Nitrogen fertiliser only increases leaf area in the lower crown of mid-rotation Eucalyptus nitens plantations

Abstract

Nutrient supplementation can stimulate growth of plantation trees through an increase in leaf area, but this link is dependent upon how the leaves are distributed throughout the crown. Due to its effect on the light environment through the crown, vertical leaf area distribution can have important impacts on leaf function with implications for tree growth. We used large-scale fertiliser experiments in three mid-rotation Eucalyptus nitens plantations across Tasmania to investigate the effects of mid-rotation nitrogen and phosphorus fertiliser application on leaf area index (LAI), light extinction and leaf traits. Nitrogen and phosphorus application rates were 0, 300, 450, 600, 1200 and 2200 kg/ha and 0 and 200 kg/ha, respectively. We found a non-linear response of LAI to N application at all sites in the year following fertiliser application, with the greatest increases in leaf area occurring in trees grown under the intermediate N treatments. Using vertical light extinction profiles through the crown to infer leaf area distribution, we found that leaf area increase in the upper crown was relatively consistent across the range of N application levels but that leaf area increase in the lower crown increased linearly with increasing N application up to 600 kg N/ha but did not increase further with higher N application rates. The addition of phosphorus increased LAI production at all sites in the first year but had no effect on light extinction and hence the vertical distribution of foliage through the crown. Leaf traits were largely unaffected by nutrient application. These results demonstrate that nitrogen fertilisers applied mid-rotation can boost LAI in E. nitens plantations, but the extra leaf area is predominantly located lower in the crown and therefore unlikely to contribute substantially to any increase in productivity.