Spatial effects of retention trees on mycorrhizas and biomass of Douglas-fir seedlings

Retention forestry places seedlings in proximity to residual trees, exposing seedlings to additional sources of ectomycorrhizal fun-us (EMF) inoculum. To investigate this, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were planted near (2-6 m) and far (16-30 m) from 44- to 72-year-old residual Douglas-fir trees in western Washington, USA. From 1998 through 2000, seedling shoot and root biomass was assessed and EMF taxa were identified using morphology and sequence analysis of internal transcribed spacer and large subunit ribosomal RNA genes. Seedlings near residual trees had significantly greater ectomycorrhiza (ECM) abundance (percent active ECM root tips), less necrotic root tips, and higher root to shoot biomass ratios. Seedlings near trees had a richness index of 4.1 EMF taxa per seedling and 42 total taxa compared with 3.5 taxa per seedling and 33 total taxa for seedlings far from trees. Proximity to residual trees may increase seedling ECM abundance and diversity.