Hoop pine (Araucaria cunninghamii Ait.), a nitrogen—sensitive rain—forest species can be grown without fertilizers as an underplanting to established stands of Pinus, on soils which will not support open plantings of pure hoop pine without fertilizing with nitrogen at the rate of 112 kg/ha per year. Stem volume growth rates of 6.6 m3/ha per year representing 28% of total stand increment have been obtained in 15—year—old underplantings. Removal of the Pinuscomponent from mixed stands of Pinus and Araucaria led to chlorosis of the remaining hoop pine but no significant decline in foliar nitrogen levels nor loss in growth over the ensuing 3 years. In contrast, 2—year—old transplants of Araucaria established on a site from which mature Pinus had been clear felled exhibited extremely slow growth and chlorosis, which could be overcome either by shading or applying nitrogen fertilizers. Underplants responded only slightly to added nitrogen compared to open plantings. Nitrogen requirements of fertilized open plants and unfertilized underplants were similar, however, being approximately 0.005 g nitrogen/g dry matter produced. The rate of immobilization of nitrogen by 8—year—old underplants established at 1,160 trees/ha was only 3.4 kg/ha per year. The source of nitrogen for underplantings is discussed, and nitrogen requirements of hoop pine are considered in relation to light intensity. It is concluded that the success of underplanting is due to a multiplicity of factors,including (1) the role of nitrogen fixation in conjunction with Pinus and Araucaria; (2) the increased availability of nitrogen as a consequence of changes in the soil—nitrogen cycle; and (3) the effect of reduced light intensity on improving the carbohydrate/nitrogen balance in Araucaria under conditions of low soil—nitrogen availability.