Review of the Effectiveness of Soil Inoculation to Improve Tree Seedling and Sapling Performance

Abstract Purpose of Review The success of large-scale forest restoration depends strongly on the establishment of tree seedlings and saplings, and soil inoculation with beneficial microorganisms is proposed as a tool to support this establishment. The purpose of this narrative review is to evaluate the recent literature to answer three main questions: whether soil inoculation improves the performance of tree seedlings and saplings; which types of inoculants are used in forestry; and which factors decide whether the inoculation is successful or not for its use in restoration. Recent Findings Mycorrhizal fungi are the most studied inoculants, and studies confirm that they can increase the growth of saplings, especially in degraded and phosphorus-poor soils. Other inoculant groups, such as the dark septate endophytes, plant growth-promoting bacteria, protists, and mycorrhiza helper bacteria, are promising, but they are lesser researched. Multi-species consortia appear to perform better than single strains and native or locally adapted inoculants generally perform better than commercial cosmopolitan products. The outcome also depends on the host species, on the mycorrhizal type, on the origin, amount, and viability of the inoculum, and on the biome. Summary Soil inoculation is a promising but not guaranteed tool to improve sapling establishment and growth. It gives the most consistent benefit when locally adapted and multi-species inoculants are applied to highly mycorrhizal-dependent hosts and on degraded and inoculum-limited soils. Future research should close the gap between the greenhouse and the field, define standards for dose and quality, and treat ecological safety as a normal part of the inoculation practice.