Second rotation decline in P. radiata plantations in South Australia has been corrected

Abstract

South Australia has a very limited area receiving adequate rainfall for commercial forestry. P. radiata (D. Don) plantations were started in the early 1900s and the State is now very dependant on these plantations for its timber. A thriving industry has developed in the lower South East Region which supports the main plantation areas. Although first rotation stands in this Region have produced average growth rates of 18 to 20 m³ ha⁻¹ pa⁻¹ the second rotation stands have shown a serious decline in productivity of 25 to 407 since replanting began in the mid-1930s. Long term predictions indicated a serious threat to the viability of the industry. Early trials to correct this problem did not succeed. In the early 1970s new trials were made with intensive weed control and fertilizer treatments on second rotation plantings. The fertilizer formula was a complete balanced mixture with a high proportion of N. These trials showed such promise that in 1976 the silvicultural prescription of integrated weed control and fertilizer treatments known as the “Maximum Growth Sequence” was prescribed for all second plantings, despite the high cost. Subsequent assessments of these stands show that they are now more productive than the first rotation stands on the same sites. Subsequent to the fertilizer formula in 1977 the number of elements added were reduced which significantly lowered the cost of fertilizer treatments. In the late 1970s trials with the same fertilizer formula in mid-rotation second plantings showed similarly promising responses. Large fertilizer programs developed from these findings. Subsequent studies show that the main cause of the decline was the loss of organic material resulting from the burning of the slash from the first rotation stands after clear felling. Parallel studies in the adjoining State of Victoria have reached the same conclusion. The fertilizer mixtures producing the good responses contain a high proportion of N. Policy changes have been made to reduce the organic matter loss after clear felling by crushing the clash in situ.