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Abstract PremiseSouthern Africa is a biodiversity hotspot rich in endemic plants and lichen‐forming fungi. However, species‐level data about lichen photobionts in this region are minimal. We focused onTrebouxia(Chlorophyta), the most common lichen photobiont, to understand how southern African species fit into the global biodiversity of this genus and are distributed across biomes and mycobiont partners. MethodsWe sequencedTrebouxianuclear ribosomal ITS andrbcLof 139 lichen thalli from diverse biomes in South Africa and Namibia. GlobalTrebouxiaphylogenies incorporating these new data were inferred with a maximum likelihood approach.Trebouxiabiodiversity, biogeography, and mycobiont–photobiont associations were assessed in phylogenetic and ecological network frameworks. ResultsAn estimated 43 putativeTrebouxiaspecies were found across the region, including seven potentially endemic species. Only five clades represent formally described species:T. arboricolas.l. (A13),T. cf.cretacea(A01),T. incrustata(A06),T. lynniae(A39), andT. maresiae(A46). Potential endemic species were not significantly associated with the Greater Cape Floristic Region or desert.Trebouxiaspecies occurred frequently across multiple biomes. Annual precipitation, but not precipitation seasonality, was significant in explaining variation inTrebouxiacommunities. Consistent with other studies of lichen photobionts, theTrebouxia–mycobiont network had an anti‐nested structure. ConclusionsDepending on the metric used, ca. 20–30% of globalTrebouxiabiodiversity occurs in southern Africa, including many species yet to be described. With a classification scheme forTrebouxianow well established, tree‐based approaches are preferable over “barcode gap” methods for delimiting new species. 

Abstract Understanding how species-rich communities persist is a foundational question in ecology. In tropical forests, tree diversity is structured by edaphic factors, climate, and biotic interactions, with seasonality playing an essential role at landscape scales: wetter and less seasonal forests typically harbor higher tree diversity than more seasonal forests. We posited that the abiotic factors shaping tree diversity extend to hyperdiverse symbionts in leaves—fungal endophytes—that influence plant health, function, and resilience to stress. Through surveys in forests across Panama that considered climate, seasonality, and covarying biotic factors, we demonstrate that endophyte richness varies negatively with temperature seasonality. Endophyte community structure and taxonomic composition reflect both temperature seasonality and climate (mean annual temperature and precipitation). Overall our findings highlight the vital role of climate-related factors in shaping the hyperdiversity of these important and little-known symbionts of the trees that, in turn, form the foundations of tropical forest biodiversity.