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Transformation of Sitka spruce stands to continuous cover forestry- ContinuFOR

Transformation of Sitka spruce stands to continuous cover forestry- ContinuFOR

Grace Jones & Ian Short, Forestry Development Department, tell us that in the 1920’s only 1% of Ireland’s land area had forests, yet now over 11% of Ireland is forested.

About half of this forest is Sitka spruce (Picea sitchensis) plantations. Sitka spruce was commonly planted due to its fast growth and suitability for available markets.  At the end of the forest rotation forests are often clearfelled and then replanted.  Over the last two decades there has been increasing interest in alternative forest management practices that do not require the felling of all the trees at the same time.  To research alternatives to clearfelling, the DAFM-funded ContinuFOR project trials different thinning options to transform an even-aged Sitka spruce plantation to continuous cover forestry (CCF).  CCF often involves periodically harvesting a single trees or small groups of trees so the forest maintains some tree cover at all times. CCF can result in a more diverse forest structure with trees of different ages, sizes, and species, providing additional benefits and functions.

In 2010, the DAFM-funded “Low Impact Silvicultural Systems (LISS)” project selected two sites of Sitka spruce ready for their first thinning – Fossy Hill and Ballycullen.  These sites were divided into three blocks, and each block had three 50×50m plots placed within it.  Three thinning treatments were then randomly assigned to the plots, so all treatments were represented in each block.  The thinning treatments were: low thinning, crown thinning, and graduated density thinning (explained here).  Trial sites were then monitored and maintained by the LISS project and later during the Teagasc-funded TranSSFor project (2017-2021).  The work continues with the four-year ContinuFOR project led by UCD with Teagasc and Maynooth University as partners and FERS Ltd. contracted for modelling. 

Drone image of Fossy Hill

Figure 1. Drone image of Fossy Hill plots 1-6 – the plots are in the early stages of transformation to CCF. Image by Derek Gibson (Teagasc technician)

Now, the ContinuFOR project team is looking to set up a third trial site.  A potentially suitable site will have six or more hectares of uniform Sitka spruce that are nearly ready for their first thinning. The Teagasc thinning ready reckoner can be used to inform whether a forest is ready for thinning. Ideally the forest will be within 100km of Dublin, but others further away can be considered, and have a medium productivity and soil.  Think this could be you?  Get in touch with Ian or Grace.

Some of the work we have been doing in ContinuFOR:

Measuring tree diameters and heights

Figure 2. Measuring tree diameters and heights (Grace and Derek)

Trees in these trials have been individually monitored since the LISS project initiated the trials. The trees are painted with identification numbers and 1.3 m DBH height marks to ensure that the same tree can be measured at the same point over time.  This allows us to report on individual tree size and how each tree has grown over the course of the trials.  The tree diameter is measured at 1.3 m, and can be used to calculate basal area and tree volume.  Basal area is the cross-sectional area of trees at 1.3m height, and is important when considering tree growing space.  Some forest management plans aim to keep the basal area per hectare below a certain level to encourage natural regeneration, which is particularly important for CCF.  We are incredibly lucky to have long-term trials that have been established, and continuously maintained and measured during earlier projects and acknowledge the ongoing support of the site owners.

Getting ready for tree marking

Figure 3. Getting ready for tree marking (Grace, Derek, Ian, Paddy Purser [Purser Tarleton Russel Ltd.] and Padraig O’Tuama [CCF Management])

The current two ContinuFOR sites are due to be thinned for their fourth time in the coming months. Prior to each thinning operation selected trees from each treatment are marked for removal with orange dashes, while trees of a superior quality (Q-trees with better form and branches) in the Crown and GDT treatments are also marked with white bands.  The aim of the thinning in the Crown and GDT treatments is to favour these Q-trees, to provide them with more growing space, and to create an irregular canopy structure. This is in contrast to the Low thinning treatment, where the objective is to increase stand quality and uniformity. A special mark is also used if there is some biodiversity consideration (such as a nest) where the tree should be retained.  The markings make it easier for the forest harvester operator to quickly select the right trees for felling, and also shows them where to avoid.  Each plot within each site had the same proportion of trees marked for removal (20% and 25% of the total basal area for Fossy Hill and Ballycullen respectively) to keep the treatments consistent.  The machines need to have a path to move along (a rack) which is usually made during first and second thinning operations, by removing every 7-9th row of trees.

A common Sitka spruce thinning regime in Ireland (Low thinning) involves removing the smallest and poorest form stems, alongside any wolf trees.  Wolf trees grow much faster than their neighbours, and have a tendency to be poor form with bad branches.  This Low thinning removes the smallest and worst stems so that the retained crop trees have more room to grow.  It should result in a stand of trees that are a similar size and have a reasonable stem quality, which suits a clearfell harvest.

In Graduated density and Crown thinning, Q-trees are selected to have a neighbouring competing tree removed.  In these treatments the Q-trees are favoured so they will have extra room to grow.  This should mean higher volume growth for the superior trees.  One difference between the two is that during the first thinnings, the Graduated density treatment aims to create a broader range of tree sizes by removing more trees from the rows immediately next to the racks, and reducing the selection intensity further from the racks.  At the second thinning, a new rack is made between the previous ones so that the thinning intensity gradient is reversed.  This is easier to see in a diagram where the rack has 100% of stems removed during thinning 1 (a) and thinning 2 (b):

Illustration of Graduated Density Thinning

Figure 4. Illustration of Graduated Density Thinning indicating proportion of trees removed per row. From Vitková and Dhubháin (2013). a) 1st thinning intervention; b) 2nd thinning intervention.

As you can see, our project team has already been very busy but with much more to do, yet this is only a small part of the overall ContinuFOR project.  Our other project members are working to, e.g: survey regeneration and ground vegetation, model stand development over time, and record canopy openness.  Future work will also look at the economic performance of the three different thinning treatments, and so we are investigating multiple aspects of continuous cover forestry.  The project will continue for at least another 4 years, so if you want to follow along or find out more feel free to check out our website.

Dr Grace JonesGrace.Jones@teragasc.ie 
Dr Ian ShortIan.Short@teagasc.ie | Twitter @IanShort_Forest