Needle age and exposure drive compositional patterns of endophytic fungal communities in crowns of coastal Douglas fir (Pseudotsuga menziesii var. menziesii)

Background/Question/Methods

Old-growth Douglas-fir (Pseudotsuga menziesii var. menziesii) forests provide habitat to diverse lineages of macroscopic life. These forests modulate microclimates across watersheds, producing complex environmental and spatial gradients along which communities assemble. Less is generally known about how fungal communities in these crowns vary, however, as accessing crowns is often intractable, and previously applied sampling methods are biased towards culturable organisms. Additionally, studies to date have relied on crown position metrics as proxies for components of the microenvironment that are potentially more relevant to community assembly. Using high-throughput sequencing approaches and leveraging airborne laser scanning data, the endophytic mycobiomes and surrounding crown microenvironments of needles sampled along the boles of eight P. menziesii trees were characterized for increasing ages of needles. We hypothesized that (1) microenvironmental metric of crown closure would account for more compositional variation than the more positional metrics of height and depth in crown, and (2) that heights characterized by different crown properties are associated with divergent community structure with increasing needle age.

Results/Conclusions

Results showed that the extent of crown closure and the depth in crown accounted for more compositional variation than height. For needles in second-year age-class, a vertically-oriented, within-tree compositional signal was observed. By the time needles reached the third- and fourth-year age-classes, the within-tree compositional signal was lost and samples began to cluster according to exposure group (defined by having less or more crown closure than expected for a given height/crown depth). Needle communities from less exposed heights became more structured with increasing needle age, while more exposed communities lost structure over four years of age-classes. These findings highlight the importance of the microenvironment and its potential influence on long-term compositional dynamics on the canopy mycobiome for a foundation tree species.