The role of stranding and inundation on leaf litter decomposition in headwater streams

Abstract

Discharge-driven shifts in the wetted area of streams can modify the amount of leaf litter resources available to stream consumers as well as the physical conditions to which leaf litter is exposed. The consequences of this continual movement from wet to dry storage for rates of organic matter processing and resource availability to benthic communities are poorly understood. We used a 30-day field experiment during the period of maximum stream contraction to examine the effects of stranding on black cottonwood (Populus trichocarpa) leaf litter decomposition rates and associated changes in microbial respiration in a forested stream in western Montana. Leaf litter was enclosed in both coarse and fine mesh bags and moved from the wetted area of the stream to the stream bank in six treatments designed to mimic a gradient of dry exposure due to stranding. We also measured existing accumulations of organic material in quadrats placed in both wet and dry areas of the stream. The total storage of litter resources (ash-free dry mass, g m2 ) retained on dry stream banks increased steadily as stream flow decreased, resulting from reductions in wetted width and continuous inputs from terrestrial zones. In contrast, total mass of stored litter submerged in the stream channel remained relatively constant. Leaf decomposition rates increased as a function of time inundated and were fastest in the presence of macroinvertebrates. Our results suggest that prolonged stranding can alter fundamental processes and energy pathways in stream food webs by shifting pools of resources from the active channel to dry storage on riverbanks where decomposition is driven primarily by microbial processes. Since the length of time that leaf litter is inundated prior to stranding alters decomposition rates, changes in stream hydrograph variability (as a consequence of land management practices or incipient climate change) has the potential to alter energy flow through stream systems. In particular, dry storage may function as a type of ‘temporal subsidy’ for stream organisms particularly if slowly decomposing stranded leaf litter is re-entrained during periods when in-stream detrital resources are otherwise scarce.