Foliar fungal diseases have a great influence on both photosynthesis and ecosystem function. However, information on how the elevation gradient (which is one of the most important biogeographic factors) affects foliar fungal diseases is scarce. Here, we did an investigation along 3200 m ~ 4000 m in an alpine meadow and arranged 30 quadrats collecting data of foliar fungal diseases, plant composition and soil properties to study the plant community-mediated (through changes in plant biomass, diversity, phylogenetic structure and community composition) and soil-mediated (through changes in soil properties) effects of elevation on community-level foliar fungal diseases. Based on linear models, we found that elevation did not significantly affect most of the individual plant species disease severity and community pathogen load. Meanwhile, a combination of community proneness and Pielou’s evenness index was the best model in predicting pathogen load. The structural equation model further confirmed that although elevation significantly changed both the plant community indices and soil properties, elevation mainly drove pathogen load via plant community-mediated effects, rather than soil-mediated effects. Hence, we provided new empirical evidence of the plant community-mediated effects on plant diseases by changing the composition and the evenness of plant community along elevation. Our study will improve the predictability of plant diseases, especially under the background of global climate change.

Our experiment was conducted along an elevation gradient ranging from 3200 m a.s.l. ( 37°36'39'' N, 101°18'16'' E) to 4000 m a.s.l. (37°42'29'' N, 101°22'27'' E) on the south slope of Qilian Mountains in Menyuan County, located in the northeastern Qinghai-Tibetan Plateau. We established thirty 0.5 × 0.5 m plots along the elevation gradient. Specifically, we established 6 plots (replicants) on flat ground at each of five elevations: 3200 m a.s.l. (37°36'39'' N, 101°18'16'' E), 3400 m a.s.l. (37°39'58'' N, 101°20'20'' E), 3600 m a.s.l. (37°41'47'' N, 101°21'34'' E), 3800 m a.s.l. (37°42'13'' N, 101°22'14'' E) and 4000 m a.s.l. (37°42'29'' N, 101°22'27'' E). The plots were randomly selected with at least 10 m buffer zone between two adjacent plots at each elevation.

In early August 2020, we harvested all the plant aboveground parts at ground level and sorted them into plant species for each plot. Then we dried them at 65℃ for 48 hours to constant mass and weighed to 0.01 g as the species aboveground biomass. We also collected four soil cores (5 cm in diameter and 10 cm in depth) from each plot and pooled them as one sample. Soil moisture content (W; %) was measured gravimetrically after 5 h of desiccation at 120℃. A pH analyzer and a conductivity analyzer were used to measure the soil pH (pH) and soil conductivity (C; ms/s), respectively. 5 grams of fresh soil were extracted with 50 ml 0.2 M KCl for 1 h at 60 rev s^-1 using a shaker, then nitrate-nitrogen (NO₃^-; mg/kg) and ammonium-nitrogen (NH₄⁺; mg/kg) were measured using an auto-analyzer (AA3, Bran-Luebbe, Germany).

We recorded disease severity (i.e. % leaf area covered by fungal lesion; V_i) from five leaves randomly selected from five individuals for each plant species in each plot. And we recorded all available leaves for species with less than 25 leaves. Pathogen identification mainly followed identification manuals including Fungal Identification Manual.