Trait functional diversity explains mixture effects on litter decomposition at the arid end of a climate gradient

Litter decomposition is controlled by climate, litter quality and
decomposer communities. Because the decomposition of specific litter types
is also influenced by the properties of adjacent types, mixing litter
types may result in non-additive effects on overall decomposition rates.
The strength of these effects seems to depend on the litter functional
diversity. However, it is unclear which functional traits or combination
of traits explain litter mixture effects and if these depend on the range
of trait values and the ecosystems involved. These uncertainties hamper
our ability to predict decomposition in plant communities. We aimed at
understanding whether and how functional diversity (measured as functional
dispersion, FDis) influences litter decomposition, and how this influence
varies among different climates and across decomposition stages. We
calculated FDis based on litter traits related to nutrient concentrations
or to litter recalcitrance, and tested whether these diversity measures
and climatic parameters (soil moisture and temperature) explained litter
mixture effects on decomposition. Additive mixture effects (i.e.
decomposition of mixtures equalling the mean decomposition of the single
litter types) were common in most of the evaluated climates. Non-additive,
negative effects were mainly restricted to the driest and warmest sites,
and decreased with time. Non-additive effects increased in magnitude with
the mixtures’ FDis, with positive effects being related to FDis in
nutrient traits and negative effects being related to FDis in
recalcitrance traits. Synthesis: Litter mixing did not have strong effects
on decomposition rates across the studied climatic gradient overall, and
the direction and intensity of the mixture effects were context-dependent.
The effects were stronger and more negative in the dryer ecosystems. Where
effects were found, functional diversity calculated from selected groups
of traits (related to nutrients or litter recalcitrance) predicted mixture
effects, especially where trait ranges were broad, though much of the
variation remains unexplained. We propose that functional diversity
metrics based on litter traits that are mechanistically relevant, applied
to diverse site-specific litter mixtures in different climates, can help
to better understand under which conditions and in which direction litter
diversity affects decomposition.