UC Santa Barbara

Ectomycorrhizal association is a prevalent symbiosis that plays a crucial role in supporting temperate and boreal forest ecosystems through the provisioning of fungal foraged nutrients for plant-derived carbon. The stability of this ecological relationship can be viewed as heavily dependent on environmental constraints and resource dynamics. To test how associations with distinct fungal communities impact the performance of Pinus muricata seedlings growing under controlled greenhouse conditions, we inoculated seedlings with ectomycorrhizal fungi using field-collected soil from the Jug Handle State Reserve Ecological Staircase in Mendocino, California. We collected field soils from two terraces which have been previously described to have distinct fungal communities, and established four inoculum treatments: one from Terrace 2 (T2), a relatively resource-replete site with tall trees and low carbon-demand fungi, one from Terrace 3 (T3), a comparatively resource-deplete site with dwarfed “pygmy” trees and high carbon-demand fungi, one with both T2 and T3 inoculum, and one control with neither. To track seedling performance under the four inoculum treatments, we performed aboveground measurements on all seedlings for a total of nine months and harvested plants in month 9 (end-of-experiment). Differences between inoculation treatments became more pronounced over time, with uninoculated control seedlings generally maintaining greater height, diameter, and growth rates relative to the inoculated treatments. Seedlings with high-resource (T2) inoculum showed greater representation of Byssocorticium and Rhizopogon occidentalis in their fungal communities, whereas under the same environment with low-resource (T3) inoculum, Wilcokina mikolae was often dominant. We found that having greater mycorrhization or diversity of fungi did not correlate with increased growth – and having any fungi, regardless of abundance on the root system, led to decreased seedling performance and growth. Our results indicate that reciprocal exchanges of nutrients and carbon are not invariably regulated by mutual benefit in ectomycorrhizal symbioses.