The knowledge regarding the impact of fungal diversity, which is inherently involved in terrestrial ecosystem functions, on ecosystem functioning remains limited. Additionally, a systematic understanding of how environmental factors influence the relationship between fungal diversity and ecosystem functions is lacking. To understand the decomposition process of organic matter, which is one of the key ecosystem functions, we conducted a decomposition experiment using wood blocks of Fagaceae species in six forest sites in Japan. The wood blocks were retrieved one year after installation, and we measured the rate of mass loss and respiration. The fungal composition and abundance were analyzed using amplicon sequencing and quantitative PCR. Additionally, we obtained environmental factors such as soil temperature at the study sites and the moisture condition of the wood blocks. We then analyzed the relationships among these factors using structural equation modeling. Cumulative temperature had a direct positive effect on the mass loss rate and, in addition, indirectly influenced both respiration rate and mass loss rate through an increase in fungal DNA copy numbers and a decrease in fungal species diversity. These results suggest that, during the early stage of angiosperm wood decomposition, higher temperatures promote fungal growth while reducing fungal species diversity, ultimately leading to a decrease in decomposition rates. This structural relationship highlights the influence of environmental factors on fungal diversity and ecosystem functions. Future temperature increases due to global warming may accelerate the decomposition rate of organic matter by altering fungal abundance and species diversity.