Microplastics and forest fungi: A review and call for comprehensive research

Fungi are the main drivers of the global carbon and nutrient cycle and act as ecosystem engineers in forest ecosystems by regulating primary production and decomposition. Moreover, fungi are among the most diverse organisms in forest ecosystems and affect almost all forest microhabitats, from the canopy to the soil. In contrast to aquatic and agricultural ecosystems, forest ecosystems have received less attention in assessing the effects of microplastic pollution on biodiversity and ecosystem functioning. However, recent studies have demonstrated that significant amounts of microplastics might reach even remote forests. Thus, we summarize our current understanding of how microplastics enter forests and move within ecosystems to assess potential risks to diverse fungal habitats. Next, we summarize the state of the art concerning how microplastics affect fungal individuals, fungal diversity, and related ecosystem processes. Our review revealed that forest management and recreational activities are the most important direct pathways, whereas atmospheric deposition is recognized as the main indirect pathway by which microplastics enter forest ecosystems. The movement and dynamics of microplastics within forest ecosystems result from complex abiotic and biotic mechanisms; however, these mechanisms remain poorly understood. On the basis of our current understanding of the different exposure pathways, we suggest that microplastic contamination is biologically relevant across all fungal habitats. Nonetheless, evidence is scarce regarding how microplastic loads affect forest fungal diversity across functional lifestyles and related ecosystem processes. Direct evidence is limited, but soil studies from agricultural areas and laboratory experiments suggest that important forest processes, such as primary production and decomposition, are affected via changes in fungal diversity caused by microplastic contamination. We argue that current microplastic research fundamentally underestimates plastic loads and the related consequences for fungal diversity and ecosystem functioning in forest ecosystems. To address this challenge, we propose a conceptual framework based on ecological theory and provide testable hypotheses. Our framework could act as a blueprint for other forest taxa and thus improve predictions about the consequences of microplastics on the many functions and services that forests provide.