Effects of Plant Litter Species Composition and Diversity on the Boreal Forest Plant-Soil System

Although most plant products eventually enter the below-ground subsystem as plant litter, relatively little is known about the effects of plant litter diversity or composition on ecosystem processes and no study has considered the responses of plant growth to these factors. We conducted an experiment in which humus substrate was collected from three field sites in the boreal forest of northern Sweden. Litter was then placed on the humus surface, and the litter used consisted of monocultures of Empetrum hermaphroditum (dwarf shrub), Betula pendula (tree), and Pleurozium schreberi (feather moss), as well as mixtures containing all the possible (two-way and three-way) combinations of these species: the experiment was maintained in out-door conditions. Although decomposition rates of this surface-placed litter differed between species few effects of litter mixing on litter mass loss were apparent. Added litter of Pinus sylvestris litter broke down most slowly when placed in E. hermaphrodnum litter but sometimes showed elevated decomposition rates when placed in some of the multiple species litter mixes. Soil microbial biomass and activity was lowest when plant litter was absent, but as long as plant litter was present on the humus surface the species composition and diversity of the litter was irrelevant. There were few effects of litter treatments on growth of seedlings of either B. pendula or P. sylvestris planted into the humus. However, for one site there were significant effects of mixing litter of P. sclueberi and E. hermaphroditum in reducing growth of both seedling species. Litter treatments generally did not alter the competitive balance between B. pendula and P. sylvestris seedlings when grown together but for all sites litter treatments had significant effects on the overall intensity of competition, and mixing of litter of B. pendula and P. sehreheri had significant non-additive effects on competition intensity for two of the three sites. The abundance of mycorrhizae on seedlings was only weakly related to litter treatment but there were some positive effects of litter mixing on one of the most abundant mycorrhizal morphotypes on both species of seedlings for one of the sites. Our results suggest that litter presence was important in influencing a range of above ground and below ground properties and processes. In some instances individual species effects and litter mixing effects were also important but few general patterns emerged, and the nature of significant effects tended to be idiosyneratic. Ultimately our results show that plant litter has important afterlife effects which need to be considered in order to develop a more complete understanding of how biodiversity affects ecosystems.