Microbial Contribution to the Sustainability of Forest Ecosystems: Mechanisms of Stress Mitigation and Growth Promotion

Global climate change is intensifying drought, salinity, extreme temperatures, nutrient limitation, and soil degradation, thereby threatening forest regeneration, seedling establishment, and ecosystem functioning. This review synthesizes current knowledge on the contribution of forest-associated microorganisms to tree growth, resource acquisition, and stress tolerance in both coniferous and broadleaved forest species. It covers plant growth-promoting rhizobacteria, endophytic bacteria and fungi, arbuscular mycorrhizal fungi, ectomycorrhizal fungi, and microbial consortia associated with diverse forest trees, including species of Pinus, Picea, Cedrus, Pseudotsuga, Thuja, Quercus, Fagus, Populus, Betula, and Acacia. Across nursery, greenhouse, field, reciprocal-soil, and climate-gradient studies, beneficial microbial associations have been linked to improved germination, root development, nutrient and water acquisition, photosynthetic performance, seedling survival, and biomass accumulation under drought, salinity, heat, cold, nutrient deficiency, and degraded-soil conditions. These effects involve enhanced root development, nutrient mobilization, phytohormone regulation, ACC deaminase activity, antioxidant defense, osmotic adjustment, and modification of plant–soil feedbacks. However, microbial effects are not universally positive and vary according to tree genotype, microbial identity, inoculum origin, soil properties, stress intensity, and biotic interactions. Microbial consortia and climate-matched native communities may provide complementary functions, although their persistence and performance under field conditions remain variable. Overall, microbiome-informed nursery production, afforestation, and ecological restoration represent promising complementary strategies for improving forest resilience. Further multi-year field studies are needed to determine whether these benefits translate into sustained forest ecosystem functioning and climate resilience.