Functional trait diversity of leaf litter and its influence on biological processes of forest ponds

Background/Question/Methods:

The diversity of functional phenotypic traits is a key driver of ecological function.  Within woodland aquatic ecosystems, the decomposition of leaf litter and its subsequent influence on nutrient cycling and ecosystem function is controlled by multiple phenotypic, chemical traits that differ substantially across all plant species.  It is hypothesized that increased chemical trait diversity results in non-additive, synergistic effects on decomposition, nutrient cycling, and overall ecosystem function.  We tested this hypothesis in outdoor, 800-L pond mesocosms by using recent theoretical advancements offering computational methods to calculate indices of dissimilarity among multiple species in multivariate trait-space.  We measured 10 chemical traits of 20 common coniferous and deciduous litter species, including traits relevant to nutrient content, structural integrity, and litter toxicity.  Using the resulting trait-by-species matrix, we manipulated the chemical diversity of mixtures containing four litter species in equal biomass.  Three levels of chemical diversity (i.e. low, medium and high) were replicated 20 times.  Litter, along with multiple species of snails, amphibians, detritivores, zooplankton, algae, and microbes were added to outdoor mesocosm.  For 3 months, we measured multiple abiotic and biotic responses relating to ecosystem function, including decomposition rate, consumer and producer density and productivity, net primary production, microbial respiration and diversity (via RFLP assays).  In general, we predicted that responses would increase as trait dissimilarity increased. 

Results/Conclusions

Contrary to predictions, most biological and abiotic responses did not change as litter chemical diversity increased.  Differences among diversity treatments were weak, although there were effects of individual litter species.  Our measurement of chemical diversity may have been affected by equal weighting of all chemical traits, as it is likely that some traits are more related to ecological processes than others.  Intriguingly, after comparing mixture responses with previous work using monoculture assemblages of litter species in similar mesocosms, mixture responses were generally additive or synergistic.  These results suggest that relatively small amounts of chemical diversity in litter mixtures are sufficient to synergistically increase, and potentially maximize ecosystem function.