Endophyte richness
Endophyte richness was most strongly influenced by host identity, with endophyte richness of one host (S. chirindensis ) being consistently lower than that of the other hosts. In contrast, several abiotic and biotic variables showed inconsistent effects on endophyte richness across the three host species. Reduced endophyte richness at higher light intensity agrees with other studies: high levels of UVB negatively impact endophyte persistence, with high UVB increasing leaf desiccation and/or the activation of the plant’s defence responses, ultimately leading to lower endophyte richness (Unterseher et al. , 2007, 2012).
Other abiotic effects on endophyte richness differed between host species which may shed some light on how such factors drive endophyte richness in these host species. Lower endophyte richness in S. chirindensis occurred in warmer BCs that this species occupied. Temperature differences within BCs may drive differences in chemistry and secondary metabolite production produced by the leaves of S. chirindensis that experience higher temperatures (Reich et al. , 1999; Veteli et al. , 2002), potentially leading to lower ASV richness (Arnold and Herre, 2003; Unterseher et al. , 2012, 2013). High temperatures can activate the plant defence response, thereby increasing the production of secondary metabolites with potential anti-microbial properties (Unterseher et al. , 2016) which decreases the observed endophyte richness by reducing the number of successful endophyte colonisations. Taller trees are usually older, and older hosts have been shown to support decreased endophyte richness compared to their younger conspecifics (Oono et al. , 2015). It has been postulated that older trees invest more in defence mechanisms which resist endophyte colonisation; alternatively established groups of endophytes in older trees outcompete newly arriving endophytes, ultimately leading to lower endophyte richness in older trees (Unterseher et al. , 2007; Oono et al. , 2015).