22 June 2022

 

Dr David Cutress: IBERS, Aberystwyth University.

 

  • Woodland creation is a core consideration for both UK and global greenhouse gas mitigation strategies and is set to increase in the foreseeable future
  • Despite the environmental benefits of woodland creation, its active management via machinery can have detrimental impacts if not planned and managed carefully
  • Considering machinery used and location, along with soil damage mitigation strategies, can further boost the environmental benefits and productivity of woodlands

 

Main impacts

Increasing our woodland area in the UK has been noted as a key carbon sequestration tool toward achieving our UK Committee on Climate Change (UKCC) targets of net-zero emissions by 2050. It was predicted in the UKCC 2019 report that woodland increases of 30,000 – 50,000 hectares a year would deliver savings of between 16.2 – 27.5 MtCO2e (metric tons of carbon dioxide equivalent) emissions annually. It was also indicated that along with new woodland creation there were added savings to be found through managing our neglected woodlands more efficiently. This level of sequestration could offset roughly half of the UK's 46.3 MtCO2e annual emissions associated with agriculture as such there is a clear drive for landowners, inclusive of farmers, to move towards supporting this woodland increase. Whilst the creation of woodland for the sake of carbon sequestration is a key driver, the running of such woodlands requires active management including regular tree clearance to optimise growth and maximise the carbon sequestration potential of the land, as well as benefit biodiversity. This management alongside the utilisation of woodlands for timber or other alternative revenue streams, such as innovative food products (whilst still maintaining woodland area via controlled replanting) tends to require that machinery has access to this land. Forestry machinery has been documented to cause impacts on soil physical properties via a variety of factors including;

 

  • Loss of total pore volume
  • Reduced saturated hydraulic conductivity
  • Increased pre-compression values
  • Loss of air-filled pore space
  • Increased bulk density
  • Increased penetration resistance

Essentially damage can be broken down to soils being affected through compaction or the formation of ruts. Ruts occur due to vehicles exceeding the soil bearing capacity leading to vertical or horizontal soil displacement to either the sides or the middle of the trail and particularly occurs in wet soils.

 

Ruts and their associated bordering bulges of displaced soil can play a significant role in water flow and soil hydrology acting as preferential routes for water runoff, particularly in steep angled terrain, this can also impact already increased issues with soil erosion and downstream/downhill waterlogging surrounding these types of topography. Compaction of soils, in general, as well as specifically in woodland scenarios are linked with impacts on greenhouse gas (GHG) emission generation (particularly Methane which has ~28x more impact on the environment than CO2) through processes such as increased anaerobic and methanogenic bacterial community emissions. Furthermore, compaction can reduce the productive growth of woodland trees and associated vegetations and take significant time to recover, limiting an area's carbon sequestration efficiency, biodiversity and revenue value.

 

Factors influencing woodland soil impacts

When considering soil physical disturbances related to machinery usage in woodlands four main areas that affect these came out of a literature-based meta-review analysis. These include considerations surrounding terrain related factors, operational planning, machine modification and soil surface amendments.

 

Terrain related factors

Terrain impact considerations include;

  • Soil water content
  • Slope (longitudinal incline or decline)
  • Organic matter (OM) content
  • Soil texture
  • Relative bulk density
  • Duff (plant litter) thickness

Many of these have already been well evaluated towards management changes to mitigate soil disruption as can be seen in the table in the next section. Whilst some of these factors impact harvesting and ongoing management of woodlands many are location specific so cannot be easily influenced. This may occur in instances where the land in question is the only available land for woodland creation or if woodlands are already historically established in these regions. One example is where slopes impact rutting and soil displacement as they lead to more of the vertical load of the vehicle being distributed across less surface area increasing soil damage. It is for these reasons (as well as preserving key sensitive areas) that governments have produced woodland opportunity maps to help advise possible new plantation schemes.

Operational planning

Researchers have posed several mathematical models for deducing soil disruption and rutting based on the track type of vehicles, expected traffic and regional soil qualities. Such tools could factor into the choice of vehicle for the available land and also into the initial processes of determining the location of new plantations. Similarly, trafficability mapping can be a vital tool for determining when to harvest timber with traditional machinery, to cause the least impact and damage whilst doing so. In some studies, this has been successful at increasing the percentage of stands able to be harvested without causing land damage by up to 30%. Models are of course impacted by many variables such as slope, land type, weather and season and can either be static (set levels which don’t change based on changing variables) or more promisingly they can be made dynamic, adapting to up to date information such as local weather data. These are relatively new systems under research and evaluation but could play an important role in the future if adapted and commercialised. Similar models have also been evaluated for assessing and minimising skidding from farm tractors which are adapted for forestry and have been demonstrated to have benefits on work time performance, productivity and reducing skid trails per hectare. Other tools seek to optimise the logistics of strip road planning by taking slope driving forward-backwards and sideways into account in their models. These have been shown to reduce the area of soil negatively impacted in one study by between 50 and 70% across three different stands tested. 

Machine specific consideration

Machine specific modifications and considerations factored into 27 of the 104 articles found in the review linked above. It was demonstrated that the main machinery factors influencing disturbance of soils were

  • Machine size
  • Axle load
  • Tyre size
  • Traction type
  • Traffic (total number of passes)

The modification of such factors will be mentioned further when discussing mitigation possibilities later in this article.

Amendments

In this instance, amendments reference the addition of something being placed on the topsoil's surface during or after harvesting to be a protective layer. As such, whilst these impact soil disturbance levels they do so positively and thus will also be considered in the next section as a mitigation strategy. The two main sources of these are brush and mulch. Brush comes from the processing phase of forestry operations and includes treetops, branches and foliage being placed on the floor to form a brush mat for machines to move across. Mulch on the other hand includes straw, litter and sawdust and tends not to include direct by-products of the woodland processing itself, thus requiring import. Of note both of these options now face more intensive consideration of their use as they have potential beneficial alternative roles through incorporation into bioenergy production, causing conflict surrounding their best use.

 

Potential mitigation tools and strategies

Studies have shown that steel flexible tracks (SFTs) added to wheeled machines can reduce impacts by increasing the contact area between the machine and the soil surface. Similarly, wider tyres and reduced inflation within tyres spread more of the load and have been assessed for their positive impacts on rutting with or without SFT addition.

Winch-assisted systems have seen rapid development and adoption in forest management where steep terrain harvesting is required. Not only has it been shown to make soil disruptions in these terrains equivalent to gentle terrain operations it also has huge bonuses concerning worker safety. Such systems essentially involve anchored support cables being placed securely higher up a slope (the anchor often being another vehicle) which helps to minimise load pressures on the soil as well as avoid possible machine tipping and slipping which can lead to fatal injuries. Several other mitigation strategies are already well discussed and considered in research in relation to various soil disturbance factors and can be seen in the table below.

 

Table adapted from Labelle et al., 2022

Factor

Country of study

Number of studies included

Mitigation recommendations suggested

Slope

Italy/Iran

3

Limit operations to gentle slope, restrict trails to slopes < 25%, importance of preplanning and using designated trails

Slovakia

1

Limit operations to gentle slope

Iran

7

Limit traffic to slopes < 20%

Soil water content

Slovakia and Czech Republic

2

Schedule operations to dry periods or frozen soil whenever possible

Iran

2

Plan logging operations when soil water content is low, limit excessive machine passes on moist soils

Soil texture

Brazil

1

Consider soil load bearing capacity

Iran

1

When possible, exclude areas with fine textured soils or apply topsoil reinforcement (i.e., logging residues, brush mats)

USA

1

Limit skidding operations to times of low soil water content

Relative soil bulk density

Germany

1

Exclude driving on sensitive areas as there is a link between relative bulk density and biomass growth impediments

Mulches are well regarded for their roles in improving different soil health factors including soil moisture levels, soil nutrient availability and in reducing erosion and compaction both in general and in forestry situations specifically. Largely, mulches have shown better benefits in their use post-harvest rather than during harvest with studies showing that when used as a post-harvest recovery solution mulches show benefits over bare soil when considering the recovery of soil penetration resistance, bulk density and rut depths.  Brush applications on the other hand have been noted in studies to provide better protection from ruts and soil bulk density increases than any other reported mulches during active traffic. It was noted in one study that brush coverage of 15-20 kg per metre squared (equivalent to 40-50cm of loose thickness cover) still offered load distribution benefits (reducing soil compaction impacts) even after 12 loaded passes. Specific study findings of both amendments can be seen below

Table adapted from Labelle et al., 2022

Amendment type

Machine weight (kg)

Amendment amount

Soil properties

Key findings

Brush

31,080

average of 12 kg m−2

RD

Exponential regression explains the relationship between rut depth and amount of brush

32,860

5, 10, 15, 20 kg m−2

BD, PR

Recommended brush mats of 15–20 kg m-2 to reduce soil density increase

10,257

10, 20 kg m−2

BD, RD

20 kg m−2 brush mats significantly reduced bulk density increases as compared to no brush statistically reduced rut depth

N/A

10, 20, 30, 40 kg m−2

Microstrains below brush

Softwood brush showed better capacity at distributing loads compared to hardwood mats

26,000

average of 27 kg m−2

BD, TP

Increases in soil bulk density were significantly higher in the upper 10 cm following operations on brush as compared to no brush

11,200

7.5, 17.5 kg m−2

BD, RD

Both tested brush amounts statistically lowered rut depth as compared to no brush. Benefits limited to 5 machine passes

29,900

1 m thickness

BD, RD

Soil compaction rates were reduced by half with the use of brush as compared to no brush.

31,800 and 30,500

15 kg m−2

BD, RD

Brush constrained rut formation after all forwarder passes and was deemed necessary wood harvesting on soft soils

Mulch

6,800

1.27 kg m−2 straw and 1.67 kg m−2 litter

BD, PR

Three years after mulch application, bulk density recovery values were significantly higher when using straw as opposed to litter mulch

10,300

3.65 kg m−2 straw

BD, TP, RD, PR

On trails with a slope of 10%, straw mulch allowed for faster recovery of bulk density, rut depth and penetration resistance

10,257

10, 20 kg m−2 sawdust

BD, RD

20 kg m−2 mats are significantly more effective in reducing adverse effects than lighter 10 kg m−2 mats

BD - bulk density, PR - penetration resistance, TP - total porosity, RD - rut depth

 

 

To further reduce soil damage increases in machine to soil contact area or reductions in machine load weights are required. This consideration was evaluated in a recent European Innovation Partnership (EIP) that took place in Wales. This project performed a comparative analysis of traditional tractors adapted for forestry utilisation against two other more innovative low impact machinery options. Whilst the study found no significant differences between the methods’ impacts on the chosen soil damage measurements over the study period, they did highlight the equivalent effectiveness of these smaller vehicles in performing the required tasks.

One method in which machine size and contact area mitigation could be improved is via machine control and robotics use in forestry. This has become an increasing focus, particularly as traditionally the forestry sector has been known to struggle with physically demanding labour and the high worker risks associated. Unique robots could reduce weight loads (as they would no longer require cabs to support an operator for example) whilst also removing workers from hazards at the same time. The three levels of control in this area are remote control (where the worker is in line of sight of the vehicle and controls it with a radio signalled controller device), teleoperation (where the worker views the work scene via some form of image transmission whether that be red, green, blue (RGB) video or even radar or light detection and ranging (LiDAR)) and automation (where there is no human control and the machine algorithms make all the decisions though there may be human supervision). Amongst these, ‘teleoperation’ using unmanned ground vehicles with or without winch-assistance technology has been discussed for playing further roles in improving timber harvesting in steep terrains and even making harvesting possible in previously infeasible terrain conditions. Where soil disturbances and robotics are debated, many options have been discussed where robot machinery could have advantages over traditional machinery. For example, “walking” forest harvesters have already been produced and could be easily adaptable for transition to robots which would have far less of a continuous impact on soils (as they can lift their feet off the forest floor) and as such could reduce soil disturbance. Other interesting considerations are robots that move from tree to tree and never touch the ground, with the suggestion that these could work in tandem to clear canopies and fell trees. Though benefits of such robots don’t take into consideration the soil disturbance linked with the subsequent removal of fallen trees following such procedures. These could, in future, work in conjunction with systems such as the Konrad (Pully) semi-autonomous steep terrain harvester which facilitates felled tree transport and was designed partly to improve soil conservation in more susceptible steep locations.

Walking and Autonomous walking harvester and tree to tree robotic concepts Parker et al., 2016; Visser and Obi 2022

Finally, a consistent factor that influences soil disturbance across studies is the machine operators’ skills and knowledge. Better operators know how to cause less damage to soils through learned observations and experiences. Research has discussed the effectiveness of different training methods such as virtual reality (VR) to assist in realistic training to provide operators with high levels of skill without risking damage to expensive machinery.

 

Barriers to woodland creation

With increasing interest in forms of woodland management, due to subsidies and recognised roles in carbon sequestration, it is often important to consider the barriers and possible strategies for overcoming some of these barriers particularly in smaller stands or in stands intended to utilise less productive steeper hillsides.

As noted above training and knowledge are vital aspects of the successful and low damaging processing of stands, as such this could be an important consideration for farmers looking to diversify in this area. Unlike large forestry operations, they would be unlikely to have the same access or the same cost value gains from taking part in such training so specific small-scale training packages need to continue to be developed and run. Information on these are available through government linked pages as well as training offered along with subsidies to Welsh landowners. The Farming Connect’s mentoring service also has a wealth of forestry management specialist advice available to be taken advantage of.

The cost of machinery can also be a big barrier to adoption as many small-scale systems can't justify the high costs associated with forestry machinery, particularly those required to harvest in a timber-based operation. Whilst contracting is an option this doesn’t necessarily facilitate the use of machinery able to minimise land-based soil impacts discussed above.

 

Summary

In all likelihood, woodland creation will continue to be pushed and incentivised for the foreseeable future as it offers such substantial potential benefits toward national and global greenhouse gas mitigation. For this reason, those increasing their woodland area or planting new woodlands must have a good understanding of the key factors that could potentially negate some of their environmental benefits as well as potentially lessen any revenue gains. A key aspect of this is the impact that woodland management machinery can have on the soil within stands as heavy vehicles can cause damage to its structure and productivity. This has a direct impact on its ability to sequester carbon and has been linked with increased emissions from other sources. Many factors should be considered to minimise impacts including consideration of machinery used and, where possible, consideration of where the woodland should be planted in the first place. 

 

If you would like a PDF version of the article, please contact heledd.george@menterabusnes.co.uk


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