The relative role of species richness versus evenness on the decomposition of multi-species leaf litter mixtures in a Piedmont stream

In detritus-based ecosystems, decay of organic matter influences nutrient cycling, soil formation, downstream nutrient transport, and is the rate that detritus becomes available to higher trophic levels.  Senesced leaf litter comprises a significant portion of the detrital pool, and the decay of this material can be governed by interspecific variation in litter chemistry.  Estimates of decay rate of litter mixtures has revealed that diverse litter assemblages can decay both in an antagonistic and synergistic manner.  While these experiments are useful, they often manipulate leaf species richness while holding evenness constant.  However, species-specific availability of leaf litter is rarely equal.  We performed a litter mixture decay study and created two sets of treatments for each leaf species combination, (1) those with species equally represented (even) and, (2) those reflecting the availability of each species in the environment (uneven).  We ascertained the latter by surveying the input rates of litter to a small, Piedmont stream over leaf fall.  Tulip Poplar, Green Ash, American Beech and Chestnut Oak comprised >75% of the inputs to this first order stream.  All combinations of 1, 2, 3 and 4-species mixtures were created as 6g leaf packs, placed in the stream in December 2006, and retrieved periodically to determine mass remaining.  Analysis of decay rate between even and uneven mixtures of the same species combinations revealed that uneven mixtures decayed faster than the even (k=0.0177/d vs. 0.0206/d).  Further analysis showed that mixtures of Tulip Poplar+Green Ash, and Tulip Poplar+Green Ash+American Beech were driving this pattern.  This work highlights the importance of leaf species diversity to litter decay in streams, and the interaction with species composition and evenness.  Loss of tree species from streamside forests has implications for in-stream organic matter processing, and could be complicated by the relative abundance of certain species.