poor job predicting variation in litter mass loss within a forest. These results illustrate the importance of quantifying local scale variation (e.g. in soil moisture) to better understand how decomposition responds to regional drivers (e.g. mean climate).