Farm Forestry New Zealand

How Should We Manage Our Radiata Pine?

Sunday, May 21, 2017, Denis Hocking's blog

At a couple of recent conferences we have had speakers urging higher stocking rates to increase total recoverable volume and returns from plantations. A recent paper by John Moore & other Scion researchers suggests an optimum stocking rate, using best genetics, of around 600 sph.

However I think this “recipe” is rather simplistic. First note that as the author state this is suggested for unpruned, “structural” regimes where volume, straightness and branch size/habit are the key determinants.

For pruned regimes diameter is also important. At this year’s conference harvesting day, Peter Martin from Waverley Sawmills emphasised this, showing some planks through several pruned logs. What he is after is a wide sheath of clearwood, but he also pointed out that small diameter, pruned logs have a much higher proportion of low value defect core than larger diameter logs with similar sized defect cores. Consider two 6.1 metre logs, one with an sed of 35 cm. and an average diameter of 40 cm., the other 45 cm. sed and an average diameter of 50 cm. If both logs have defect cores of 20 cm. the smaller log is 25% defect core while the larger log is 16% defect core. With 25 cm. defect cores the figures are 39% and 25%. Add in the losses to outside slabwood and sawdust plus extra handling costs with small logs and you can see why he wants large diameter pruned logs - he has less of this expensive waste wood. This translates into lower stocking rates for the plantations. Note though that while Waverley can handle logs up to 120 cm. in diameter, many other sawmills are limited to 90 and even 85 cm..

So there are advantages for structural regimes in having high stocking rates for maximum volume and for pruned regimes in having lower stocking rates and larger diameter trees. What seems to be missing from the debate is recognising the versatility of radiata pine. If we could get high productivity from lower stocked stands without excessively large branches we could get the best of both worlds. And perhaps we can.

In the 1980s, in response to Rogernomics, I started production thinning my teenage pine plantations to generate an early cash trickle. At the time FRI/Forest Research was advocating 200 sph for pruned regimes, but I carried on and thinned down to 100 – 150 sph.

In 1992 Forest Research installed a series of permanent sample plots (PSPs) to measure performance at these “very low” stocking rates. The results were interesting. Eight PSPs were measured for between 3 and 8 years (2 to 7 increments). Stocking rates ranged from 90 to 133 sph. on sand dunes and flats that had rather modest site indices between 22.8 and 29.4. This was before the introduction of the P300 productivity index. The average

annual volume increment, the amount of wood added each year on these plots, was a very respectable 25 cubic metres per hectare with a range of 16 to 30. And this was with paleolithic 1963 to 1970 genetics.

This increment is exactly the average measured by John Moore et al. for post 1975 sites measured round the country 15 years later at higher to much higher stocking rates. Certainly they measured some sites with annual increments up to 40, and one near Hawera at 60 m3/ha./year, possibly some top land that had escaped from dairying.

There were some other pluses for these stands of mine. The early confinement meant good branch control on second and third logs while continued good diameter increments through their 20s meant these trees had a higher proportion of the stiffer, denser, adult wood compared to the lower density, unstable, juvenile wood contained in the first 10 to 12 to 15 growth rings. (Note we are not talking pruning defect cores here, juvenile wood is an undesirable but inevitable feature of radiata pine’s early, annual growth rings.) The downside was that recovered volumes at harvest were a modest 350 – 500 m³/ha. Since they were still adding plenty of valuable wood I should perhaps have left them longer than the 28 to 31 years they had.

So what do I see as the lessons here?:

1. Radiata pine is a versatile species that responds very well to thinning, with remaining trees picking up much of the growth lost by removing some, perhaps many, of the original trees. Wink Sutton says his early silvicultural trials demonstrated that this endearing feature of radiata pine was not shared by most other forestry species, though I would claim that the cypresses also respond well.
2. The timing of the thinning is important. Thin too early and the trees will branch heavily, downgrading the second and third logs; thin too late and there will be insufficient green crown to maintain productivity and the trees don’t seem to grow new crown as vigorously. I have found thinning in the teens, 13/14 to 20, perhaps early 20s, has worked well.
3. Production thinning is not an option on many sites and is not a big cash generator. However it comes early and certainly pays all my establishment and silvicultural costs, with quite a bit left over in years like this.
4. I would add the comment that you shouldn’t look for recipes for silviculture; rather you should understand the principles and trade-offs and manage your trees in a manner appropriate for your situation.

Comments please!!!! 

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