The impact of hemiparasitic plant litter on decomposition: direct, seasonal and litter mixing effects

1 Nutrient-rich litter of hemiparasites (and N-fixers) has the potential to influence ecosystem-level decomposition and nutrient cycling, which may be important in ecosystems where productivity is nutrient limited. This influence may be both direct, and indirect via impacts on the decomposition of other species' litters in mixed-species litter layers. 2 We investigate the importance of these direct and indirect effects of sub-arctic hemiparasite litter in a litterbed study and a field litterbag experiment, and compare these data with those from a microcosm study also involving litter mixtures. 3 Field-placed litterbags demonstrate for the first time the direct importance of hemiparasite litter on decomposition and nutrient cycling in a field setting: litter of the hemiparasite Bartsia alpina lost 18 times more N over 2 years than Betula nana litter. 4 The majority of this N was released during the growing season, strongly supporting the proposal that hemiparasites can have a substantial direct impact on nutrient availability to other plants. The short spring and autumn periods were important for litter mass and N dynamics, with consequences for predicting effects of environmental change. 5 However, hemiparasite litter did not indirectly affect the decomposition rate or nutrient release from litter mixtures in field-placed litterbags. Furthermore, the direction and magnitude of litter mixing effects on mass loss varied strongly between methods; interactions were absent or negative in the litterbed, non-significant in field litterbags and positive in microcosms. 6 We thereby demonstrate the importance of the decomposition environment, rather than species-specific litter quality, in determining litter-mixing interactions, with implications for evaluating the impact of particular species on ecosystem carbon and nutrient fluxes via litter mixing 7 We conclude that hemiparasites have a particular functional role in sub-arctic heath systems: in effect they 'short circuit' generally slow nutrient cycles, with potential impacts on resource patchiness and local scale biodiversity.