Can greenhouse inoculation with native mycorrhizae improve planted jack pine and black spruce survival and growth? A 25-year retrospective assessment

We evaluated the long-term effects of greenhouse inoculation with native ectomycorrhizal fungi on the survival and growth of planted jack pine ( Pinus banksiana) and black spruce ( Picea mariana) in boreal Ontario, Canada. Between 1997 and 1999, operational plantations were established with inoculated (IC) and non-inoculated (NI) seedlings, and we reassessed ten paired trials 20 to 25 years later. We evaluated both stand-level (i.e., tree density, basal area, and gross total volume) and individual-tree attributes (i.e., total height and QMD) across 195 fixed-area (200 m 2 ) plots (96 – IC, 99 – NI) and compared responses between species and treatments. Jack pine plantations inoculated with Hebeloma spp., established on dry, nutrient poor sites, showed sustained stand-level advantages, with significantly higher crop tree density (+17%), basal area (+26%), and gross total volume (+29%) relative to the NI controls. Inoculated jack pine blocks also supported fewer deciduous competitors, contributing to greater wood volume accumulation. In contrast, black spruce plantations, established on more nutrient rich, mesic sites, displayed modest and non-significant increases in stand-level metrics (+12–13%), alongside higher ingress of non-crop conifers such as balsam fir ( Abies balsamea). For both species, quadratic mean diameter and height did not differ between treatments, indicating that benefits arose from enhanced early survival rather than accelerated growth in inoculated individuals. Our results demonstrate that inoculation can improve long-term plantation performance on dry, nutrient-poor sites, commonly planted with jack pine, while responses on mesic sites, more commonly planted with black spruce, remain variable and context-dependent.