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The importance of selecting for vigour when growing hardwoods

Broadleaf woodlands are a very important resource for our local environment and rural communities. Broadleaf woodlands can also produce valuable hardwood timber, ranging from firewood to high quality furniture grade. Forestry Development Officer Jonathan Spazzi gives advice on growing broadleaves.

There is no doubt that broadleaf woodlands, both existing mature woods on farms or arising from more recent planting, are a very important resource for our local environment and rural communities. They provide natural connectivity with our hedgerows and fulfil a wide range of services, from water protection, landscape, biodiversity-enhancement to recreation, all essential to society well-being.

Broadleaf woodlands can also produce valuable hardwood timber, ranging from firewood to high quality furniture grade. Growing quality hardwood timber can be fully integrated with ecosystem services provision and in many cases will involve long term landscape retention through continuous cover management.

Read more about Continuous Cover Forestry here

Selective Management

Growing quality hardwood timber from our broadleaf woodlands requires a selective management approach and it is therefore important to identify early the quality trees within the woodland.

These trees are known as Potential Crop trees (PCTs).

PCTs are selected trees of high quality that are favoured during thinning operations. Some of these trees will go on to produce the final and intermediary quality hardwood saw-logs. Timber from initial hardwood thinning can be profitably marketed as firewood. 

A Woodland Improvement grant scheme funded by the Department of Agriculture Food and the Marine is available to broadleaf woodland owners to help with thinning management costs. Read more about Tending and Thinning of Broadleaves here

Thinning is the removal of a proportion of trees to provide space and resources to enable the selected quality trees to continue their steady development.

It is normally done for the first time, when the height of the woodland is approximately 8-10m

If the broadleaf species was planted in mixture with a conifer species which has grown taller than the broadleaf, thinning may be required earlier to remove this conifer nurse species and protect the broadleaf.

The timing of first thinning is very important. Too early intervention will drastically reduce the length of usable final log and too late intervention will cause a loss of vigour with future loss of productivity (see picture).

Twenty year old oak in County Limerick released by thinning at the right time. Note a clear 6m straight stem and a healthy vigorous crown.

Over time and through repeated thinning the selected trees are then brought to commercial size for harvest. 

This process in many cases is expected to be considerably longer when compared, for examples, with our commercial spruce plantations which matures at around 30 years of age.

For this reason, and as a way to ensure vigorous growth leading to profitability,  it is important that the selected broadleaf trees are chosen first from the most vigorous quality trees within the woodland also called “dominant”(see picture). This are assessed by their larger canopies which will function as the “engine” of timber production.  As a second choice, quality “co-dominant” trees can also be selected. “Suppressed” trees should never be selected as a PCT and if a quality “intermediate” tree “is selected, it is with the knowledge that productivity for that specific tree will be reduced.

Trees “social classes”: D=dominant, C=co-dominant, I= intermediate, S=suppressed. Note the blue arrows denoting different growth rates

The majority of our broadleaf plantations have been planted in the last 30 years

As most species will require 40-80 years to reach saw log size (for most species 40-60cm diameter at breast height) we don’t yet have direct experience of the full maturity process for our farm broadleaf woodlands.

However we can gain considerable insights from broadleaf management across Europe where woodlands have been actively managed for hardwoods production for centuries.

Classic management versus new tree-focused management

Classic management for the production of hardwood timber across Europe has focused historically on producing very long straight stems. This has been achieved by partially releasing the crown of selected trees, therefore sacrificing some of its potential vigour, in an effort to maximise the length of a clear stem. This process has results in “cathedral” like woodlands, with slender PCTs with relatively narrow crowns and, importantly, very long maturity periods often in excess of 100 years.

In more recent decades, supported by new research and trials across Europe, a new approach to hardwood production has emerged. New evidence indicates that by providing greater space to the crown of PCTs to grow and develop we can  substantially accelerate growth and gain opportunities for earlier and possibly greater returns despite shorter sawlog lengths. This approach is often referred to as “tree-focused” management (see picture).

Comparison between classic silviculture (left) and new tree-focused management (right) for a beech stand in Belgium (source: Baar, 2010). Note the different maturity timing and different crown dimensions. Saw log dimensions are achieved from 40-50 cm diameter-at- breast-height (dbh) up to 70-80cm dbh.

In terms of profitability, the reduced log length is , in this case, more than compensated by the faster maturity. Additional important benefits are also less defects due to vigorous growth, greater pest resistance and overall resilience. This last aspect is becoming particularly important in the face of biotic and abiotic disturbances induced by climatic changes.

The majority of hardwood timber value is in the bottom 6m log. Prices are indicative ranges. Question marks relate to the under-developed nature of current hardwood sawmilling.

In Ireland we are now at the very beginning of a new hardwood production value-chain. We see some small commercial developments taking place but much of the sawmilling capacity will still need to be developed as hardwood saw log timber will become available in years to come. While we need to further refine and further process evidence based best practice for effective hardwood production for our main broadleaves species, the importance to selecting for vigour from an early stage when choosing PCTs is coming to the fore.

Mature 60 year old beech tree in County Limerick approaching sawlog harvest.  Note the 6-8m clean sawlog length and the well-developed healthy crown resulting from regular thinning.