Journal of Ecology 2004 © 2004 British Ecological Society Blackwell Publishing, Ltd. The impact of hemiparasitic plant litter on decomposition: direct, seasonal and litter mixing effects HELEN M. QUESTED*, TERRY V. CALLAGHAN*†, J. HANS C. CORNELISSEN‡ and MALCOLM C. PRESS* *Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK, †Abisko Naturvetenskapliga Station, S 981 07 Abisko, Sweden, and ‡Department of Systems Ecology, Faculty of Earth and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1087, 1081 HV Amsterdam, the Netherlands Summary 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; inter- actions 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. Key-words: ecosystem processes, functional groups, hemiparasitic plants, litter mixtures, nutrient cycling, plant–soil interactions, seasonal decomposition, sub-arctic, winter processes Journal of Ecology (2004) doi: 10.1111/j.1365-2745.2004.00951.x Introduction In sub-arctic habitats, plant growth is characteristically limited by nutrients, in particular nitrogen, and com- munities are typically dominated by slow growing, long lived perennials (Berendse & Jonasson 1992; Callaghan & Jonasson 1995; Aerts & Chapin 2000; Wookey 2002). Evidence is accumulating that the functional attributes of plant species can have important implications for ecosystem properties, for example decomposition rates and nutrient availabilities to plants (Hobbie 1992; Wardle et al. 1997; Lavorel & Garnier 2002; Eviner & Chapin 2003). Many plants typical of sub-arctic habi- tats have long leaf life spans and low tissue nutrient concentrations, resulting in low nutrient losses from Correspondence: Helen M. Quested, Department of Botany, Stockholm University, S 106 91 Stockholm, Sweden (tel. + 46 8 163757; fax + 46 8 162268; e-mail helen.quested@botan.su.se).