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Soft soils

Soft soils

Scoping the challenges of sustainable
forest operations in soft soils

Maintaining continuous year-round forest operations while ensuring both sustainable and sound environmental practices can pose challenges, especially as the weather appears to become much more variable.

The following steps will help you find material and knowledge that will help to address these challenges.

First step: Scoping the issue

The necessary first step is to define the problem at hand and what goal needs to be achieved. The following section will review some terminology that will help the user link to relevant material.

WHAT IS SOIL OR SITE DISTURBANCE?

Soil or site disturbances can take many forms. Each jurisdiction or landowner may choose to regulate these using specific indicators, such as rutting for example, define its metric and set limits not to be exceeded. In other instances, operations may have to commit themselves to a strict documentation process where results are recorded and improvements need to be shown year-over-year.

Definitions of soil or site modifications

Soil disturbance

It is typically caused by harvesting or silviculture activities that can negatively affect the physical, chemical, and biological properties of the soil. They can be further classified in different categories such as the area occupied by corduroyed trails, the compacted areas, areas of dispersed disturbance, and unrehabilitated temporary access structures including excavated or bladed trails of a temporary nature. Dispersed disturbance is non-continuous, and can include dispersed trails, gouges (deep versus wide), and scalps (wide versus very wide).

Result: Temporary access structures, compacted areas, and corduroyed trails require rehabilitation. Rehabilitating these areas may exclude them from the site disturbance tallies towards any set limits.

Soil degradation

Where rehabilitation has not been applied to bring the soil condition back to a productive and functioning state, the area occupied by the disturbance is considered permanently degraded. Typically, within a management unit with a stated net area to be reforested, there is a set limit to the amount of disturbance allowed.

Result: The higher the degraded area in a harvested block, the less area can be successfully reforested not affected by the negative impacts of disturbance (compaction, rut, etc.). Natural resources management strives to ensure renewability of its resources and the environmental sustainability of its land base.

Soil rutting

Ruts are the trenches or furrows created by machine tires or tracks. Rutting displaces soil and damages its structure. Rutting is caused by repeated passes of equipment over the same trail. If soil strength is low, as is the case with saturated soils, rutting can occur even after only one pass of equipment.

Result: Rutting displaces the soil. The resulting changes in soil structure and strength mean that root development and drainage are severely affected. The effects of ruts make it difficult for roots near the surface to develop and penetrate soil. Shallow ruts can damage or break roots near the soil surface. When roots are damaged or broken, trees are more susceptible to disease and windfall. Lateral flow of water through the soil can be partially impeded or blocked, causing localized increases in the water table and pockets of saturated soil or ponding. Water flowing along the rut can contribute to soil erosion.

Soil compaction

Compaction is the compression of soil caused by a load (or by pressure) that exceeds the resisting strength of the soil . As a result, the soil particles and aggregates are pushed together, and the large pore space is reduced, resulting in an increased density of the soil.

Result: The reduced pore space and slowed or modified drainage of water in soils can reduce the exchange of gases and the amount of oxygen available to plant roots. Over the long term, fewer nutrients become available. In addition, slower infiltration of water into the soil can lead to increased surface flow and soil erosion. The reduced pore space also makes it hard for the tree roots to penetrate the soil hampering plant or tree establishment.

Exercise: Define in a single sentence what your issue is.

WHEN DOES YOUR EXCESSIVE SOIL DISTURBANCE OCCUR?

  1. At the beginning or end of the winter season
  2. During the fall
  3. During excessively wet summers
  4. During warm spells in winter
  5. On a particular contractor’s operation, or with specific machine operators
  6. When a particular class of equipment or type of treatment is used
  7. Other:

Exercise: Do you recognize patterns in the occurrences or do they appear completely random? For example, think of the last 5 years.

WHERE DOES PROBLEMATIC DISTURBANCE OCCUR?

Here is a list of locations where problematic disturbances often occur. The list is not exhaustive, and some elements may be overlapping. Can you narrow down a few locations or site characteristics where you experienced problems?

  1. Bottom of slopes or low-lying areas
  2. Local lowland pockets
  3. Organic soils
  4. Fine-textured soils
  5. Approaches to roadside landings and decks
  6. Other: (specify) ___________________

Exercise: try to estimate the proportion of harvesting sites that are affected by problematic disturbance.

WHO IS INVOLVED IN OPERATIONS?

Harvest operations involve many stakeholders, all of whom may have different perspectives on the issue of site disturbance. Who is involved in operations with you? (hint: ideally all should be):

  1. Landowner/province
  2. Certification authority
  3. Customer/mill
  4. Managers and planners
  5. Supervisors
  6. Contractors
  7. Machine operators

WHY IS THE PROBLEM OCCURRING?

Use the Five whys to identify the root cause of a problem. The principle is that asking why enough times, will lead to establishing the root cause of the problem. As an example, this is how you would approach it:

  1. Why is there a rut in this location? Answer: Because the skidder drove over the same area several times.
  2. Why did the skidder drive several times in this location? Answer: Because there was no way to travel up the hill to get to the bunches as was originally planned.
  3. Why could the skidder not make it up the hill as was initially planned and expected? Answer: Because of the snow load.
  4. Why was a bulldozer not used to alleviate the snow load by making trails? Answer: The bulldozer was off doing another job and not available.
  5. Why was a bulldozer doing another job? Answer: The bulldozer had to push trucks up a poorly designed slope.

In this example, it was the unavailability of a bulldozer to break trail for the skidders that caused the operators to look for an alternate route to keep the skidding operation working, resulting in excessive site disturbance.

Once the problem scoping is completed, continue to the framework for analysis below.

Second step: Framework for analysis

Once you have completed the scoping of your problem, it is proposed you take a systematic look at what you have been doing until now to manage your soft soils problems.

This process serves to identify what you have done so far in each of the three timeframes: before, during and after harvest operations.

Before harvest operations: planning

Two main aspects of planning forest operations are important to minimize soil disturbance:

  1. Site selection and scheduling (e.g. summer vs winter)
  2. Harvesting system selection (equipment team)

These two elements do not cover all the factors that can lead to soil disturbance but they are a necessary first planning step to minimize risks.

1. Site selection and scheduling

Here are some questions to help assess the current state of your planning process with respect to minimizing soil disturbance:

  • Did the planning take into account the susceptibility of harvest blocks to soil disturbance?
  • Do you have a seasonal (or frozen vs non-frozen / dry vs wet) classification of your harvest blocks?
  • Is the main concern of your winter sites the low volume of wood per kilometer of road or is it the bearing capacity of the ground?
  • Does the scheduling and site selection process include learnings from previous experiences and allow for input from the entire team?

Example of reports to provide knowledge about soil sensitivity

This report shows how using Lidar-derived ground surface elevation data can provide forest operations with improved trafficability predictions.

Predicting soil trafficability: Project summary of FPInnovations contribution – TR2015 no.20

Most jurisdictions will have developed a matrix of their various forest stand and soil types. The figure below such an example for Quebec.

Various forest stands and soils types with their susceptibility to rutting
Various forest stands and soil types with their susceptibility to rutting (Grondin, 2005).

 

When such information is available it can be incorporated into a decision support tool such as shown in the next illustration.

The crosshatched zone on the orthophoto is an area susceptible to rutting. A linear programming approach was used to schedule several such sites for winter harvest with a goal to minimize rutting
The crosshatched zone on the orthophoto is an area susceptible to rutting. A linear programming approach was used to schedule a number of such sites for winter harvest with a goal to minimize rutting. (Lacroix, S. 2005. Modèle intégré de sélection du moment et du lieu de récolte en fonction des coûts d’opérations et des impacts au niveau des sols. Master’s thesis. Laval University. QC.)

The previous example is from Québec, a jurisdiction that regulates rutting thresholds as one of its main indicators of healthy forest conditions. Similar work could be done with any regional ecosystem and the metrics chosen to indicate sustainable forest practices.

Even if decision support tools propose a particular solution, other constraints may override the proposed scenario, for example, the additional expense of opening a road in the winter to access a low-volume harvest block. Resolving cost versus impact dilemmas like these will require the judgment of the operations planners.

Weather-based susceptibility of soils to disturbance

It is common knowledge that certain sites will experience rutting and soil disturbance issues regardless of the rainfall, such as lowland black spruce stands growing on organic soils. Conversely, others will never have any moisture-related problems such as sandy, well-drained pine stands growing on upland sites. These extreme cases are obvious, most sites are somewhere in-between. Some sites support forest operations well during dry weather but experience excessive disturbance after a rainy period. These in-between stands and blocks present the biggest challenge to planning forest operations.

Many studies have reported the level of disturbance observed as a function of harvest system and season of harvest. With climate change, however, winters can no longer be depended on to provide consistently frozen ground due to warming temperatures in many regions. During summers, these climate changes can be seen more often in the form of droughts or intense rainstorms.

2. System or machine selection

System selection

Many studies have shown that leaving slash on the cutover, especially concentrated on extraction trails, helps reduce soil disturbance such as an increase in bulk density. Slash is left on the cutover when using a cut-to-length at the stump system. It is difficult to predict how this approach would affect other metrics of soil disturbance, such as rutting. A well set-up comparative trial could provide some answers.

An example of such a comparative study was done in northwestern Quebec in 2000. The study was controlled but no repetitions were done. The soil disturbance metric used was the extent of ruts deeper than 20 cm measuring longer than 4 m (a similar metric is used in other jurisdictions). The study showed that slash on the cutover did help reduce the amount of rutting but did not completely eliminate it.

Ability of three harvesting systems to limit rutting on a clay site – Advantage Vol. 2 No. 46

comparison of three harvest systems used on a clay-based site
This picture shows a comparison of three harvest systems used on a clay-based site. The extraction trail on the left-hand side had slash laid in the trail at a 45° angle by a stroke delimber. The middle trail was used by a harvester and 6WD forwarder with the slash laid at 90° to the trail. The trail to the right was created by a full-tree system with a grapple skidder and a delimber at roadside.

Machine selection

The use of specialized equipment is often considered when looking for ways to reduce rutting or other soil disturbances. The guides and reports section contains many references describing modified or specialized equipment be they wide tires, tracked skidder, or cable yarding on flat ground. When considering such solutions, it is important to take into account the full yearly utilization of the equipment and factor this into the cost. For example, what is the economic impact of using wide tires for only half of the year? Also, what is the added cost of logistical complications with the need to bring a specialized team for sensitive sites?

Finally, trials of system configurations or machines that are not used in the region of interest will require considerable effort to organize. Therefore, it will be worthwhile ensuring a good experimental design so that a maximum of relevant knowledge can be drawn from such trials.

During harvest operations

Having standard operating procedures (SOP)

Some operators have a better understanding of machine mobility and maneuverability on wet sites and are less prone to get stuck compared with others. However, this does not necessarily equate to less soil disturbance. It is commonly understood that some practices such as avoiding sharp turns, preventing wheel slip, and selecting an appropriate load size (i.e., drag size for a skidder) can help prevent soil disturbance. Yet it would be unreasonable to expect that operating practices or techniques alone could prevent all soil disturbance. Operator awareness (resulting either from training or from practical experience) is an important factor to ensure that when disturbance does occur, the planned mitigation steps are executed. These can include appropriate feedback and notification to supervisors or even operational shutdowns if the plan requires such actions. These situations can occur quickly, in a matter of hours due to storms. Therefore, actions need to be taken immediately when excessive disturbance appears. There are guides available for training and quick reference in the field to help aid operators and supervisors including:

Establishing a clear course of actions

After establishing thresholds when disturbance is considered excessive, a decision process needs to be implemented:

    • Who does an operator report to when he observes a developing soil disturbance problem?
      • Operators must have a clear point of contact for reporting excessive disturbance conditions. Also, they must feel that reporting is the right thing to do above and beyond productivity targets.
    • Do you have alternatives operating areas nearby?
      • A well-constructed site plan will clearly identify potentially problematic sites (i.e., by soil type, slope location, site indicators, etc.) and should have less susceptible alternate operating areas.
    • Is it realistic to complete the planned work in the problem areas after the conditions improve?
      • Favor a productive and proactive team to bring ideas forward for discussion and a re-adjusted plan should be discussed.
    • If you have no alternative operating areas nearby:
      • Do you park the operation?
      • Do you change sites and move to a remote contingency site?
      • With these two options, the costs must be considered and weighed against all other options.

Every harvest plan has some uncertainty because many indicators for soil disturbance are hard to anticipate. However, a well-thought-out and easily executable decision process will provide the operators and supervisors with clear directions and a smooth series of executable events (i.e., reporting, adjustment, shutdowns, etc.). It is always preferable to have a plan in place built by reflecting on the issue beforehand.

Trees felled but there is already a rut
In this picture, the trees are felled but there is already a rut: what are the options for extraction?

After harvest operations

Every jurisdiction or company has follow-up procedures once harvesting operations are completed, including those for assessing soil disturbance and if needed, a site disturbance survey. If possible, it is advisable to analyze disturbance data over a broad area because some trends may only appear when aggregating multiple sites or when linked with overall terrain characterization. Notwithstanding, some jurisdictions may require a soil disturbance assessment at the harvest block level.

The common problem with post-harvest assessments and follow-ups is the amount of time needed to compile survey data before it is communicated back to the harvesting team when much of the operation’s context will have faded from memory. This limits what can be learned from the results at the block level, good or bad, with the people who actually harvested the block.

A monitoring mechanism that allows for rapid feedback about current site disturbance conditions while operations are in process (considering the machines in use, operating techniques, weather, soil type, etc.) provides a better opportunity for drawing useful learnings compared to a monitoring process which is completely disjointed in time.

Here are some questions that should be asked:

  • Was the equipment adequate?
    • The planning should take into consideration equipment.
  • Are there incentives to ensure that objectives will be met? Are they effective?
    • The crew is trained, informed and rewarded for their diligence in achieving good environmental performance.
  • Were there contingency sites planned or used or would stopping the operation have been the only solution. If so, who bears the cost?
    • Planning for contingency should be the answer if stopping the operation is not an option.
  • How would various mitigation scenarios have compared from a cost perspective: temporarily shutting down the operation, moving to another location or remediation costs if such a practice is acceptable?
    • Comparing costs of alternatives can be done with a basic spreadsheet (see figure: costing alternatives)
  • Can new work methods be identified?
    • Continuous training and leveraging existing guides and knowledge exchange events can add to the crew’s ability to think outside of the box and bring unique site-specific solutions forward.
  • Was the approach to document any issues effective? Does it provide an effective feedback-loop to the before harvesting phase?
    • The outcomes were compared to the predictions; the soil type was predicted accurately by location; the slope aspect was considered appropriately; deviations from the plan were analyzed. Key learnings were identified and shared with appropriate staff.

Problems often appear obvious once they have occurred yet predicting them before they occur is far more difficult. For this reason, a feedback-loop should be included as one of the more important phases of the planning process. In other words, how well did the planning turn out?

Example of a spreadsheet to help determine costing alternatives
Example of a spreadsheet to help determine costing alternatives (click to download)

Third step: Finding useful resources

Resources in this section are organized by some of the common themes when dealing with soil disturbance and harvest operations. Under each theme are links to the listed resources.

Comparative studies

System comparison

Equipment comparison

Special equipment

Wide tires

Downsizing

Modified practices

Best practices guides

FPInnovations brochures

Guidelines development

Developing predictability tools

Equipment ground pressure

Operations and physical properties of soils

NOTE: Much of the earlier work done on soil disturbance was conducted by measuring the impact of operations on physical soil properties such as bulk density, resistance to penetration, and others. The more recent approaches favor visually observable criteria and metrics rather than instruments or lab-based measurements.

Rehabilitation

Regional guidebooks - external links

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