Multiple mechanisms for trait effects on litter decomposition: moving beyond home-field advantage with a new hypothesis

1. Evidence is growing that leaf litter generally decomposes faster than expected in its environment of origin, owing to specialization of litter and topsoil decomposer communities to break down litter encountered most often. Nevertheless, this home-field advantage (HFA) in decomposition is inconsistently supported by experimental data and fails to account for situations where contrasting qualities of litter coexist within the same litter matrix. 2. In contrast to the HFA hypothesis, which expects a positive interaction between every litter species produced locally and the local decomposer communities irrespective of litter species quality, we define here an alternative substrate quality-matrix quality interaction (SMI) hypothesis that expects a continuum from positive to negative interaction between specific litters (substrates) and decomposer communities as specific litters and the ecosystemlitter layer (i.e. the matrix, which drives local decomposer community composition) become increasingly dissimilar in quality. 3. To test this hypothesis, we conducted a reciprocal transplant decomposition experiment of eight leaf, six fine-stem and nine fine-root litter species from three neighbouring ecosystems of the subarctic biome: dry forest, riparian forest and forest-surrounded pond; and characterized the quality (represented by lignin content and an integrated measure of carbon/nutrient economics) of each litter species and each ecosystem litter layer. 4. We found substantial overall effects of SMI on decomposition rates of leaf (20% explained variance), stem (14%) and root (15%) litters, although this effect was lower than the single effects of litter quality and microclimate (remaining explained variance). Despite being partly inconsistent across litter species, likely due to the complexity of litter quality-decomposer community relationships, the SMI hypothesis appeared more broadly applicable than the HFA hypothesis. 5. Synthesis. We demonstrate here that plant traits, likely via their control on litter and topsoil decomposer community composition, have indirect effects on litter breakdown rates, not only at the interface between ecosystems but also within ecosystems, with likely implications for many other ecosystems world-wide. These results suggest functional variation in decomposer communities between ecosystems with respect to their efficiency to degrade litters with contrasting qualities, such as different lignolytic and detoxification activities but also contrasting efficiencies to degrade non-recalcitrant tissues.