Inoculation and Successful Colonization of Whitebark Pine Seedlings With Native Mycorrhizal Fungi Under Greenhouse Conditions

Efforts to maintain and restore whitebark pine (Pinus albicaulis) forests in western North America have increased dramatically over the last two decades and now include the planting of nursery-grown rust resistant seedlings in openings and burned areas. Over 200,000 nursery seedlings have been planted in the western U.S. but survival rates are low and in many areas approach zero. One possibility for enhancing seedling survival is application of mycorrhizal fungi in the greenhouse before out-planting. All pines require ectomycorrhizal fungi (ECM) to survive in nature, including whitebark pine. Non-mycorrhizal seedlings are at risk of dying when planted in soil lacking appropriate mycorrhizal fungi; this might include ghost forests, severe burns, dry habitats and areas not previously in pine. This study screened 25 isolates of native ECM fungi (primarily suilloids) from whitebark pine forests in the Greater Yellowstone Area as a preliminary step in development of an effective inoculum for whitebark pine seedlings grown in forest nurseries. Most are ‘suilloid’ fungi specific for 5-needle pines. A majority grew well in vitro and selected strains were then used to develop various types of mycorrhizal inoculum. Four basic inoculation methods were tested under greenhouse conditions using spore slurries and soil inocula. Spore slurries added to soil produced the highest rate of mycorrhizal colonization (100 per cent frequency) in the shortest time (3-5 months), but colonization also occurred with soil inoculum (mycelium). Fertilization was found to suppress mycorrhizal colonization at least at the high levels tested. Soil substrate was found to be an important factor in ECM colonization and some soil mixes suppressed or prevented colonization. Commercial inoculum is not recommended since it risks introduction of alien fungi, it may not favor 5-needle pines, and therefore has the potential to disturb sensitive whitebark pine systems. Here we report results from preliminary trials and discuss on-going research to provide up-to-date information.