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 considere d the responses of plant growth to these factors. We conducted an experimen t in which humus substrate was collected from three field sites in the bore al forest of northern Sweden. Litter was then placed on the humus surface a nd the litter used consisted of monocultures of Empetrum hermaphroditum (dw arf shrub), Betula pendula (tree), and Pleurozium schreberi (feather moss), as well as mixtures containing all the possible (two-way and three-way) co mbinations of these species; the experiment was maintained in out-door cond itions. Although decomposition rates of this surface-placed litter differed between species few effects of litter mixing on litter mass loss were appa rent. Added litter of Pinus sylvestris litter broke down most slowly when p laced in E. hermaphroditum litter but sometimes showed elevated decompositi on rates when placed in some of the multiple species litter mixes. Soil mic robial biomass and activity was lowest when plant litter was absent, but as long as plant litter was present on the humus surface the species composit ion and diversify of the litter was irrelevant. There were Few effects of l itter treatments on growth of seedlings of either B. pendula or P. sylvestr is,is planted into the humus. However, for one site there were significant effects of mixing litter of P. schreberi 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 is seedlings w hen grown together but for all sites litter treatments had significant effe cts on the overall intensity of competition, and mixing of litter of B. pen dula and P. schreberi had significant non-additive effects on competition i ntensity for two of the three sites. The abundance of mycorrhizae on seedli ngs was only weakly related to litter treatment but there were some positiv e effects of litter mixing on one of the most abundant mycorrhizal morphoty pes 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 a nd below ground properties and processes. In some instances individual spec ies effects and litter mixing effects were also important but few general p atterns emerged, and the nature of significant effects tended to be idiosyn cratic. Ultimately our results show that plant litter has important "afterl ife effects" which need to be considered in order to develop a more complet e understanding of how biodiversity affects ecosystems.