Effect of thinning, pruning and nitrogen fertiliser application on light interception and light-use efficiency in a young Eucalyptus nitens plantation

Thinning, pruning and fertiliser application are used by forest managers to increase growth rates of retained trees and to improve log quality. Understanding whether growth responses are due to changes in light interception, light-use efficiency (LUE) or both, and whether they are subject to treatment interactions could assist the design of more productive and light-use efficient plantations. Two levels each of thinning, pruning and nitrogen fertiliser treatments were applied in factorial design to a Eucalyptus nitens plantation in south-eastern Australia at age 3.2 years, and responses measured to age 8.1 years. Treatments were: unthinned, or thinned from ca. 900 to 300 trees ha⁻¹; unpruned, or 50% of the live crown length pruned of the largest-diameter 300 potential sawlog crop trees ha⁻¹; and nil, or 300 kg ha⁻¹ N fertiliser. Light-use efficiency was defined as annual above-ground biomass (AGB) or stem-wood growth per unit of absorbed photosynthetically active radiation (APAR). Growth responses were associated with changes in both light interception and LUE. There were interactions between thinning and pruning: thinning effects were weaker for APAR, and stronger for LUE for pruned than unpruned trees. During the first year after treatment, thinning increased AGB growth of the 200 largest-diameter sawlog crop trees (SCT₂₀₀) by 34% (7.1–9.5 Mg ha⁻¹ year⁻¹), APAR₂₀₀ by 24% (0.77–0.95 GJ m⁻² year⁻¹), and LUE₂₀₀ by 13% (0.93–1.05 g MJ⁻¹); fertiliser application increased AGB₂₀₀ growth by 23% (7.4–9.1 Mg ha⁻¹ year⁻¹), APAR₂₀₀ by 6% (0.83–0.89 GJ m⁻² year⁻¹), and LUE₂₀₀ by 13% (0.93–1.05 g MJ⁻¹). Thinning and fertiliser application both increased APAR₂₀₀ by increasing tree leaf areas, and increases in LUE were associated with increases in photosynthesis. Pruning reduced tree leaf area by about 75%, but AGB₂₀₀ growth and APAR₂₀₀ were only reduced by 12% (8.8–7.7 Mg ha⁻¹ year⁻¹) and 37% (1.06–0.67 GJ m⁻² year⁻¹), respectively, and LUE₂₀₀ increased by 36% (0.84–1.14 g MJ⁻¹) during the first year after treatment. Pruning increased LUE by removing the least efficient lower canopy foliage, and by increasing the efficiency of the remaining foliage. This study shows how changes in stand structure, crown architecture and tree nutrition can be used to alter APAR and LUE, and improve our understanding of responses to silvicultural interventions in eucalypt plantations managed for solid-wood products.