University of Tasmania
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Growth and physiology of Eucalyptus nitens in plantations following thinning

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posted on 2023-05-26, 00:08 authored by Medhurst, JL
In Australia, a new activity is the management of eucalypt plantations for solid wood products. Thinning is one option for improving log sizes without the need for long rotation periods. The timing and intensity of thinning operations has a large bearing on the final products. This thesis describes a series of experiments in which the growth and physiological responses to thinning in Eucalyptus nitens (Deane and Maiden) Maiden plantations were identified. The thinning treatments, in three plantations, included an unthinned control and a range of final stockings from 600 to 100 trees per hectare. Diameter growth was significantly improved by thinning. In general, the more trees removed by thinning, the higher the growth response by the retained trees. Dominant and co-dominant trees showed the greatest growth response to thinning. Plantations of low to medium productivity will benefit from early-age thinning. Based on stand and individual tree responses, a final density of 200 to 300 trees per hectare may be ideal. The hydraulic conductivity of sapwood increased with stem height and the ratio of leaf area to stem sapwood area increased with stem size. Thinning did not alter these relationships. A broader study across a wide range of E. nitens plantations found strong relationships between stem sapwood area and tree leaf area that were independent of site, age and silvicultural treatment. Crown structure and the development of stand leaf area index following thinning were investigated. Larger branches were found in the lower crown of thinned trees and the increase in leaf area as a result of thinning occurred on the northern aspect of the crown. For trees in unthinned stands, the vertical distribution of leaf area was skewed towards the top of the crown and was correlated with live crown ratio. The vertical leaf area distribution of trees in a thinned stand was less skewed and was unrelated to tree size or dominance. Leaf area index, as estimated from light interception measurements, increased at a constant rate soon after thinning regardless of residual stand density. In the longer term, residual stand density had a strong influence on leaf area increase per tree and was correlated with changes in crown length. Water use of retained trees increased after thinning through changes in the radial variation in sap flow across the sapwood. Stand-level water use after thinning was reasonably predicted from the Penman-Monteith equation scaled by a simple canopy size factor. Increases in photosynthetic capacity and changes in foliar nitrogen distribution occurred after thinning. These responses were well correlated with measured changes in the light regime and were due primarily to changes in specific leaf area. Foliar nitrogen content was positively related to light levels throughout the crowns of trees in thinned and unthinned stands. Thinning induced both short- and long-term physiological changes. A modelling analysis demonstrated that increases in crown light interception and light-use efficiency drive growth response. The relationship of these changes with environmental factors suggests that, in order to maximise individual tree and stand productivity, thinning regimes for E. nitens plantations should be designed to maintain adequate light levels for individual crowns between thinning and harvest.


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