Not seeing the tree for the Forest: Scattered trees can be unexpected hotspots of fungal diversity

Scattered trees are hubs of biodiversity. Here, we present a study in which fungal communities in the caulosphere of scattered individual trees were compared to those of woodlands. We compared alpha and beta diversities of wood- and bark-inhabiting fungi from the stems of trees across seven vegetation types belonging to three species: Pinus muricata , Quercus agrifolia and Notholithocarpus densiflorus . Scattered pines were among the investigated vegetation types. Correlations between stand density, or other tree features, and alpha diversity matrices, were studied with six Generalized Linear Models (GLMs), while the effect of spatial dispersion of trees was studied by correlating Ripley's K values with diversity. Results show that scattered trees represent a hub for biodiversity of wood and bark fungi. We found that caulosphere fungal richness in scattered pines was higher than that of pines growing in stands and was as high as that in notoriously biodiverse oak woodlands. Beta diversity analyses showed that the high fungal diversity in scattered pines is explained by the large number of taxa unique to pines, by a significant overlap of fungi between scattered pines and other vegetation types, and by a significant number of fungi unique to scattered pines. The GLMs showed significant correlations between high species richness, Shannon's and Simpson's indices and low forest density. Finally, we show that the fungi in or on the stems of more dispersed vegetation types are more diverse. These surprising results suggest that preserving or planting scattered trees is a cost-effective strategy that could support as much, or even more, caulosphere fungal biodiversity than entire woodlands. Arrows indicate sharing of caulosphere fungal taxa among oak woodlands, pine stands, other sources and scattered pine trees. Color of the arrows refers to discrete and unique fungal communities, such as those pertaining to a host and/or to a specific location. Multiple colors indicate multiple sources. Thick arrows indicate a significant portion of the community is shared, including abundant taxa. Thin arrows indicate sharing of a limited number of taxa, mostly represented by non-abundant taxa. Scattered trees result highly diverse because, thanks to their dispersed distribution bordering both oak woodlands and pine stands and other vegetation types, they can share taxa from multiple sources. Note that direct sharing among scattered pine trees or between scattered trees and pine stands may be absent. • In a rapidly changing world, preservation of biodiversity is a challenge. • Fungi are key, versatile and hyper-diverse organisms of most ecosystems. • The mycobiome is extremely diverse in tree stems. • The caulosphere mycobiome of scattered trees is as or more diverse than that of trees in woodlands. • Preserving scattered trees across the landscape may enhance fungal biodiversity.