Because reproductive propagules of endophytes are airborne, it is plausible that they may disperse across landscapes and thus traverse boundaries of proximate but distinct forest types, with little definition due to landscape-level gradients climate or seasonality. However, recent studies argue that despite apparent host generalism in terms of affiliations, many fungi in tropical forests demonstrate strong functional specialization, interacting with particular host species, or with hosts that have particular traits, in distinctive and ecologically important ways 7. At a larger scale, environmental factors such as temperature, precipitation, and vegetation are relevant to endophyte community structure in all biomes surveyed to date 21, 22, 23, 24, underscoring the interplay of abiotic and biotic drivers in shaping assemblages of these hyperdiverse and important symbionts.Įndophytes in tropical forests rarely demonstrate strict-sense host specificity, instead forming distinct communities in co-occurring host species 25, 26. They typically are transferred horizontally as airborne spores and hyphae, forming communities that are structured at local scales by host phylogeny and traits such as secondary metabolites and leaf phenology 15, 18, 19, 20. These foliar endophytic fungi (hereafter, endophytes) are abundant and highly diverse in tropical forests, where they play important roles in protecting plants against pathogens 15, altering leaf water relations 16, and influencing leaf photosynthetic efficiency 17. Tropical forest trees support and interact with a tremendous diversity of associated organisms, with particularly high diversity occurring among the fungal symbionts that affiliate with living tissues such as leaves. Similar patterns also are observed in plant-associated animals such as pollinators (e.g., hummingbirds and butterflies 10, 11), emphasizing the importance of climate, both in absolute terms and in terms of seasonality, as a key driver of biodiversity across not just the tropics but on a planetary scale (see also refs. These factors come together to define broad patterns in the distribution of tropical tree communities at local to landscape scales, with classical work showing that climate (temperature and precipitation) and seasonality (intraannual shifts in temperature and precipitation) play an essential role: wetter and less seasonal tropical forests typically harbor higher tree density and diversity compared to drier and more seasonal tropical forests, partly because of intense pressure from natural enemies under stable and productive environmental conditions 3, 8, 9. In highly diverse tropical forests, tree diversity is structured by the interplay of edaphic and climate characteristics, and by associated biotic interactions with dispersers and natural enemies such as herbivores and pathogenic fungi 3, 4, 5, 6, 7. Understanding how species-rich communities persist has roots in ecological studies of biotic interactions and abiotic factors at local to landscape scales 1, 2. 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. Endophyte community structure and taxonomic composition reflect both temperature seasonality and climate (mean annual temperature and precipitation). Through surveys in forests across Panama that considered climate, seasonality, and covarying biotic factors, we demonstrate that endophyte richness varies negatively with temperature seasonality. 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. 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. Understanding how species-rich communities persist is a foundational question in ecology.
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