The complex socio-spatial architecture of Rhizopogon spp. mycorrhizal networks in xeric and mesic old-growth interior Douglas-fir forest plots

Mycorrhizal networks (MNs) can influence tree establishment and resource competition but little is known regarding their underlying architecture in situ. This study examined the socio-spatial architecture of MNs between Rhizopogon spp. genets and interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) trees in an old-growth forest. MN features were contrasted between plots with xeric versus mesic soil moisture regimes as a proxy for changes in site water stress anticipated with climate change. My objectives were to: (1) describe the fine-scale spatial patterns and autecological traits of R. vesiculosus and R. vinicolor mycelia systems and compare these between xeric and mesic plots; (2) describe the spatial patterns and architecture of Rhizopogon spp. MNs at the forest stand scale; (3) contrast MN architectures between phytocentric and mycocentric perspectives and between xeric and mesic plots, and identify critical determinants of MN architectures. Rhizopogon vesiculosus mycelia occurred deeper, were more spatially prolific, and colonized more tree roots than R. vinicolor mycelia. Both species were associated with moist microsites within plots, and had more prolific mycelia in mesic compared to xeric plots. The occurrence of R. vesiculosus shifted in the presence of R. vinicolor towards deeper soil horizons, suggesting competition and foraging strategy are important for niche partitioning between these species. At the forest stand scale, Rhizopogon spp. genets spanned tens of metres and colonized up to 19 trees, but R. vesiculosus genets were larger and linked more trees than R. vinicolor genets. Multiple tree cohorts were linked, with saplings and mature trees sharing the same fungal genets. Across all plots, the physical size of individual trees or fungal genets was positively related to their MN connectivity. This together with size asymmetries among different genets and trees resulted in the self-organization of complex, hierarchical scale-free MN architectures. The MNs appear robust to random perturbations but susceptible to the loss of large trees or fungal genets. No MN structural differences were found between phytocentric and mycocentric models or between xeric versus mesic plots. The pervasive mycelia and extensive MNs formed by these Rhizopogon spp. could influence interior Douglas-fir stand dynamics and resistance to water stress.Forestry, Faculty ofGraduat