See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/286450945 Soil disturbance resulting from stump harvesting Article · December 2015 READS 27 4 authors, including: Clare A. Wilson Andy J Moffat University of Stirling University of Reading 50 PUBLICATIONS 275 CITATIONS 166 PUBLICATIONS 1,043 CITATIONS SEE PROFILE SEE PROFILE All in-text references underlined in blue are linked to publications on ResearchGate, Available from: Andy J Moffat letting you access and read them immediately. Retrieved on: 28 April 2016 20 STUMP HARVESTING © RSFS SCOTTISH FORESTRY VOL 69 NO 2 2015 PEER REVIEWED Soil Physical Disturbance resulting from Stump Harvesting Jeff Collison,1 Clare A Wilson,1 Andy J Moffat2 and John Gallacher3 1Biological and Environmental Sciences, School of Natural Sciences, University of Stirling, Stirling, FK9 4LA, UK 2Forest Research, Alice Holt Lodge, Farnham, Surrey, GU10 4LH, UK. [email protected] 3Tilhill Forestry, 1 Duckburn Park, Stirling Road, Dunblane, Perthshire, FK15 0EW, UK Introduction The harvesting of tree stumps and major roots for use as bioenergy feedstocks has taken place in various parts of the world over the last few decades. Interest in this commercial opportunity in the UK began in the 2000s and experimental and operational scale trials took place, mainly in Scotland, to explore the technology and its logistics (e.g. Saunders, 2008). In parallel, reviews of the possible environmental consequences of stump harvesting were commissioned (e.g. Walmsley and Godbold, 2010). These pointed to the likely increased effect on soil disturbance that stump harvesting could cause. They also identified the need for further ‘field scale research…. to ensure that the desire to source local biomass is fully compatible with other efforts to maintain the functioning of forests ecosystems and the vital The site. Summary services that they provide us’ (Walmsley and Godbold, This paper describes a detailed study of stump 2010, p. 33). harvesting in Lamloch Forest in north Dumfries and The assessment of soil physical disturbance resulting Galloway from 2010 to 2014. The study explored both from stump harvesting is the focus of this study. To the nature and extent of soil disturbance resulting address this question, an intensive integrated research from stump harvesting using a variety of standard and campaign was carried out at an operational harvesting innovative techniques. Stump harvesting disturbance site in Scotland, which also allowed comparison of was compared with that of other forestry practices. disturbance levels with other forestry operations (trench To complement the two-dimensional and subjective mounding and direct planting). The importance of nature of visual assessment techniques, a radiometric evaluating the impact of stump harvesting on other approach was adopted, utilising residual Chernobyl aspects of the forest environment, for example carbon 137Cs fallout to determine the degree of soil mixing. To support bulk density measurements, micromor- and nutrient dynamics, is readily acknowledged here. phological analyses of soil thin sections were carried However, resources were inevitably limited and it was out to investigate the impact of compressive force on considered that an important first step in understanding pore space. Low-cost tracer devices were deployed in overall impact was to establish the nature of soil the soil around stumps prior to extraction to permit disturbance itself. This has been little studied to date. the monitoring of soil lateral movement during stump In the context of this study, a stump is defined as extraction. both the above-ground stump remaining after stem The study showed that stump harvesting followed by harvest, and the below-ground extractable root mass. restock, when carried out under current guidelines, For conifers, and using modern extraction technologies, disturbed around five times the volume of soil these typically constitute around 25% of the biomass of compared to that disturbed by trench mounding. the tree (Eriksson and Gustavsson, 2008). Extracting Stump harvesting also resulted in a net reduction in this resource from the soil requires considerable force soil bulk density. Suggestions for modification of stump harvesting operations are made to reduce soil and invariably involves some degree of soil disturbance disturbance, including avoiding raking over the site (Moffatet al., 2011), the latter defined by the UK Forestry following stump harvesting which is estimated to add Commission’s Forests and Soil Guidelines (2011) as “any a further 10% to the volume of soil disturbed. activity that mixes and moves soil material”. PEER REVIEWED © RSFS SCOTTISH FORESTRY VOL 69 NO 2 2015 STUMP HARVESTING 21 Table 1. Operational Dates Activities Materials and methods schedule for Site and operational description July 2010 – Feb 2011 Stem harvesting and timber removal Compartment 51 The experimental work was carried out at Lamloch April – May 2011 Ground preparation and drain forestry operations. Forest in north Dumfries and Galloway, (National Grid construction coordinates NX 51480 97920), within a privately owned June 2011 Stump harvesting (research site only) plantation managed by Tilhill Forestry. The site had July 2011 Replanting completed been ploughed and planted with Sitka spruce (Picea sitchensis (Bong.) Carr.) in the mid-1970s, and was being being transferred to the stump windrow. The excavator actively harvested during the study. The research site is advanced uphill, and subsequently reversed back along located in Compartment 51 on the east-northeast slope the same track lines, raking over the soil behind it in the of Cullendoch Hill in an area of 0.71 hectares. Elevation process. Stump windrows were formed along either side rises from 250 to 280 m O.D. The average gradient of an existing extraction rack, following industry across the site is 13.3º, with shallower gradients of around guidance (Forestry Commission, 2009). 8º in lower areas, rising to almost 20º in the upper reaches. With stump harvesting carried out and restocking of The latter is just within the current UK guidelines for the the entire site completed in August 2011, the next phase, permitted gradient for stump harvesting (Forestry during autumn 2011, was to measure by various means Commission, 2009).The soil was categorised as predom- the resultant disturbance in each of the three inantly an upland brown earth (Paterson and Mason, treatment zones. 1999; Kennedy, 2002), with a sandy silt loam texture and Research methodology pH of 3.5. Disturbance levels were measured in four ways: Following stem harvesting three different treatment (a) visual ground disturbance survey, (b) soil bulk areas were established: 1) trench mounding, 2) stump density measurement (supplemented by soil thin section harvesting, and 3) direct planting. Table 1 summarises analyses), (c) radiometric determination of the degree of the schedule of operational activity on site. Trench soil mixing, and (d) soil movement tracking. Taken mounding (Forestry Commission, 2002) involved the together, the outcomes from the methods provided a excavation of spoil trenches down a line of uprooted comprehensive comparison of the level of soil stumps, and planting mounds were created using disturbance following stump harvesting, compared with material excavated from the trenches (Morgan and other forestry operations, for the upland brown earth Ireland, 2004). This was carried out in accordance with soil type studied. the contemporary UK guidelines (Forestry Commission, (a) Visual Ground Disturbance Survey 2009). For stump harvesting, a Cat 21B excavator fitted Ground disturbance surveys sample the selected area at with a Pallari KHN-60 destumping head was used defined intervals, and allocate one of four disturbance (Figure 1). The jaws penetrate beneath the stump while classes (Table 2) to each sample point based mainly on gripping it with the shear “thumb”. Vertical force is the categorisation system of Bockheim et al. (1975), with applied to lift the stump and roots from the ground, subsidiary disturbance states allocated to brash-covered followed by shaking to release adhering soil. Larger and stump-occupied sampling points. Such surveys stumps are split into a number of fragments by closing were carried out both before and after the application of the thumb onto the jaws. The fragments are stacked by differential treatments, denoted as “Harvested” and the excavator into adjacent stump windrows prior to “Restocked” surveys respectively. Transect sampling transfer to the roadside by forwarder. The destumped lines following McMahon’s (1995) guidance were set up area was then direct planted. spanning the three treatment zones (Figure 3), these being Destumped (DS), Trench Mounded (TM), and Direct Planted (DP). Sampling points were set approxi- mately one metre apart, in locations that equated to the pre-existing ridge and furrow apexes. A Chi-Square test was applied to test for significant differences between the Harvested and Restocked surveys. (b) Soil Compaction A total of 71 soil samples to determine bulk density were taken at various depths through the profile (max depth 50 cm) from soil inspection trenches located in each of the treatment zones. This permitted a comparison Figure 1. Destumping shear head. between treatment zones of soil compaction/ decompaction levels. Samples were extracted using steel An additional area was also direct planted without it coring