Pruning and thinning eucalyptus - Stringybark eucalypt silviculture
New Zealand Tree Grower November 2006
|Equipped with a harness, pole-pruner and ladder|
Pruning stringybarks is faster per tree than radiata, and the timber is worth more at the end. So wheres the problem? I believe the biggest issue influencing value and returns is the quality and spacing of final crop trees.
StockingFirstly lets look at the area of most concern to me, stocking, and how that relates to silviculture. How many seedlings do we plant, do we need to think about this or just follow radiata regimes of around 1000 sph? At a high stocking of 2000 stems per hectare we get real selection from a typical unimproved Australian seed line, allowing us to keep just the biggest and straightest trees and without too many gaps in the canopy. At this stocking, branch size is kept to a minimum and upward growth is encouraged. Be aware that timely repeated thinning operations
|High stocking means small branches|
Heres another option. By planting trees close within the row (I have found 1.5m is the optimum spacing) a lower initial stocking still allows for good selection. That is, at 7m between rows the initial stocking is only 950/ha, but because selection and rejection takes place only within the row, and in two dimensions, the whole process of selecting crop trees is simpler and more effective. Eucalypts are very happy to be this close initially; and the otherwise complex three dimensional problem of progressively thinning evenly spaced trees down to 250/ha without leaving big gaps in the forest canopy is simplified.
With eucalypts straight trees mean valuable logs. Any wobbles in the trunk mean lower recovery and lower quality when milled. This is because of such issues as compression wood in the core, radial vs tangental shrinkage and a gradient of compression and tension in the trunk. What makes our eucalypts grow wonky? Firstly, high fertility can give them the "speed wobbles", even in fairly sheltered valleys. Finding the optimum fertility and selecting the right species can be a bit hit-and-miss without experience. Genetics is also at play, both at a species level (E.pilularis and E.agglomerata are the worst) and within a seedlot. Again, lots of trees gives lots of selection.
|This double leader has split open. The tree has no value|
|Cutting back a double leader allows one leader to take over and straighten|
|Modern "conically ground" pull-saw|
Another question is whether the small loose dead branches should be removed thus allowing the socket to callous immediately, or left to be shed by the tree. I am not aware of any research on this, but I prefer to pull them out rather than cut them off or leave them to shed by themselves.
|"Wolf" brand professional pole pruner head. Robust tools are important|
RotationPresently unpruned Eucalyptus saligna (Sydney Blue Gum) is sawn in Northland down to a small end diameter of 35-40cm. So what is the target log size for stringybarks, with their vastly superior timber? I believe this should be decided at the start, because final crop spacing combined with the rotation length gives you the target diameter. The problem is that we don't really know how sawmill technology and market demand will influence future demand for smaller logs. My gut feeling is that well pruned trees which yield straight small-diameter logs will be in high demand, allowing for final crop stocking of as many as 400-600 stems per hectare which may become suitable for harvest in 20 year rotations.
Selecting for fast growth and straight stems is the objective and I would argue that presently the best way of achieving this is with high initial stocking. I am not aware of any work being undertaken to improve the genetic resource of solid-timber eucalypt species in New Zealand, which to me is a real shame, and perhaps the number one constraint to establishing a commercial-scale industry.
Dean Satchell, Sustainable Forest Solutions, is based in Kerikeri and manages eucalypt plantations in Northland.