Context dependent effects of ectomycorrhizal species richness on tree seedling productivity

While there has been much recent interest about the relationships between p lant diversity and plant productivity, much remains unknown about how the d iversity of mycorrhizal fungi affects plant productivity. We investigated t he effects of ectomycorrhizal fungal community composition and diversity on the productivity and growth characteristics of seedlings of two tree speci es (Pinus sylvetris and Betula pendula) as well as their interactions with each other. This involved setting up a mycorrhizal fungal diversity gradien t from one to eight species using a design previously demonstrated to be ab le to separate diversity effects from compositional effects. We found that the eight mycorrhizal fungal species differed in their effects on seedling productivity and that the nature of effects was determined by the fertility of the substrate. Fungal species richness effects were also important in a ffecting seedling productivity over and above what could be explained by "s ampling effect" but only in some situations. For B. pendula in a low fertil ity substrate there were clear positive causative effects between fungal sp ecies richness and productivity with the eight species treatment having ove r double the productivity of any of the eight monoculture treatments; no di versity effects were, however, detected in a high fertility substrate. For P. sylvestris in a high fertility substrate there were significant negative effects of fungal diversity on productivity while in a low fertility subst rate no effects were apparent. The possible mechanistic bases for these res ults are discussed. The growth of P. sylvestris relative to that of B. pend ula when grown in combination was unaffected by mycorrhizal treatments. Our results provide clear evidence that effects of mycorrhizal fungal diversit y on productivity are context dependent and may be positive, negative or ne utral depending on the situation considered.