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Summary The scope of plant control over its microbiome is a central question in evolutionary biology and agriculture. Leaf traits are known to shape pathogen colonization and disease development, but their impact on the broader community of largely non‐pathogenic fungi that colonize plant leaves remains an open question.We used reciprocal common gardens of the model tree,Populus trichocarpa(black cottonwood), to examine relationships between leaf traits and the leaf mycobiome in two strongly contrasting environments. We measured six leaf traits (stomatal length, stomatal density, carbon‐to‐nitrogen ratio, leaf thickness, leaf dry matter content, and specific leaf area) and used fungal marker gene sequencing to characterize leaf fungal communities for 57 tree genotypes replicated in one mesic and one xeric common garden (809 trees).Several leaf traits covaried with the leaf mycobiome, yet one relationship was paramount: plant genotypes with longer, sparser leaf stomata hosted a greater richness and diversity of more similar fungal species compared to plant genotypes with shorter, denser leaf stomata.These relationships, while modulated by the environment plants were sourced from and grown in, suggest that stomatal traits may be a general mechanism through which plants and the leaf mycobiome influence one another. 

Abstract Habitat fragmentation resulting in habitat loss and increased isolation is a dominant driver of global species declines. Habitat isolation and connectivity vary across scales, and understanding how connectivity affects biodiversity can be challenging because the relevant scale depends on the taxa involved. A multiscale analysis can provide insight in biodiversity patterns across spatial scale when information on dispersal ability is not available, in particular for community‐level studies focusing on multiple taxa. In this study, we examine the relationship between arthropod diversity, patch area, and connectivity using a multiscale approach. We make use of a natural experiment on Hawai‘i Island, where historic volcanic activity has transformed contiguous native forests to lava matrix and discrete forest patches. This landscape of patches has persisted for 150 yr, and we selected 10,000 ha consisting of 863 patches to analyze landscape connectivity using a graph theory approach. We collected arthropod samples fromMetrosideros polymorpha tree canopies in 34 forest patches during multiple years. We analyzed the relationship of arthropod diversity with area, as well as with connectivity across increasing scales, or dispersal threshold distances. In contrast to well‐established ecological theory as well as prior work on birds and fungi in this system, we did not find support for a canonical species–area relationship. Next, we calculated connectivity across spatial scales and found lower Shannon diversity with higher connectivity at small scales, but no effect at increased dispersal threshold distances. We examined the landscape structure and found all habitat patches connected into three subnetworks at a 350 m threshold distance. All patches were connected at 700 m threshold distance, indicating structural dispersal limitation only at small scales. Our findings suggest that canopy arthropods are not dispersal limited at scales shown to impact both soil fungi and birds in this system. Instead, Hawaiian canopy arthropods may perceive the landscape as a connected area where discrete forest patches and the early‐successional matrix contribute resources that vary spatially with regard to habitat quality. We argue for the utility of multiscale approaches, and the importance of examining maintenance of biodiversity in fragmented landscapes that persist for hundreds of years.