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Thinning, pruning and fertiliser application are often used to accelerate the growth of crop trees and to improve wood quality and value in plantation forests that are managed for solid wood or knot-free wood products. These treatments can mimic natural disturbances and the influence of soil fertility on forest growth. Thinning might also be used to reduce stand water use and susceptibility to drought. Many studies have examined the growth responses to each of these silvicultural treatments as well as some of the mechanisms behind them. However, these treatments are sometimes carried out simultaneously. In spite of this, interactions between these important treatments have received little attention. For example, to make the most of pruning, thinning and/or fertiliser application can be used to increase growth rates of the retained trees and hence the production of knot-free or clearwood. Thus knowledge of how silvicultural treatments interact with each other and with site quality can assist with optimising financial investment and returns.
Main findings
*The interactions between these treatments can be large (2,3,7,9,14)
*Pruning had larger effects in thinned stands than in unthinned stands, because in unthinned stands the lower branches are shaded, less efficient and contribute less to a trees’ photosynthesis than the comparable branches in thinned stands (2,3,4,7,14).
*Absolute and relative thinning responses increased with site quality (9). However, there was still a lot of variability that site quality did not explain.
*Stand structure, in terms of the skewness or variability of diameter distributions in unthinned stands, often explained as much, or even more of the variability in thinning responses than site quality (9). Thinning responses were greater in stands that were more negatively skewed (higher proportion of large trees) and less variable. In those stands the large potential crop trees were competing with other large trees. The effects of site quality on thinning responses were found to be stronger than the effects of thinning age on growth responses.
*Unlike the interaction between thinning and site quality, there was no significant interaction between planting density and site quality for the same species and ranges in site quality (13). This contrasting result might be because interactions are more likely when competition is suddenly reduced, such as by thinning, causing tree physiological responses and changes in biomass partitioning and crown architecture.
*Many eucalypt species have the ability to coppice and this ability is often used in short-rotation forestry. However, coppice growth from stumps in thinned stands can potentially compete with, and reduce, the thinning response of retained trees. In our experiments the coppice reduced the response of the retained trees. In one of these plantations the competitive effect was so strong that there was no thinning response (5). This study demonstrated the importance of coppice control when the aim is to produce large-diameter logs.
*The production ecology equation was a useful framework to examine the processes that drove the growth responses (14)
Journal articles related to this project:
1. Forrester, D.I., Medhurst, J.L., Wood, M., Beadle, C.L., Valencia, J-C., (2010). Growth and physiological responses to silviculture for producing solid-wood products from Eucalyptus plantations: An Australian perspective. Forest Ecology and Management 259, 1819-1835. doi:10.1016/j.foreco.2009.08.029
2. Forrester, D.I., Baker, T.G. (2012). Growth responses to thinning and pruning in Eucalyptus globulus, Eucalyptus nitens, and Eucalyptus grandis plantations in southeastern Australia. Canadian Journal of Forest Research 42, 75-87. doi:10.1139/x11-146
3. Forrester, D.I., Collopy, J.J., Beadle, C.L., Baker, T.G. (2012). Interactive effects of simultaneously applied thinning, pruning and fertiliser application treatments on growth, biomass production and crown architecture in a young Eucalyptus nitens plantation. Forest Ecology and Management. 267, 104-116. doi:10.1016/j.foreco.2011.11.039
4. Forrester, D.I., Collopy, J.J., Beadle, C.L., Warren, C.R., Baker, T.G. (2012). Effect of thinning, pruning and nitrogen fertiliser application on transpiration, photosynthesis and water-use efficiency in a young Eucalyptus nitens plantation. Forest Ecology and Management. 266, 286-300. doi:10.1016/j.foreco.2011.11.019
5. Forrester, D. I., Bertram, C. A., Murphy, S. (2012). Impact of competition from coppicing stumps on the growth of retained trees differs in thinned Eucalyptus globulus and Eucalyptus tricarpa plantations in south-eastern Australia. Canadian Journal of Forest Research. 42, 841-848. doi:10.1139/x2012-040
6. Binkley, D., Campoe, O. C., Gspaltl, M., Forrester, D.I. (2013). Light absorption and use efficiency in forests: Why patterns differ for trees and forests. Forest Ecology and Management. 288, 5-13. doi:10.1016/j.foreco.2011.11.002
7. Forrester, D.I., Collopy, J.J., Beadle, C.L., Baker, T.G. (2013). Effect of thinning, pruning and nitrogen fertiliser application on light interception and light-use efficiency in a young Eucalyptus nitens plantation. Forest Ecology and Management. 288, 21-30. doi:10.1016/j.foreco.2011.11.024
8. Alcorn, P.J., Forrester, D.I., Smith, R.G.B., Thomas, D.S., James, R.N., Nicotra, A.B., Bauhus, J. (2013). Crown structure and vertical foliage distribution in 4-year-old plantation-grown Eucalyptus pilularis and Eucalyptus cloeziana. Trees-Structure and Function 27, 555-566. doi:10.1007/s00468-012-0809-1
9. Forrester, D.I., Elms, S.R., Baker, T.G. (2013). Tree growth-competition relationships in thinned Eucalyptus plantations vary with stand structure and site quality. European Journal of Forest Research. 132, 241-251. doi:10.1007/s10342-012-0671-0
10. Alcorn, P.J., Forrester, D.I., Thomas, D.S., James, R.N., Smith, R.G.B., Nicotra, A.B., Bauhus, J. (2013). Changes in whole-tree water use following live-crown pruning in young plantation-grown Eucalyptus pilularis and Eucalyptus cloeziana. Forests 4, 106-121 doi:10.3390/f4010106
11. Alcorn, P.J., Forrester, D.I., Smith, R.G.B., Thomas, D.S., James, R.N., Nicotra, A.B., Bauhus, J. (2013). The influence of site quality on timing of pruning in Eucalyptus pilularis and Eucalyptus cloeziana plantations. Australian Forestry 76, 24-35. doi:10.1080/00049158.2013.776923
12. Forrester, D.I., Elms, S.R., Baker, T.G. (2013). Relative, but not absolute, thinning responses decline with increasing thinning age in a Eucalyptus nitens plantation. Australian Forestry. 76, 121-127 doi:10.1080/00049158.2013.817043
13. Forrester, D.I., Wiedemann, J. C., Forrester, R. I., Baker, T.G. (2013). Effects of planting density and site quality on mean tree size and total stand growth of Eucalyptus globulus plantations. Canadian Journal of Forest Research 43, 846-851. doi:10.1139/cjfr-2013-0137
14. Forrester, D.I. (2013). Growth responses to thinning, pruning and fertiliser application in Eucalyptus plantations: a review of their production ecology and interactions. Forest Ecology and Management 310, 336-347. doi:10.1016/j.foreco.2013.08.047
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