Also the Volcanic Plateau region has the largest area of man made forest in the Southern Hemisphere. Nearly all the trees are "Pinus radiata" which were bought in for trials at Rotorua in 1899 from Kew Gardens, London. The tree, a native of Monterey, California, grew very fast in the poor soils. Mass planting was initially carried out by prisoners in the 1920s and then by the unemployed on a large scale during the Great Depression. After World War II scientists from Norway settled in the region and helped develop ways of treating the soft pine to prevent rotting using pressure treatment with chemicals. They also introduced selective breeding methods which was the standard way of improving the trees until tissue culture was introduced in the 1990s. Trees take 25–30 years to reach full height. Most plantations use growth factor (GF) trees to guarantee very high quality (straight and knot-free) timber, however treatment by chemical salts is still required to stop rotting. This is done by either dipping or more usually, pressure treatment. New Zealand has pioneered the use of Radiata timber and plywood in small craft boatbuilding. Untreated clear timber or plywood is used that is then encapsulated in epoxy resin after construction. The key reason is the low cost of the wood rather than any inherent properties of the wood which is of moderate weight but fractures easily when bent, when dried to 12% moisture. One of the largest ply factories in New Zealand is located on the Volcanic Plateau at Tokoroa. Although planting by hand is still done on steeper land, most seedlings are now planted with the help of machines.