Growth or ring width responses to nitrogen, phosphorus and the two combined fertilizers were significant over five out of seven years that followed treatment (post-treatment period) in thinned, mid-rotation radiata pine (Pinus radiata D. Don). Responses were similar between nitrogen and phosphorus treatments and largest in the nitrogen plus phosphorus (NP) treatment. Fertilizer increased the proportion of wood formed in mid-rotation relative to juvenile wood thereby increasing tree density in comparison to the control. Fertilizers, other than nitrogen, significantly lowered density compared to the control in the posttreatment period only. Density was higher in nitrogen than phosphorus and similar between phosphorus and NP. Density responses lasted for up to 5 years and were inconsistent between treatments. Ring width and density were weakly correlated. Ring width and density responded differently to climatic variables within and between sites, signalling independent physiological controls and highlighting the importance of site specific differences.

Effects of nitrogen (N) and phosphorus (P) fertilizers on average fibre radial diameter, tangential diameter, wall thickness and number of fibres per unit area were studied over 7 years of growth in radiata pine. Stands were in mid-rotation, thinned and treatments replicated 4 times at 3 sites. Compared to a thinned, unfertilized control, N decreased and P increased fibre radial diameter. Only N decreased fibre radial diameter, thereby increasing the number of fibres per unit area. All fertilizer treatments decreased fibre wall thickness. Responses in ring width and fibre properties, other than fibre tangential diameter peaked 2 years following fertiliser application, before gradually disappearing after 4–5 years. Fertilizer effects on fibre properties diminished at different rates, causing significant interactions with time. Ring width had little effect on density. Changes in density and fibre coarseness were attributed to changes in fibre wall thickness more than fibre radial diameter and tangential diameter. The largest decrease in density and fibre coarseness in the combined N and P treatment was explained mostly by thinner walls.