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, a recent study demonstrated that airborne microplastics are present even in remote forests, averaging 11.9 ± 7.1 particles m⁻³ of air. Thus, we summarize our current understanding of how microplastics enter forests and move within these 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. Furthermore, while a few studies have investigated microplastic effects in forest ecosystems, their number is currently too small to support robust conclusions or any broader generalization regarding impacts on fungal diversity and related ecosystem processes. However, from other ecosystems, particularly agricultural soils, we have evidence that microplastic changes fungal diversity and influence key ecosystem processes, including primary production and decomposition, with the direction and magnitude of these effects depending on microplastic properties, concentration, and environmental conditions. From this, we hypothesize that microplastic contamination should be biologically relevant across fungal forest habitats with consequences for fungal diversity and the associated ecosystem processes. To address this challenge, we propose a conceptual framework based on ecological theory and provide testable hypotheses. Although taxon-specific mechanisms will require tailored sampling and analyses, our framework could serve as a general blueprint for assessing microplastic impacts across forest ecosystems and improving predictions of their effects on biodiversity, ecosystem functioning, and ecosystem services.