There is high potential for the formation of common mycorrhizal networks between understorey and canopy trees in a mixed evergreen forest

1 The patterns of ectomycorrhizal (ECM) host specificity between understorey and canopy trees were investigated in three mixed evergreen forest stands in northern coastal California. ECM root tips from the dominant canopy ( Pseudotsuga menziesii ) and understorey ( Lithocarpus densiflora ) trees were sampled from 18 soil cores (six per stand) and identified using molecular techniques (PCR, RFLP, and DNA sequencing of the rDNA ITS region). 2 We found 56 ECM taxa; 17 on both hosts, 27 solely on Pseudotsuga and 12 on Lithocarpus . There were no significant differences in ECM taxon richness or diversity across stands, although ECM taxon richness was significantly higher on Pseudotsuga than Lithocarpus . ECM taxa similarity across stands was low. 3 Multiple-host ECM taxa had significantly higher abundance than single-host ECM taxa and 13 of the 17 multiple-host ECM taxa were present on both hosts within at least one core. Twelve of the 14 cores had at least one ECM taxon that was present on both hosts, although the specific taxon varied between cores and stands. In addition, shared ECM taxa often had unequal relative abundances on the two hosts. 4 Taken together, our results suggest that there is high potential for common mycorrhizal networks to form between Lithocarpus understories and Pseudotsuga canopies in mixed evergreen forests.