The availability of limiting nutrients plays a crucial role in shaping communities of endophytes. Moreover, whether fungal endophytes are host-specific remains controversial. We hypothesized that in a harsh and nitrogen (N)-deficient area, diversity and community composition of foliar endophytic fungi (FEFs) varied substantially among plots with experimentally elevated levels of macronutrients, and thus, N availability, instead of host species identity, would have a greater influence in structuring fungal communities at different scales. We also expected an important subset of taxa shared among numerous host species and N gradients to form a community-wide core microbiome. We measured the leaf functional traits and community structures of FEFs of three commonly seen species in an alpine meadow nested with a long-term N fertilization experiment. We found that host plant identity was a powerful factor driving the endophytic fungal community in leaves, even in habitats where productivity was strongly limited by nitrogen (p < 0.001). We also found that within the same host, nitrogen was an important driving force for the composition of the endophytic fungi community (p < 0.05). In addition, the leaf carbon content was the most important functional trait that limited the diversity of endophytic fungi (p < 0.001). Finally, we documented a distinct core microbiome shared among our three focal species and N gradients.