What type of diversity yields synergy during mixed litter decomposition in a natural forest ecosystem?

Investigating the relationship of biodiversity and ecosystem function in natural forests allows incorporation of established feedbacks between long-lived plants and soil processes. We studied forested stands in northern Arizona that vary in dominant species richness across small areas. We examined the effects of natural variation in dominant tree biodiversity on ecosystem parameters, particularly litter decomposition. We determined not only whether plant species decompose in mixture as predicted by their individual decomposition rates but also: (1) how particular species affect the decomposition rate of each other in mixture; and (2) whether litter decomposes more rapidly at its site of origin; i.e. is there a "home field advantage" to decomposition? Over a 2-year period, litter mixtures of functionally similar tree species decomposed more rapidly than expected from rates of the individual species alone. Mixtures of conifer species litter decomposed up to 50% faster than expected, with individual conifer members of those mixtures decomposing up to 85% faster than expected. In contrast, more functionally diverse mixtures of litter, which included a deciduous species, did not show synergistic effects during decomposition. We found no significant "home-field advantage" to decomposition. Our study is the first to demonstrate that litter mixtures from more closely related plant species give rise to the most synergistic effects of biodiversity on litter dynamics, indicating that more taxonomically and functionally diverse plant assemblages do not always drive greater emergent effects on ecosystem function.