Tree seedlings are dependent on symbiotic associations with microorganisms including bacteria, fungi, and actinomycetes for normal growth and development. Nitrogen fixing leguminous and non-leguminous trees form symbiotic relationships with Rhizobium (bacteria) and Frankia (actinomycete), respectively. Specialized soil-borne fungi from many different taxa form mycorrizal relationships with trees. Because the benefits derived from different species or isolates vary for a given site or tree, manipulation of these microorganisms can improve uniformity and yield of nursery crops. Inoculation of tree seedlings with symbiotic associates has dramatically improved the early growth and survival of tree seedlings on adverse reforestation sites around the world.