Forest Service BC
Forest Renewal BC
 

Lesson 3: Prescription Design Concepts

Learning Objectives

Home Page

In this lesson, participants will:

  • be aware of the importance of assessing impact potential before determining treatment priorities
  • be aware of key prescription elements which increase or decrease treatment risk.

This lesson contains 17 pages. Much of the content is presented as slides with accompanying text descriptions. Several of the key graphics have been produced from the FS 712A. As well, read the linked reference articles for additional background information.

 

Background

Background Page

Windthrow assessment provides information on the intrinsic windfirmness of portions of a site prior to harvest or access development. Depending on how the block is developed, the actual risk of windthrow occurring can be increased or decreased. The basic principles of designing prescriptions to reduce windthrow are to reduce the wind loading on edge or residual trees, and to increase the resistance of these trees to wind loads. At the landscape scale or development planning stage such elements as block location, size, shape and access, and timing of removal of adjacent areas should be considered. At the block scale or engineering/cruising/silviculture prescription field work stage such elements as boundary location and orientation, reserve size and location, and edge modification should be considered.

For partial cutting, removal levels and leave tree criteria, and re-entry timing are additional elements to be considered. The different silvicultural systems provide a range of residual canopy structures and re-entry schedules. The strip shelterwood and strip selection systems were designed for use in windy climates.

 

Illustration of Terms

Page 2 - Risk Definitions
  • ‘Biophysical Hazard’ is the combination of the topographic, soils and stand hazard components. It represents the intrinsic windloading and wind stability of trees on the site prior to treatment.
  • ‘Treatment Risk’ is the way in which a particular treatment increases or decreases the windloading or wind resistance of trees.
  • ‘Windthrow Risk’ is the likelihood of damage from endemic winds. It is the combination of Biophysical Hazard and Treatment Risk.
  • ‘Windthrow Impact’ refers to the potential harm windthrow could cause if it occurs. Impact is negative if wind damage results in management objectives not being met. If some level of damage is acceptable, this should be indicated in the original silviculture prescription.
Levels of Damage

This table summarizes the expected levels of damage (uprooting or stem breakage) due to endemic peak winds along cutblock boundaries with a high treatment risk (downwind end of opening and at right angles to dominant wind direction), or to dispersed residual trees in uniform partial cuts.

Biophysical hazard class

Expected damage on ‘high’ treatment risk boundary
or in partial cuts

None

No stand present to be damaged by winds, or no damage expected.

Low

Little or no damage. Less than 10% of trees damaged within first tree length of cutblock edge, or within partial cut.

Moderate

Partial damage. Between 10 and 70% of trees damaged within first tree length of cutblock edge. 10–30% of trees damaged within partial cut.

High

Heavy damage. More than 70% of trees damaged in first tree length. 30–70% of trees damaged within partial cut.

Very High

Very severe damage. More than 70% of trees within the first and second tree lengths of cutblock edge. More than 70% of trees damaged within partial cut.

Page 4 - Component Hazards

The following schematic provides a step-by-step visual guide to the component hazards on an imaginary site that together make up windthrow risk.

Read through the schematic layers and read the accompanying text to learn how each component hazard contributes to the determination of relative windthrow risk.

#1 Topographical Exposure

  • This imaginary area is proposed for a clearcut block. Walking transects across the area at the SP stage enabled the classification of topographic exposure as shown here.

 

#2 Soil Hazard

  • An evaluation of soil conditions enabled the classification of soil hazard shown here.

 

#3 Stand Hazard

  • An evaluation of stand conditions enabled the classification of stand hazard shown here.

 

#4 Biophysical Hazard

  • Integrating the three sides of the windthrow triangle (TOPOGRAPHIC EXPOSURE + SOIL = SITE HAZARD); (SITE HAZARD + STAND HAZARD = BIOPHYSICAL HAZARD) using the box grids yielded the map of biophysical hazard shown here.
  • This map represents the intrinsic windfirmness of these stands in the pre-treatment condition.

 

#5 Treatment Hazard

  • The cutblock proposed for this area has the shape shown here.
  • The diagnostic question for treatment risk is ‘how does the treatment affect the wind loading on residual trees?’
  • Looking at the orientation and location of the cutblock and reserve boundaries, you can see that some boundaries will pick up more wind after harvesting.
  • Boundaries which will have greatly increased wind loads have a higher treatment risk.
  • Boundaries where windloads are not increased much have a low treatment risk.

 

#6 Windthrow Risk

  • Boundaries which have a high treatment risk and which pass through areas of high biophysical hazard have a high windthrow risk.
  • Boundaries which have a high treatment risk and which pass through areas of low biophysical hazard have a low windthrow risk.
  • Boundaries which have a low treatment risk and which pass through areas of high biophysical hazard have a low windthrow risk.

 

Cutblock Design Strategies

Page 5 - Openings Parallel to Prevailing Winds

  • Reducing the length of the opening parallel to wind direction reduces wind loading on trees at the downwind boundary, thereby reducing treatment risk.
  • While results vary for different studies, to maintain a sheltering effect, the opening length should be less than 5 tree lengths.

Openings parallel to prevailing winds.
Openings parallel to prevailing winds.

Openings Perpendicular to Prevailing Winds
  • Reducing the width of the opening perpendicular (at right angles) to wind direction also affects wind loading on trees at the downwind boundary.
  • A sheltering effect occurs for openings narrower than 1.5 tree lengths.
  • Between 1.5 and 4 tree lengths wide, there is a funnelling effect.

Openings perpendicular to prevailing winds.
Openings perpendicular to prevailing winds.

 

Page 6 - Wind Loading

  • Isolated trees in large openings experience 40% more windload than trees along downwind boundary.
  • A 90º corner increases windload 30% compared to straight edge.
  • Loading is reduced for a 60º corner, but if neighbour trees are removed, loading increases.


Parallel openings.

 

Reserve Design Strategies

Page 7 - Boundary Orientation

Effects of boundary orientation on windthrow risk

  • As the angle of the boundary moves towards windward (perpendicular to dominant wind direction), wind load increases.


Boundary orientation.

 

Wind Exposure Index
  • Boundaries are often exposed to more than one dominant wind direction.
  • The combined loading from two directions can be estimated using the ‘Wind Exposure Index.’

 

Reserve Design Strategies

Page 8 - Reserve Size Risk

Effects of reserve size on windthrow risk

  • Windthrow severity is typically highest nearer the edge, and diminishes with distance into the stand.

 
Narrow Reserves
  • Damage is highest in narrow (less than 10 metres) reserves.

Narrow reserves.
Narrow reserves.

 

Page 9 - Residual Inter-tree Distance Considerations

Considerations in designing uniform partial cuts and thins:

  • Wind loads on residual trees in uniform thins increase in proportion to inter-tree spacing.

Cuts and thins considerations.
Cuts and thins considerations.

 

Page 10 - Tree Acclimation

  • After thinning, vigorous trees begin to adjust in form by thickening at the base and reducing height increment.
  • The crown centre of gravity also shifts down.
  • Consider delaying fertilizing slender stands for several years after thinning to allow them time to acclimate.

Tree acclimation.
Tree acclimation.

 

Lesson 3 Exercise

Exercise Background

Refer to the background material for the field exercise in Lesson 3, and for the elements of prescription design exercise.

There are five steps in prescription design:

  1. Review management objectives from higher level plans (HLP).
  2. Diagnose site and stand condition, capability and constraints.
  3. Integrate HLP objectives with site and stand condition by designing an appropriate ‘target stand’ condition.
  4. Consider the silviculture system and treatment regime necessary to move the stand from its current condition towards the target condition.
  5. Prepare the prescription documents and maps.

For this exercise, select target stand which would meet HLP objectives for the site. Consider this target in terms of windthrow risk and probability of prescription success, and suggest modifications to the target stand/treatment regime that make the desired outcome more likely.

For the RMA, review the ecological function of RMAs. Consider whether these functions are being met in areas with partial damage. Note the location of uprooted trees/mineral soil exposure relative to the stream channel, and consider under what stand, bank or channel situations impacts might be greater/lesser. Consider RMA design approaches that could reduce windthrow risk, and the pros and cons of salvage, and impact mitigation.

There is a shortage of well-designed trials on the impact of windthrow in riparian areas. For those that do exist, impacts are mixed. In most studies, windthrow along streams delivers little sediment unless trees are rooted into the bank or channel. However, some studies noted bank failures which delivered large quantities of sediment. Depending on the configuration of the side slopes, windthrow may be suspended well above the stream. Suspended stem material may deposit into the channel over the course of several decades as logs decay. Review the gully assessment course for further discussion of gully management issues.

 

Page 11 - Windthrow Assessment. Reducing Windthrow in Partial Cuts and RMAs.

Purpose of this exercise is to:

  • provide learners with further practice in field assessment including recognition of hazard indicators
  • have learners consider biophysical hazard, treatment risk and potential impacts and comment on prescription modifications for different target stands (e.g., uniform thin vs group openings).
Learning Objectives
  • learn how to recognize indicators of biophysical hazard in the field
  • learn how to assess treatment risk in the field
  • gain experience in recommending prescription modifications to reduce windthrow risk in partial cuts and RMAs

This exercise was designed to be carried out in the field. Be sure to prepare properly by wearing all appropriate field gear including a hard hat. You will be working with the FS 712 field cards. Work with a local expert to arrange visits to suitable sites in your area. Contact a colleague familiar with windthrow hazard and history in your area and arrange to take a tour together. The text that follows is for information only.

 

Page 11.5 - Exercise Background information

Refer to the background material for the field exercise in Lesson 3, and for the elements of prescription design exercise.

There are five steps in prescription design:

  1. Review management objectives from higher level plans (HLP).
  2. Diagnose site and stand condition, capability and constraints.
  3. Integrate HLP objectives with site and stand condition by designing an appropriate ‘target stand’ condition.
  4. Consider the silviculture system and treatment regime necessary to move the stand from its current condition towards the target condition.
  5. Prepare the prescription documents and maps.

For this exercise, select target stand which would meet HLP objectives for the site. Consider this target in terms of windthrow risk and probability of prescription success, and suggest modifications to the target stand/treatment regime that make the desired outcome more likely.

For the RMA, review the ecological function of RMAs. Consider whether these functions are being met in areas with partial damage. Note the location of uprooted trees/mineral soil exposure relative to the stream channel, and consider under what stand, bank or channel situations impacts might be greater/lesser. Consider RMA design approaches that could reduce windthrow risk, and the pros and cons of salvage, and impact mitigation.

There is a shortage of well-designed trials on the impact of windthrow in riparian areas. For those that do exist, impacts are mixed. In most studies, windthrow along streams delivers little sediment unless trees are rooted into the bank or channel. However, some studies noted bank failures which delivered large quantities of sediment. Depending on the configuration of the side slopes, windthrow may be suspended well above the stream. Suspended stem material may deposit into the channel over the course of several decades as logs decay. Review the gully assessment course for further discussion of gully management issues.

 

Page 12 - Overview from Previous Workshop – for information only

This lesson will take place in the field at 2 stops – Stop 1 is the ‘proposed partial cut,’ Stop 2 is an existing RMA.

Stop 1. The whole group will first inspect the boundaries of an opening edge and work through the field cards to calibrate the biophysical hazard assessment and discuss the impacts of windthrow on prescription success. The group will then walk into the adjacent stand.

In the stand, the group will split into the same small groups of 3–4. Each small group will be asked to design two partial cutting prescriptions, one for a target stand with uniform removal, a second for a target stand with group removal. They should consider biophysical hazard and modify their prescriptions to reduce risk to an acceptable level. They should also consider impacts of windthrow and salvage strategies. The entire group will get together and small groups will present their findings.

Stop 2. The group will inspect an existing RMA with windthrow. Start by having them go through the cards and evaluate windthrow risk. Discuss whether the outcome was as they would have predicted. Discuss potential/observed impacts and whether they are acceptable given prescription objectives. Discuss alternative RMA design strategies, and potential for salvage/mitigation of impacts.

 

Page 13 - Exercise – STOP 1: Partial Cut

Action – Using overview map of field site, describe windthrow activity.

Key points

  • point out or have participants point out any past windthrow activity in vicinity of field site
  • discuss age, location, orientation, damage extent and any observed associations with topographic, soils or stand features
  • identify if possible, the dominant direction(s) of damaging winds
  • have the participants discuss which boundary orientations if any appear to have higher/lower treatment risk.
Action – Go over field work checklist.

Key points

  • steps are summarized on checklist
  • remind participants of steps.
Action – Using block map and prescription summary, describe management objectives and 2 target stands.

Key points

  • have the participants turn to the prescription summary in their packages
  • briefly summarize the history, management objectives of the proposed block
  • summarize the target stand and proposed first harvest entry plan.
Action – Have the whole group inspect adjacent edge and calibrate the assessment. Look for trends in damage by tree type.

Key points

  • with the whole group, inspect a nearby high treatment risk cutblock boundary with similar biophysical attributes
  • go through field cards and integrate to yield biophysical hazard
  • classify existing edge damage
  • discuss whether damage is consistent with expected damage for biophysical hazard and treatment risk
  • adjust/calibrate if necessary: make sure participants are comfortable with the rationale for calibrating
  • look for trends in damage by tree type, noting: species, crown class, form, rooting medium, pathogens
Action – Have small groups inspect proposed block and complete hazard assessment cards, and consider biophysical hazard, treatment risk of proposal, expected damage, impacts of windthrow, and potential modifications to reducing treatment risk.

Key points

  • have small groups work through the cards at a representative place in proposed partial cut
  • assist them in evaluating proposed target stands and consider which treatment elements they can modify to reduce risk.
Action – Have the whole group discuss: biophysical hazard, treatment risk of proposal, expected damage, impacts of windthrow.

Key points

  • have the whole group discuss: biophysical hazard, treatment risk of proposal, expected damage, impacts of windthrow on target stand and management objectives
  • for each of the assessment items, have one small group present their findings
  • check to see if other groups agree and discuss any differences.
Action –For the proposed block, discuss with the whole group: buffering outside of unit; group size, shape, and removal sequence for group removal systems; removal level and leave tree attributes for uniform systems; salvage strategies; re-entry timing.

Key points

  • discuss with the whole group: buffering outside of unit; group size, shape, and removal sequence for group removal systems; removal level and leave tree attributes for uniform systems; salvage strategies; re-entry timing
  • have each small group present their suggested modifications
  • check to see if other groups agree and discuss any differences.
 
PAGE 14 - Exercise – STOP 2: RMA
Action – View RMA with partial windthrow.

Key points:

  • have participants assess windthrow risk along edge of RMA
  • discuss whether level of damage is consistent with assessment of risk
  • discuss ecological objectives of RMA
  • discuss potential/observed impacts and their acceptability
  • discuss pre-harvest modifications to RMA design
  • discuss pros/cons of slavage or damage mitigation.