Defoliation effects on plant and soil properties in an experimental low arctic grassland community - the role of plant community structure

In Northern Fennoscandia, sub-arctic and arctic grasslands are commonly grazed by the semi-domesticated reindeer. Reindeer grazing is known to affect plant production and belowground processes, such as nutrient mineralization in these grasslands, but little is known of the role of plant Community Structure in the response of plant and soil properties to the defoliation of plants. Using soil and seeds from a low arctic meadow, we established a 23-week greenhouse experiment to test whether communities of different plant species richness (one, two or four species) and composition (three different replicated compositions within each richness level) respond to defoliation in a different way. We defoliated plants twice to 2 cm above the soil surface in half of the replicates of each community and analyzed the response of different communities in terms of changes in plant mass accumulation and the abundance of soil organisms (microbes, protozoa and nematodes). We predicted that these parameters and their resistance to defoliation will: (1) differ among plant species compositions; and (2) increase with increasing plant species richness. We found that plant species composition affected shoot mass accumulation (comprising both defoliated and harvested shoot mass), harvested root mass (in nondefoliated systems) and the abundance of decomposer fungi in the soil. Defoliation increased the numbers of fungal-feeding and predatory nematodes regardless of plant community structure, while the effects on roots and decomposer fungi varied across plant species compositions. Of the measured variables, only the abundance of omnivorous Aporcelaimellus spp. nematodes responded to plant species richness and no evidence was found that the effects of defoliation on any variable were modified by species richness. Soil bacteria, arbuscular mycorrhizal fungi, protozoa and bacterial- and root-feeding nematodes did not respond to any treatment. Our results suggest that defoliation effects may to some extent differ between different plant species compositions in the low arctic grasslands grazed by the reindeer. However, it appears that they may not depend on plant species richness. (C) 2008 Elsevier Ltd. All rights reserved.