A synthesis on the role of mycorrhizal fungal traits for understanding forest tree functions

ABSTRACT Almost all tree species are associated with mycorrhizal fungi, which help their hosts to acquire soil resources and influence the nutrient economy of the ecosystem. However, despite the fundamental nature of this symbiotic interaction, our understanding of the relationship between fungal properties and host functions is still in its infancy. We compiled a unique database of fungal properties and examine the properties and traits of mycorrhizal fungi in relation to the three main functions of forest trees: resource acquisition, tree production, and carbon release (C). Using quantitative synthesis and meta-analysis, we highlight the current strengths and knowledge gaps regarding tree species, spatial coverage and the relationships between fungal properties and tree functions. Most studies showed that the properties of fungal community assembly, host-symbiont interactions and soil exploration have a positive influence on the resource acquisition and production of host trees. According to a meta-analysis, the composition of the fungal community had a greater influence on resource uptake and tree production than taxon-specific effects. However, some properties of fungal community assembly were negatively related to root carbon (C) exudation (with low certainty), and the influence of these properties and soil exploration on carbon (C) release by host trees through root exudation and litter decomposition remained unclear. We conclude that the influence of mycorrhiza on important tree functions remains an important area for future research, particularly with regard to fungal properties relating to soil exploration and tree properties relating to C release. Such research should progress beyond laboratory measurements of soft traits and increasingly focus on the mycorrhizal fungal properties that are most important for ecosystem functioning.