Shaping fungal communities in Cenchrus setaceus: host fitness and habitat filtering

We investigated the fungal leaf communities of Cenchrus setaceus across plant fitness categories (high vs. low) and environmental zones (coast vs. hill; trade-wind exposure) on Tenerife (TF) and La Palma (LP). We hypothesized that community assembly reflects both host-driven deterministic filtering and abiotic promotion of richness in favourable environments via two mechanisms: i) high-fitness plants promote stable, guild-structured communities; ii) humid, topographically buffered zones enhance fungal richness, especially for endophytes and saprotrophs. Nanopore sequencing and functional guild annotation revealed island- and zone-specific fungal assemblages. In TF, low-fitness plants hosted stress-tolerant or pathogenic genera, while high-fitness plants, especially in humid northern hills, supported niche-associated saprotrophs and pathogens. In LP, high-fitness plants in eastern hill zones were associated with distinct taxa, while drier western coastal low-fitness plants were enriched in stress-related fungi. OTU richness (Hill0) was consistently higher in low-fitness plants (TF: 146 vs. 95; LP: 94 vs. 76; p < 0.05), while Shannon diversity diverged: greater in high-fitness plants on LP (3.29 vs. 2.98), but lower on TF (3.10 vs. 3.28; p < 0.05). Community structure was shaped primarily by fitness in TF (PERMANOVA R² = 8.6%, p < 0.05), and by zone in LP (R² = 15.0%, p < 0.05). Fitness effects were signficant across all fungal guilds in TF (χ² = 11.7–38.8, p < 0.001), while zone × fitness effects shaped richness patterns in LP, with saprotrophs richness increasing 2.7-fold in humid hills. Overall, high-fitness plants supported less diverse but compositionally stable fungal communities, while favourable environments enhanced guild richness independently of host condition.