Role of mycorrhizal networks in facilitating seedling establishment

Interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) is an ectomycorrhizal (EM) tree species with broad host receptivity. In the field, Douglas-fir seedlings can be colonized with native EM fungi by linking into a mycorrhizal network (MN) with neighboring trees. Failure of Douglas-fir to regenerate on clearcuts in the Interior Douglas-fir zone (IDF) may be partly due to poor early EM and MN formation. Variable retention forestry (i.e., where trees are retained during harvest) may help conserve EM fungal diversity and allow an MN to form, permitting resource transfer from retained trees to seedlings, and increasing seedling survival. In 2004, two field experiments were established where Douglas-fir seedlings were planted 0.5 m from retained Douglas-fir saplings. Seedlings were planted either with links into the MN of saplings, or into mesh bags with different size pores designed to exclude roots and EM hyphae of different sizes. Saplings and seedlings were pulsed-labeled with ¹³CO₂ and ¹⁴CO₂ during the summer of 2006 to quantify net carbon transfer via the MN. Initial results show that MNs increased seedling survival, water status, EM species diversity, and EM community similarity between residual saplings and seedlings. These results suggest that MNs are ecologically significant for regenerating forests in dry climates.