Effect of thinning, pruning and nitrogen fertiliser application on transpiration, photosynthesis and water-use efficiency in a young Eucalyptus nitens plantation

Interactions between thinning, pruning and fertiliser application in forestry are rarely examined, even though these treatments are often applied simultaneously in practice. Understanding these interactions can facilitate the design of regimes to best exploit such silvicultural interventions. The effects of these treatments on stand transpiration (E), photosynthesis and water-use efficiency (AGB-WUE, defined as the above-ground biomass production per unit transpiration) were measured in a Eucalyptus nitens plantation in south-eastern Australia. Two levels of each treatment were applied at age 3.2 years and transpiration was measured between ages 5.3 and 6.3 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 300 largest-diameter potential sawlog crop trees ha(-1); and nil, or 300 kg ha(-1) N fertiliser. There were no significant treatment interactions on growth, E or AGB-WUE. Thinning and pruning reduced E by 45% and 12%, respectively, and fertiliser application increased E by 21%. Transpiration was linearly related to stand leaf area, which explained more than 90% of the variation across treatments. Thinning and pruning also increased AGB-WUE by 23% and 21%, respectively, while fertiliser application had no significant effect. There was a small increase in AGB-WUE with increasing tree size, such that in unthinned stands the largest 50% of trees were 7% more efficient than the smallest 50% of trees. Thinning increased AGB-WUE by increasing the light available to the lower canopy and pruning increased AGB-WUE by removing the least efficient lower canopy foliage and increasing the efficiency of the remaining foliage. All treatments also modified the hydraulic architecture of the trees by changing leaf area to sapwood area ratios and radial sap flux density profiles. This study shows how silvicultural treatments can be used to modify stand transpiration and AGB-WUE of E. nitens plantations, potentially reducing their drought susceptibility while making more efficient use of the sites water resources. (C) 2011 Elsevier B.V. All rights reserved.