A very short, yet pretty thorough introduction to RPM (root production method)
The RPM system (Root Production Method) is a multistep production system of container tree production that places primary emphasis on the root system because the root system ultimately determines the tree's survival and performance in its outplanted environment. This particular container production system has been developed to facilitate volume production, in a high-quality tree with good height-caliper balance. Approximately 80% of our production consists of native trees, many of which present transplanting difficulties using conventional nursery growing systems.
It's basically a method of exposing sapling roots to the air to produce strong fibrous roots (this method is called air root pruning), combined with specified selections of seeds and the growing medium. The benefits are trees that grow extremely quickly!
With air-pruning, a plant’s roots come in contact with the air as they grow. It is a natural process where the individual root tip dries out and stops growing. The plant responds to this by producing even more fibrous lateral roots. What you end up with is a healthy plant with a dense fibrous root system, ready to take nutrients and water to your plant.
(source of the second pic)
Some excerpts from the original paper:
Superior trees growing on specific sites are selected for seed collection. Experience has taught us that most species have ecotypes that are site specific. We look towards the wetlands or floodplains as a prime seed source for native tree species that are found growing on both wetland and upland sites. Since they have evolved under stress we find they will consistently outperform their upland counterparts on virtually any site, particularly on highly stressed sites.
Our standard growing medium consists of 40% composted rice hulls, 40% pine bark, 20% sand plus slow release fertilizer, micronutrients, and a wetting agent. The medium is also inoculated with mycorrhizae spores.
Thank you to @cernoid-leporidae for introducing me to RPM and sending me excerpts from Farming the Woods