The response of mid-rotation Eucalyptus nitens to nitrogen fertiliser is non-linear and not influenced by phosphorus application

Maximising the growth and productivity of commercial plantations is critical for meeting global demands for wood and paper products, without resorting to unsustainable harvesting of native forests or increasing the plantation estate at the expense of other primary production systems. In Australia, productivity of plantation eucalypts is often limited by low nutrient availability and maintaining high growth rates is dependent on external nitrogen (N) and phosphorus (P) inputs. Therefore, an important management tool for maximising productivity is optimising fertiliser use. To do this, an improved mechanistic understanding of the productivity response to nutrient supplementation is needed. Here, we used three large-scale fertiliser experiments in Eucalyptus nitens plantations in Tasmania, Australia, to test the effects of six levels of N fertiliser (0, 300, 450, 600, 1200 and 2200 kg/ha) and two levels of P fertiliser (0 and 200 kg/ha) on annual diameter increment and annual wood volume increment over a two-year period. We found that trees responded strongly to N application both at the individual tree level (diameter) and at the stand level (volume), but not until the second growing season post fertiliser application. The application of N fertiliser increased stand volume increment but not linearly, highlighting the existence of an optimal N application rate to maximise productivity, which was remarkably consistent between sites. Further, we found the relationship between N application rate and daily DBH growth followed a predictable seasonal pattern, indicating an influence of abiotic conditions on the response of tree growth to N application. In general, phosphorous application had no impact on growth at either the tree- or stand-level. These results show that applying mid-rotation N fertiliser can boost wood growth, and that amounts greater than those currently applied could be beneficial on many sites, if it is economically and environmentally feasible to do so.