Farm Forestry New Zealand

Why pruning/clearwood is one of our major competitive advantages

Tuesday, June 14, 2022, Denis Hocking's blog

Most members will be only too aware of this, but sometimes I feel it is worth restating and discussing the obvious.

I will start by pointing out that knot free clearwood is a premium product. The Accoya and Kebany woods with their much more environmentally friendly treatment processes, also the thermally treated Abodo wood, are all climbing onto the top shelf and they all want clearwood for their top shelf products.

Producing good clearwood from plantation grown trees requires two distinct steps:

1. We need to cut the branches off before the tree diameter gets too large, and do it neatly just outside the collar that surrounds the base of the branch. This leads to quicker healing in the occlusion zone.
2. Just as important is the second step - thinning the trees so that they grow faster and put on a good sheath of knot free clearwood surrounding the knotty core and occlusion zone. I was told by one ‘old school’ experts that we should aim for about a 15 cm. clearwood sheath which requires quite a substantial log. I have achieved it occasionally, but never regularly.

This growth has to be relatively fast because time is money and while we may accept a 20 year delay in getting a return on the significant investment needed for pruning, 50 years would be another story. So we need tree species that can grow fast along with a climate and stocking rates that allow them to reach, or at least approach, their potential.

As we all know, different tree species and even provenances, grow at different, or very different, rates and show varying responses to environmental changes, including stocking rates. Wink Sutton, our former patron and long time advocate of pruning and clearwood, tells an interesting story about the range of responses to thinning in his trials. While heavily thinned radiata pine rapidly picked up the slack, with the remaining trees growing at 2, even 3 times the rates of un-thinned trees, this wasn’t true of all species in his trials. Some, such as Corsican pine showed remarkably little response. Fortunately, many of our popular production species do behave like radiata pine and respond well to thinning. This includes the cypresses though pruning is harder work with all their rather prolific branch habits.

There are not a lot of countries that share our advantages of fast growth and short rotations to make pruning attractive. The obvious ones are Chile and Australia, but neither has a history of pruning with regimes that instead focus on pulp and structural timber. We do lead the world in pruning, but it was a bigger lead 20 years ago.

I also believe there are other advantages in lower stocking rate, clearwood regimes. I stumbled across the “Wink Sutton” effect, without any planning I should add, when I started production thinning teen-age plantations in the ‘80s in response to “Rogernomics”. I was very impressed by the growth rates of the remaining trees and, rather late in the piece, I also noticed that the branches on the second and third logs were, for the most part, still carrying green foliage but not really growing. I think the term is “moribund” but it meant that above the pruned but I was growing above average diameter structural logs that still had tight green knots. And these trees were adding much of this diameter in their 20s ensuring that it would be the denser, stiffer adult wood, grown by older trees.

In the 1990s, Mark Dean put 10 permanent sample plots into plantations here; stands ranging from 100 to 150 sph. and 20-30 years old. The average volume increment over 2 to 10 years prior to harvest was 25 cubic metres per hectare per year. This would be a very respectable growth increment for stands at three to four times that stocking, and especially when you considered they were 1960’s genetics. Translate all this into log volumes and prices and the annual value increment left, and still leaves, sheep and beef farming for dead.

While this sort of silvicultural manipulation is not applicable to all sites, it does demonstrate the versatility of radiata pine and also other species that respond in similar ways. Yet the message coming out of Scion in recent years, heard 2 or 3 times at NZFFA Conferences, is to lift stocking rates to maximise total yield. While there was some acknowledgement that this might not apply to pruned regimes the overall message seemed to be chase quantity not quality. I find this reminiscent of the meat industry in the 1980s, when, thanks to Muldoon’s subsidies, we wound up with cool stores all around the country full of skinny little lamb carcasses that the world just didn’t want.

Let me repeat - there was nothing very original in my regimes. In many ways they were similar to the Silmod recommendations of the early 80s to prune and then take the final stocking down to 200 sph. The difference in my case was that I did the thinning later and in multiple steps. But regardless of how you get there, the idea of low stocking rates, (200 sph and below), seems to be an anathema for the current generation of forest researchers. I think they are too narrow minded and I am also worried that we may be looking at another case of “institutional amnesia”. There don’t seem to be the old heads to educate young researchers on what we already know, or should know, and how to fit it all into the big picture. Now, there is some bait for you other opinionated farm foresters. Let’s hear your views.

Disclaimer: Personal views expressed in this blog are those of the writers and do not necessarily represent those of the NZ Farm Forestry Association.