Careful pruning can add value to stands of Sitka

September 25 2012 04:44 AM

A criticism levelled at Irish Sitka spruce is that timber quality can be lower than desired. Mostly it's a bread-and-butter product, fast-grown and harvested early, and therefore best suited to rough construction and carcassing. Sitka rarely makes joinery grade material, as it suffers from too many dead knots which weaken the timber as well as reduce its visual appeal.

Pruning can make a considerable difference to timber quality, provided it is carried out correctly and at the right time. Some time ago, partly because of the belief that pruned timber should yield a premium price, interest in high pruning Sitka spruce gathered some momentum.

It was also helped by the introduction of a pruning grant, but there was not much take-up. This was partly because of the sawmills not committing to paying a premium, and the grant has now been withdrawn.

One of the main practical difficulties with high pruning is that the ratio of pruned timber to knotty core must be a minimum of 2.5:1 for the operation to add any value.

Given that most of our mills are not geared up to take in particularly large logs, the pruning has to start early in the rotation. A tree with a diameter at breast height of 16cm must be grown to at least 40cm to achieve the ratio and this is roughly the target diameter that the mills require.

In a typical first thinning, a proportion of the stems will already exceed 16cm. So if high pruning is the aim, it must be done with an early first thinning and about 500 potential crop trees per hectare identified for the treatment. High pruning is normally carried out in two 'lifts' -- the first to 3.5m and the second a few years later to 6m. No more than one third of the live crown is removed each time to avoid impairing growth.

While the mills are unlikely to give a guarantee of a premium, owners should still consider high pruning their crops. No mill will turn down a better quality product, which could be your saving grace if you are marketing a small or isolated wood lot.

But pruning is not restricted solely to producing clean, knot-free lengths of quality timber.

At the most basic level, brashing inspection paths through a young plantation involves pruning and even this apparently simple operation can cause a lot of damage if done incorrectly. A tree's vital organs are just under its bark, and any scarring or poor pruning can expose the tree to fungal infection and disease, and degrade the timber.

I also frequently see examples where the jagged remains of the pruned branches are left protruding several inches from the stem. At best these will result in large dead knots and degraded timber. At worst, one small stumble could end in the loss of an eye.

Preparation of sample plots is another area where pruning will pay dividends. Accurate sampling is the key to producing an inventory of the crop and determining thinning frequency and intensity. 10m square plots picked at random throughout a plantation will give a far more representative picture of the growing crop than, for example, plots of two rows of trees, 4x25m long, even though both cover the same area.

If every tree in the plot is pruned to head height, it will lead to easier, and consequently more accurate, measurement. It will also help towards the overall package when it comes to negotiating a deal at harvest time.

Willie O'Brien of the Irish company PruneEX has developed and patented a very effective tool to assist forest owners and contractors with pruning by chainsaw.

Called the Brashing Bar, it comprises a buffer and an anti-kickback device as an integral part of an adapted standard chainsaw bar. The buffer ensures that the saw is placed at precisely the right distance from the tree, so that when held at a 90 degree angle to the branch, the chain cannot come into contact with the tree itself.

This ensures a perfect cut that will heal quickly and prevents damage to the bark. Safety is enhanced by the upper tip of the nose of the guide bar being completely shielded, which prevents this part of the chain coming into contact with the branch, and thereby eliminating kickback.

The machine will be on display at the Ploughing Championships this week.

William Merivale is national secretary of PEFC Ireland and a forestry consultant based in Cork. Email: william@cjandco.net