 SRF is the practice of cultivating fast-growing trees that reach their economically optimum size between eight and 20 years old.Species used are selected on this basis and include Alder, Ash, Southern Beech, Birch, Eucalyptus, Poplar, Willow, new varieties of paulownia elongata, Paper mulberry, Australian Blackwood and Sycamore.Trees are planted at widths that allow for quick growth and easy harvesting.