Altering light availability to the plant host determined the identity of the dominant ectomycorrhizal fungal partners and mediated mycorrhizal effects on plant growth

Abstract

Variation in light availability likely impacts the processes that determine the identity of ectomycorrhizal fungi associated with the host plant, and the resulting changes in fungal composition may modify the plant’s growth response to light. Our two field surveys and two field experiments using Abies balsamea (L.) Mill. seedlings show that the identity of the dominant ectomycorrhizal fungus changed in response to natural and experimentally induced variation in light. Plants in low light were mostly dominated by a Cenococcum -like morphotype, and plants in high light were mostly dominated by a Lactarius -like morphotype. The patterns of absolute abundance show the key role of plant size: the absolute abundance of the Lactarius-like morphotype increased with increasing light and increasing root system size, whereas the absolute abundance of the Cenococcum-like morphotype was unrelated to both variables. Root system size increased with light availability. With increasing light, growth of plants dominated by the Lactarius-like morphotype decreased with respect to average plants, and therefore, as the Lactarius-like morphotype was increasing in dominance, it was decreasing in mutualistic effectiveness. Plants dominated by Cenococcum had lower growth compared with plants dominated by Lactarius in low light. However, the effects of the two morphotypes were indistinguishable in high light. Our results are likely driven by an interaction of priority effects and light-limited plant growth.