Jeff Collison

PhD Research Student

BSc Geography. Glasgow University
MSc Environmental Management. Stirling University

Dr Clare Wilson (Stirling University), Dr Andrew Tyler (Stirling University)
Dr Andy Moffat (Forest Research), Mr John Gallacher (UPM Tilhill)

Start Date: 1st April 2010
fax: +(44) 1786 467843
email: Jeff Collison

Research Project

Disturbance to forest soil resulting from stump harvesting

It is fairly commonplace in Scandinavia for tree stumps from plantation forests to be extracted, chipped and used as forest fuel in biomass boilers. In the UK, this use is much more limited due to the twin concerns of transportation costs and the potential for loss to the atmosphere of sequestered soil carbon resulting from the disturbance created by stump removal from the ground. The aim of my research is to characterise the degree of this soil disturbance, comparing it with the disturbance generated by normal timber harvesting and forest replanting operations. The focus is on mineral soils, as industry guidelines disallow stump harvesting on deep peat. This research work is funded by Forest Research, the research arm of the Forestry Commission, and UPM Tilhill, a major UK forest management company and part of the international forest industries UPM group, and also by the University of Stirling.

Stump harvesting for biomass energy purposes in the UK is covered by guidelines drawn up jointly by industry and Forest Research (Stump Harvesting: Interim Guidance on Site Selection and Good Practice. (2009)  Forest Research, Farnham, UK). It is hoped that the findings of this research will inform subsequent revision of these guidelines.

There is no generally accepted measure of soil disturbance. The approach taken here starts with the component forces at work in disturbance, and attempts to measure each of these. It may then be possible to develop a more general rating by synthesis. The components of applied force are essentially compressive, rotational and those generating lateral translocation. Compression can be monitored by changes to soil bulk density and analysis of soil pore characteristics, the voids forming the pore system being more maleable in the presence of compressive/decompressive force than the solid soil component. Rotational mixing will be monitored using the radiometric methods discussed below. Lateral translocation will be measured by monitoring the movement of tags pre-positioned around stumps prior to their removal..

Field research is currently taking place on an operational harvest site in Lamloch Forest, near to Loch Doon in Galloway, southern Scotland, courtesy of UPM Tilhill. The approach is to develop a soil chronosequence of the site as it changes from standing forest to clearfell site, and then on through ground preparation, restocking and subsequent soil settlement. Two adjacent tracts of forest have been identified, and in one of these stump harvesting has now been carried out, whilst in the other a normal sequence of forest management operations has taken place which does not include stump harvesting. A variety of methods will be employed to compare the resultant soil impact.

Harvesting at Lamloch Forest began in early summer 2010, and was largely complete by year end. The following two photographs illustrate the site transformation over that period. Stump harvesting was carried out on one of the two adjacent tracts in June of 2011, with mounding in preparation for restock being carried out in the same timeframe on the control tract.

Characterisation of soil and site was a key activity during summer 2010. The site, including both the area to be stumped and an adjacent control area, was set out in 10 metre squares, with soil augering and ground inspection taking place in each. Six soil pits were developed across the site. This established that the soil throughout the site had a high degree of homogeneity. It is predominately a mineral soil of loamy texture, with a depth generally in excess of 300mm, in the presence of glacial till. The surface had a well established litter layer with a sharp transition to mineral soil in evidence.

The site was also surveyed to establish its radionuclide characteristics. Where 137Caesium is deposited from the atmosphere, it preferentially sorbs with the surface litter layer and tends to remain there without significant leaching. If the ground is subsequently disturbed, this radioactive material is buried with the pre-existing surface layer. Analysis of the degree of radiation forward scattering detected above ground can be used to infer the mean depth of burial, and thereby provide a measure of depth of soil mixing, an important aspect of soil disturbance.

An initial radionuclide site survey was carried out using an in-situ NaI gamma ray spectroscopy detector mounted both at backpack height and ground level. This showed a high and fairly uniform level of 137Cs deposition across the site. Subsequent laboratory analysis showed clear evidence of deposition from both 1960's nuclear weapons testing and the Chernobyl incident in 1986. By comparing the above ground forward scattering patterns with the sub-surface 137Cs profiles obtained in the laboratory, it has been established that a 137Cs in-situ survey can be used to determine depth of soil mixing.

Further field trials on levels of disturbance were carried out after the trees have been felled, but before stump harvesting. This compared the outputs from a visual survey of surface Disturbance Class with the results of a radiometric survey. Over 350 points were surveyed along five transects both visually and for radiometric signature. An additional transect was also developed across an area of extensive windthrow. The results show a good level of correlation between visual and radiometric methods. The radiometric method also pointed to a greater level of sub-surface disturbance in the windthrow area, even where this was not necessarily visible to surface inspection.

To measure the volume and direction of disturbed soil when stumps are removed from the ground, tags similar to that shown on the right were placed around selected stumps in a three dimensional matrix. They were carefully set in the ground in a 5x5 array, each tag 0.8 metres apart, and at three different depths. This gave a total of 75 tags per monitored stump. A system for pre-placement was devised so that less than 1% of the soil in the volume of interest was disturbed. Each tag had been individually colour coded to facilitate tracking. The metal tags, formed from truncated 6" nails, were sealed and enveloped in Gel Wax to improve soil adherence. Subsequent detection to be directed by the use of a Pulse Induction metal detector. Initial detection tests carried out immediately after stump harvesting showed generally positive results.

Stump Harvesting took place in early June, under very inclement conditions. About 75% of the intended area was successfully harvested, but due to the weather-induced soil conditions and the slope of the terrain, it was not considered prudent to attempt the remaining area. This should not impact the project objectives.

The locational framework of transect lines and other control points has been re-established. As well as the two primary treatment areas subjected to stump harvesting and normal mounding management practice, opportunity has been taken to introduce two other treatment areas, these being windthrow affected and direct plant into undisturbed ground. A total of eight soil trenches have been opened up spread across all four areas, and over eighty soil samples collected and processed, initially for Bulk Density and Soil Moisture content. These results have been assessed and key findings established.

The detection of buried tags has worked well, with magnetic sightings equating to around 45% of the placed cohort of tags. Many of these sightings will result in the recovery of multiple tags. The high number of rainfall days in 2011 summer and autumn created taxing conditions for retrieving pre-placed tags. Local puddling and rapid water ingress into any excavation has limited retrieval to date. Summer 2012 weather has not been much better! Three stump sites have been excavated, and at two of these sites approx. half of the pre-placed tags have been retrieved. Retrieval sites have been mapped, and some rather interesting results are emerging.

The in-situ radiometric surveys have been re-run, and analysis of results carried out. These have shown the technique to have a high degree of sensitivity. Whilst many of the predicted outcomes have been achieved, some results have been intriguingly anomalous.

A number of samples are currently progressing through preparation in the Thin Section Lab to facilitate subsequent analysis of pore structure under disturbance.

Next Steps:

Field studies are now coming to an end, and writing up has begun.
Carry out Particle Density analysis in the soil lab and Radiometric Cs measurement of sample soil profile material in the radiation lab.
A further linked undergraduate Honours project is underway, comparing the growth environment created by mounding with that generated by stump harvesting.
Radiometric aspects of this project were presented at COGER 2011 (Coordinating Group on Environmental Radioactivity). An award was received for the best PhD presentation to the conference.
I would like to acknowledge the active support of industry supervisors, Dr Andy Moffat of Forest Research and John Gallacher of UPM Tilhill, as well as the great encouragement given by my university supervisors, Dr Clare Wilson and Dr Andrew Tyler.

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