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ContinuFOR project update: four thinnings into our continuous cover trials

ContinuFOR project update: four thinnings into our continuous cover trials

Professor Áine Ní Dhubháin at UCD and Dr. Ian Short, Teagasc, have been involved in projects that have actively managed and monitored two continuous cover forestry (CCF) trial sites for over 10 years!

The ContinuFOR project is led by Professor Áine Ní Dhubháin at UCD, in partnership with Teagasc, Maynooth University and FERS Ltd. Some of the background to the project has been outlined in a previous Teagasc Daily article here

Now these stands of Sitka spruce have had a fourth thinning to open up the canopy and encourage natural regeneration in the understory. Here, our Teagasc team report on the progress of our recent ContinuFOR activities, and highlight some of our 2024 plans.

The two study sites are in Co. Wicklow and Co. Laois, each is a Sitka spruce stand with a Yield Class (productivity index) of 22 m3 ha-1 year-1. The trials were set up in 2010 as part of the UCD led LISS (Low Impact Silvicultural Systems) Project. There are three thinning types: low thinning (industry standard to remove smallest and inferior trees), crown thinning (favouring the best tree forms) and graduated density thinning (a fairly novel method). Graduated density aims to open up the stand more, and create a wider range of tree sizes, by removing more trees closer to the machine paths (racks: a row of tree removed to let machinery enter a stand). Each thinning type was applied to three plots per site, with more details on the ContinuFOR webpage and in our last Teagasc Daily article.

Dr. Lucie Vitkova (LISS) and Ted Wilson (in the TranSSFor Project) both maintained these stands during their PhD studies, with help from industry CCF experts Paddy Purser and Padraig O'Tuama to mark the trees for the different thinning types. The ContinuFOR project has six main tasks and a large group of project partners researching many aspects of transformation, from biodiversity and regeneration to the economic outcomes of the different treatments. One task is to share our work and findings, so we’ve been actively hosting field days, writing articles, and attending other knowledge transfer events.

Tree growth is being monitored over time through regular stem diameter and height measurements. The diameter at breast height (DBH: 1.3 m from the ground) of every tree is measured, and a subsample of heights are taken, making sure each tree is clearly numbered and marked at DBH. As well as these ongoing field measurements, this year we collected some Time-of-Flight (ToF) values using a non-destructive testing tool called a TreeSonic timer. ToF (time taken for an acoustic wave to travel through a medium) and these acoustic measurements can be paired with wood density to get an estimate of wood stiffness, and both density and stiffness are important factors for different timber uses. For instance, when building you may want stiffer timber for a shelf so it does not sag, or denser wood for flooring to reduce dents. The TreeSonic timer measures time taken for a stress wave, created by a hammer-hit, to travel a known distance between two metal spike transducers.

To measure the wood density, wooden cores were taken at DBH height and weighed in the Teagasc Ashtown laboratory. The weight of the oven-dried wood compared to the wood’s saturated volume gives a value for basic density.

Grace weighing oven dry wooden cores at Teagasc laboratory.

Grace weighing oven dry wooden cores at Teagasc laboratory

Trees were marked for the fourth thinning early 2023 and then a subsample were measured for ToF and stem core sampling for wood density. The fourth thinning at Ballycullen (Co. Wicklow) was in spring 2023, while for Fossy Hill (Co. Laois), the fourth thinning was in the autumn. We also have measurements for the felled logs from the thinning in Fossy Hill, using a resonance-based log grader (Hitman HM220) and the TreeSonic timer. We collected discs (short sections of wood) from the felled logs to investigate ring width, or similar properties, and compare these between the three thinning treatments. We also have data derived from the harvester head’s computer on the volumes and products removed during the thinnings which will be compared between treatments.

Image of the harvester machine at Fossy Hill. The head has a chainsaw and delimber to fell and process stems, and the machine runs on hydrogenated vegetable oil (HVO)

Harvester machine at Fossy Hill. The head has a chainsaw and delimber to fell and process stems, and the machine runs on hydrogenated vegetable oil (HVO)

What are the benefits of this work? Well, we are checking wood properties to identify whether there is any significant difference in wood stiffness or density, and thus potentially wood quality, between the three thinning types. The trend is that more forests will be managed under CCF regimes in the future, and it would be beneficial if wood stiffness, density, and quality are not lower under CCF thinning regimes. This will be of great interest to forest growers and wood processors alike.

Grace sawing wooden discs to help prevent the wood cracking when stored

Grace sawing wooden discs to help prevent the wood cracking when stored

We are now in the process of setting up a third site! We’ve found a beautiful stand of 15 year old Sitka spruce in Co. Roscommon where the owner is keen to transition towards CCF management. We intend to apply the same treatments at this third site which has different growing conditions. Not all sites will be suitable for CCF management. There is already a general idea of what makes sites suitable for CCF management, in Ireland and internationally. From Fossy Hill we know wind and waterlogged soils can be an issue, but then where windfall has occurred we are beginning to get seedlings sprouting. Growth rate at Fossy Hill has remained fairly high, and it is challenging to determine if windblown trees are due to management or site conditions. In Ballycullen there has been practically no windblow, and we see seedling regeneration only near the edges of the plantation. Ballycullen is also characterised by deep brown soils, and the site slightly slopes downwards away from our trial, making it quite free-draining. It has proven difficult to keep the created canopy gaps open with the high growth rates, and it is likely Ballycullen will need another thinning treatment again soon.

Getting in and thinning early seems to be very important for CCF with Sitka spruce. We’ve been very fortunate to work with engaged forest owners who are really in tune with the condition and status of their forests. There are also many helping hands who have provided valuable insight and advice, and helped with field work. Seeing how these stands develop gives some idea of what continuous small-scale harvests could look like in Ireland.

More information

Short, I. and Jones, G. 2023. Transformation of Sitka spruce stands to continuous cover forestry (CCF): Synergies and trade-off. Forestry & Energy Review 13(1): 14-16.

Wilson, E., Ní Dhubháin, Á. and Short, I. 2020. Transforming Sitka spruce plantations. TResearch 15(1): 32-33.

Wilson, E., Short, I., Ní Dhubháin, Á. and Purser, P. 2018. Continuous Cover Forestry: The rise of transformational silviculture. Forestry Journal 288: 38-40.

Wilson, E., Short, I., Ní Dhubháin, Á. and Purser, P. 2018. Transforming Sitka spruce plantations to continuous cover forestry. Forestry & Energy Review 8(1):38-40.

ContinuFOR website

Transformation of Sitka spruce stands to continuous cover forestry – ContinuFOR. Teagasc Daily article, 24/03/2023

Dr Grace Jones  Grace.Jones@teagasc.ie  
Dr Ian Short  Ian.Short@teagasc.ie Twitter @IanShort_Forest