Factors influencing lotic leaf litter decomposition rates: results of a large scale study in Western streams

T. Simmons and C.P. Hawkins. AWER Department and The Ecology Center, Utah State University, Logan, Utah 84322

Leaf litter decomposition is an essential ecosystem process in heterotrophic streams. Accordingly, there is currently much interest in using litter decomposition as a measure of functional integrity or impairment in stream bioassessment. However, relatively little is known about how biotic and abiotic factors influence decomposition rates, even in relatively unimpacted streams. To begin to address this question, we measured aspen leaf decomposition rates in 51 reference-quality streams in Utah, Wyoming, Montana and Idaho using a standardized protocol. We also collected aquatic macroinvertebrates and a variety of physical habitat and climate data. Decomposition rates varied greatly, with exponential decay rate constants ranging from .002 to .045. We were unable to identify any simple relationships with habitat or climatic factors that might explain this variability. Similarly, the relationship between decomposition rate and various biologically based variables (e.g., macroinvertebrate species richness and compositionally derived NMDS ordination axis scores) was complex. However, the inclusion of biologically based variables allowed us to predict membership of streams into 3 broadly defined decomposition rate classes with either linear discriminant analysis or classification trees. These results suggest that biological differences among streams are at least as important as physicochemical processes in influencing litter decomposition rates.