Wood density of radiata pine: effect of nitrogen supply

In a comparison of 27-year-old Pinus radiata D. Don on a nitrogen deficient coastal dune and growing with or without lupin plus fertiliser, mean wood density, earlywood density, and latewood density in individual annual growth rings were significantly lower in treated as compared to control stands. Latewood percentage showed a similar pattern of variation in relation to treatment and tree age when the ring was laid down. Wood density and latewood percentage increased radially from pith to bark, both increasing more rapidly in unfertilised compared to fertilised trees. Mean density, earlywood density, and latewood percentage were significantly related to tree nitrogen status, as reflected by foliar analysis undertaken in the year rings were laid down, and to ring age. Other foliar nutrients tested (P, K, Ca, Mg, B, Mn, Zn, Cu) did not appear to influence wood density when analysed in the presence of N. Wood density was high at this trial site by New Zealand standards, even in fertilised stands, which can be explained at least partly by the low foliar N levels at Woodhill Forest. These exceeded 1.2% only in stands younger than 5 years old. Thereafter, foliar N decreased to ≈1.2% in fertilised stands receiving 100 kg N/ha of fertiliser annually for 10 years, and 0.8% in unfertilised controls. Foliar N levels on recent sands typically fall in the range of 0.9–1.2% at Woodhill Forest, compared to 1.2–>1.5% in most other soil types in NZ. Effective rainfall at Woodhill also tends to be lower than other forest growing areas in New Zealand. At Woodhill, low rainfall during autumn/winter/spring was associated with high latewood percentage and, hence, high ring mean density, while low summer rainfall had the opposite effects.