Successional patterns
Across the common and rare endophyte assemblages of three host tree species, evidence for deterministic succession was only found for the rare endophyte communities of one of the three woody host species (E. crispa ) and in none of the three assemblages of common endophyte species. Therefore, we find no consistent evidence of deterministic succession of endophyte communities within individual hosts across a woody-plant successional gradient. Under a scenario of deterministic succession, similar directional shifts in fungal endophyte community composition would have been observed since abiotic environmental conditions changed fairly consistently with BC size (Jamison-Daniels et al. , 2021). The lack of consistent directional changes in endophyte composition with BC size, but significant effects of the host identity on species composition, suggest that the effect of the plant host on endophyte composition trumps the impact that the consistent changes of environmental variables has on fungal endophyte communities. A large percentage of variation in the community assembly models remain unexplained, suggesting that unmeasured or random processes may contribute to the assembly of endophyte communities (Hubbell, 2001).
The lack of strong evidence for deterministic succession within endophyte communities does not preclude succession of endophyte communities of all trees per bush clump. Our study specifically assessed changes in endophyte composition within single host species across a tree successional gradient. Because tree composition changed in a directional manner with BC size (Jamison-Daniels et al. , 2021), and host identity is an important determinant of endophyte composition and richness (Liu et al. , 2019; U’Ren et al. , 2019; Harrison and Griffin, 2020) (Figure 2; Figure 4; Table 1), endophyte communities of entire BCs (i.e. across different tree species or within tree genera or families) could show stronger evidence of deterministic changes in endophyte composition. As a certain suite of tree species are associated with a specific successional stage in this system (Jamison-Daniels et al. , 2021), their associated endophyte communities may also be associated with the associated tree successional stage. Under this scenario, the succession of endophyte communities would be driven by changes in host species composition with successional stage, and not changes in environmental factors with successional stage.