Background/Question/Methods Controls over the diversity and distribution of fungal groups are poorly known. This is especially true for saprotrophic fungi, despite their central role in many ecosystem processes such as nutrient cycling and carbon release into the atmosphere. One major driver of saprotrophic (decomposer) fungal diversity and function that may be generally important across ecosystems and latitudes is plant diversity, as the diversity of compounds found in different types of litter may determine the extent of resource heterogeneity for decomposer communities. Thus, we hypothesized that an increasing diversity of plant litter causes an increase in saprotrophic activity, which results in more enzyme activity, faster nutrient transformations, and increased CO2 release into the atmosphere. To test this hypothesis we examined ecosystem functions mediated by fungi in six inventory plots that represent a natural gradient in plant diversity in Alaska, California, and Panama. In each plot we measured extracellular enzyme activity, nutrient release, and soil respiration. To directly examine the effect of plant diversity, we experimentally recreated a plant diversity gradient in litter decomposition bags in each site with representative plants from each ecosystem. We examined mass loss and extracellular activity in each litter bag. Results/Conclusions Across ecosystems we found a significant effect of plant diversity on enzyme activity (P < 0.05), but not on soil respiration. Enzyme activity increased with plant species richness in the upper, organic layers of the soil (Oi-e), but not in the deeper horizons of soil. These results indicate that plant species richness may exert a direct effect on fungal function in the early stages of decomposition, but that these effects are muted when only the most recalcitrant compounds in soil remain. In litter bags with fewer species of plant litter, decomposition was significantly slower than in litter bags with greater numbers of species (P < .05). Enzyme activity followed the same trend, suggesting that fungal activity was also higher in bags with more plant species. Together, these results indicate that decreases in plant diversity that have been observed globally due to human influences may have a detrimental effect on ecosystem functions mediated by fungi across latitudes. Future research includes sequencing of fungi in plots and litter bags to determine if decomposer fungal diversity can be directly linked to the plant diversity and ecosystem function relationships observed in this study.