Magnitude and duration of growth and wood quality responses to phosphorus and nitrogen in thinned Pinus radiata in southern Western Australia

Abstract

Summary The responses to phosphorus (P) and nitrogen (N) following thinning were investigated in a first-rotation Pinus radiata D. Don plantation on an ex-native forest site in southern Western Australia. A factorial P x N trial with up to 360 kg P ha−1 and 540 kg N ha−1 was used to monitor tree growth for 10 y after application of fertiliser. The primary deficiency was P, as there was no response to N in the absence of P. Without the application of N the magnitude of the response to P was limited, and the optimum P application was lower than when N was applied. The optimum response was achieved by combined applications of P and N. In the first 4 y after application there was an 86% increase in basal area increment at the optimum rate of P and N. The response to N lasted a maximum of 4 y at the highest rate of N application. The response to P continued for 10 y, although the response at the lower rates of P declined towards the end of this period. This decline in growth and the decline in concentration of both bicarbonate-and HC1-extractable P in the soil indicated that, on this lateritic soil, P availability declined overtime. Fertiliser application resulted in an extra 100 m3 ha−1 of wood over the 10 y of the study, and increased the periodic annual increment from 16 to 26 m3 ha−1. Nitrogen application reduced wood density for the duration of the growth response to N, but the wood density remained above the minimum density necessary for structural- grade wood. Fertiliser had no detrimental effect on stem form, as the proportion of the volume classified as sawlog and small sawlog was unchanged. The movement of logs from a smaller and hence lower- value category (small sawlog) to a larger, higher-value category (sawlog) due to an increase in log diameter, enhanced the value of the response to fertiliser above the increase in total volume.