Effects of invertebrates and local environment on decomposition rates of dead plant and animal biomass along an elevation gradient

Decomposition of necromass, that is, dead biomass of plants and animals, is an essential process across ecosystems, primarily influenced by climate, necromass characteristics, and decomposer communities, but local environmental conditions like vegetation and soil can also modify large‐scale patterns. While single necromass types have been studied intensively, it remains poorly understood how patterns and drivers of decomposition vary across necromass types originating from plants and animals. Here, we quantified how elevation, as a proxy for macroclimate, and local conditions (microclimate, vegetation, soil) interactively affect decomposition rates and invertebrate effects across necromass types. We conducted a comprehensive decomposition experiment including five types of plant or animal necromass (carrion, dung, leaves, beech wood, and spruce wood) along an elevation gradient (600–2000 m above sea level [asl]) in the German Alps. To assess the effects of invertebrates on decomposition of different necromass types, we implemented two treatments that either gave access to invertebrates or excluded them. We found that invertebrates had idiosyncratic effects on decomposition across necromass types, with positive effects for carrion and spruce wood, negative effects for dung and leaves, and no effect for beech wood. Decomposition rates declined significantly with elevation for carrion and beech wood, whereas the other necromass types showed no significant relationship with elevation. Local environmental conditions, especially higher tree cover and stronger microclimatic buffering led to lower decomposition rates of carrion, leaves, and beech wood, but higher decomposition rates of dung and spruce wood. Soil pH was particularly relevant for the decomposition of carrion and beech wood. For carrion, the contribution of invertebrates to decomposition significantly declined with increasing elevation. Our results demonstrate that patterns and drivers of decomposition show little generality across necromass types in temperate mountain forests. Local environmental conditions played a more important role in determining decomposition rates of all studied plant and animal necromass than the macroecological gradient of elevation. Invertebrates are important for decomposition of most necromass types but they can either accelerate or decelerate decomposition. This highlights that both necromass characteristics and local environmental conditions need to be considered to better understand decomposition processes at the ecosystem level.