Forest Service BC
Forest Renewal BC

Lesson 2: Windthrow Concepts and Assessment Principles

Learning Objectives

Home Page

In this lesson, participants will:

  • learn the role wind damage plays in BC's forests and the impacts it can have on integrated resource management prescriptions
  • become familiar with the mechanics of windthrow
  • know the three broad approaches to windthrow risk assessment
  • know the environmental factors that contribute to biophysical hazard for windthrow
  • know how these environmental factors can be assessed to evaluate relative biophysical hazard for windthrow.

This lesson contains 28 pages. Much of the content is presented as photographs, charts or illustrations with accompanying text descriptions. Several of the key graphics have been produced from the FS 712A. Use the hot links to find more information about a topic. As well, read the linked reference articles for additional background information.

Click here to view Lesson 2 Background information.


Background Page

Windthrow is complex and may appear random at first glance. Windthrow researchers break down this complexity by separating damage caused by endemic and catastrophic winds, and by evaluating the role of environmental factors separately. The relative hazard assessment method presented here was developed for this workshop and uses environmental indicators with which managers are familiar. It has been adopted as the basis for the FS 712 Field Cards. It uses an ecological/physiological model of windfirmness rather than a mechanistic model. The underlying premise is that trees can adapt to endemic peak winds, and that lack of windfirmness results from some site/stand limitation (see Mitchell, 1998 for details).

The environmental factors which contribute to endemic windthrow risk can be broadly grouped into topographic exposure, soil and stand properties. These factors are integrated to yield an estimate of 'biophysical hazard.' Each component hazard (soils/topographic/stand) is assessed by asking a 'diagnostic' question (e.g., for soil hazard: 'is root anchorage restricted?'). 'Windthrow risk' is the combination of biophysical hazard and 'treatment risk.' Treatment risk refers to the change in wind loading on residual trees caused by a particular treatment. Treatments which result in major increases on residual trees are high risk. Both assessment of biophysical hazard and treatment risk require an estimate of the damaging wind direction(s). Historic windthrow patterns are useful indicators of damaging wind direction and site influences.


Page 2 - Terminology


Likelihood of Damage = Environmental Factors + Management Factors



Windthrow Risk = Biophysical Hazard + Treatment Risk


Windthrow Risk:

is the likelihood of damage from endemic peak winds.

Biophysical Hazard:

is the intrinsic stability of the stand in its pre-treatment condition.

Treatment Risk:

is the way in which a particular treatment (e.g., cutblock boundary orientation) increases wind loading on residual trees.


Windthrow Impacts

Page 3 - Wind Damage

What role does wind damage play in BC forests?

  • Windthrow is a natural disturbance agent and influences the structure and composition of stands and their distribution over the landscape, especially where fire frequency is low.
  • What impacts might windthrow have on IRM prescriptions?
  • Windthrow impacts include damage to cutblock boundaries, partial cuts, riparian reserves and bark beetle outbreaks.

Patch of catastrophic wind damage near
Patch of catastrophic wind damage near Rennell Sound QCI.


Page 4 - Example of Impact in Hemlock–Amabilis Types

In moist/cool areas, wind has a major influence on stand structure and the distribution of stands across the landscape.

HA (hemlock–amabilis) and CH (cedar–hemlock) are two well known coastal types. HA stands have a history of windthrow. Some stands have historically been repeatedly blown down. CH stands lose individual trees while veteran redcedar is very windfirm.

HA types are predisposed to windthrow because of high density and uniformity; also due to frequency of trees growing on rootwads/logs. Stands become more unstable as they grow taller, eventually failing.

Example of HA types.

Example of HA type on the left and CH type
on the right.


Page 5 - 1992 MOF Windthrow Census by Region
  • A provincial census of wind damage in 1992 indicated that the amount of timber damaged by wind was similar to that damaged by fire or by insects (4–5% of AAC in each case).
  • Like fire and insect damage, wind damage is episodic and the quantity will vary from year to year.

1992 MOF windthrow census by region.

1992 MOF windthrow census by region.


Page 6 - Damage to Visual Quality Buffer

This site had been planned using digital terrain modelling (DTM). Catastrophic damage occurred with substantial breakage. The stand was subsequently salvaged and feathered.

Damage to visual quality buffer.
Damage to visual quality buffer.

Patch Retention, Bioreserves

A study by Dave Coates found more damage:

  • in patches less than 0.5 ha
  • in wet areas
  • in spruce
  • located near roads.

Patch retention, bioreserves.
Patch retention, bioreserves.


Page 7 - Commercial Thinning

  • Crop trees should have good (less than 90) height/diameter ratios
  • Leave enough trees (usually more than 40% basal area) to maintain a canopy which prevents the wind from increasing substantially on individual residual trees. If the residual trees are at great risk, the number of residual trees may need to be higher.

Abbott Heights. Example of windthrow in a thinned stand.
Abbott Heights. Example of windthrow in a thinned stand.

Streamside Management Areas


  • branches in creek, and
  • erosion along bank,

by ensuring trees are windfirm.

Streamside management areas.
Streamside management areas


Page 8 - Boston Bar

Windthrow provides bark beetle habitat.

Boston Bar.
Boston Bar.


Windthrow Concepts

Page 9 - Convective cells

Convective cells (thunderstorms) generate strong local winds.

There are two major frontal systems that cause windthrow events.

Peak winds are generated by synoptic weather systems such as Pacific low pressure systems, and Arctic outflows, on a regional scale.

Pacific Front

Low pressure cells from Pacific spiral anti-clockwise and stretch out as they pass across coast, causing SE or NW winds; white dots are weather stations, flag shows wind direction.

Pacific front.
Pacific front.


Arctic Front

Arctic fronts move south over interior and out across coast range causing outflow winds in major coastal/interior valleys; white dots are weather stations, flag shows wind direction.

Arctic front.
Arctic front.


Page 10 - Endemic and Catastrophic

  • Endemic windthrow is caused by peak winds which recur every 1 to 3 years, causing more uprooting than breaking.
  • Catastrophic windthrow is caused by peak winds which recur infrequently and cause breakage.

In general, for windthrow risk assessment, focus management on endemic first.

Windthrow Mechanics

Page 11 - Applied Forces

What do you already know about how wind interacts with trees?

  • Trees are like big levers. They are attached at the bottom and free swaying at the top.
  • The crown of the tree is like a sail. Wind acting on the crown creates a 'drag force.' This drag force is multiplied by the lever arm to produce a turning moment around the point of attachment.
  • Because trees are flexible, the crown is displaced. The mass of the crown is multiplied by the length of displacement to produce an additional turning moment.
  • As trees grow taller, the length of the lever arm increases. This means that the turning moment will increase even if the crown stays the same size.

Applied forces.

Applied forces.


Page 12 - Resisting Forces

  • Bole strength and root/soil strength enable trees to resist breakage or overturning.
  • Strong, tapered stems resist bending and stay straight, which reduces the turning moment due to crown displacement.
  • The resistance of the root/soil system to uprooting and overturning depends on strong roots, a good rooting arrangement, a large root/soil mass, and a cohesive bond between roots and soil.
  • Bole strength is not usually affected as the tree sways back and forth during a wind storm. However, the root/soil bond can break down, especially if the soils are low strength or wet.

Resisting forces.
Resisting forces.

Resisting Forces

Wind induces drag on crown which, multiplied by distance to stem base, yields horizontal 'turning moment' (=tendency to rotate around point of attachment); trees are flexible and wind causes crown to bend, weight of crown multiplied by displacement distance yields additional vertical 'turning moment.' As the tree grows taller, even if crown stays the same size (e.g., codominant), the lever arm gets longer and the applied moments (horizontal and vertical turning movements) increase.


Page 13 - Trees on Right-of-way

Our focus is on stand-grown trees; neighbouring trees provide shelter and intercrown contact (damping). If some neighbours are removed, wind load increases and trees move around more increasing stress on root system.

Trees on right-of-way.
Trees on right-of-way.

Trees on Right-of-way

Tree resists wind-induced torque. Resisting moment is a function of stem dimensions, wood strength, root strength and root/soil bond. The maximum applied torque a tree can resist is called critical turning moment; stem and roots are similar in strength under average soil conditions; root-soil bond is more likely to break down as a tree sways.


Hazard Assessment

Page 14 - Windthrow Hazard Classification

What different approaches could a forest manager take towards assessing windthrow hazard?

  • There are three basic approaches to windthrow risk assessment: observational, mechanical, and empirical.
  • Observational approaches use a checklist of indicators.
  • Mechanical approaches predict the critical windspeed for overturning from winching and wind tunnel studies, and the probability of critical wind speed from wind mapping/modelling work.
  • Empirical approaches use regression techniques to predict the probability of damage as a function of environmental and management variables.
  • The diagnostic method described below is 'observational' but includes some elements of the empirical approach.

The British system is an advanced example of the mechanical approach. It was developed for Sitka spruce plantations in upland Britain and based on winching tests, wind tunnel work with whole tree crowns, wind mapping using tatter flags. Their work allows prediction of stand height at which endemic winds will produce critical turning moments and trees will fail. Note orientation of trees – blown down in rows.

Windthrow hazard classification system.
Windthrow hazard classification system.


Critical turning moment.
Critical turning moment.

Critical turning moment is based on winching tests of trees of different species and sizes on different soil types.

Crown drag.
Crown drag.

Crown drag is measured in wind tunnels; wind speed over terrain is measured in the field or modelling in wind tunnels or with numerical models.


Page 15 - Biophysical Hazard Assessment

Use the windthrow triangle to understand component hazards and to uncover the role they play in determining windthrow risk.

The diagnostic method described below is 'observational' but includes some elements of the empirical approach:

  • The environmental factors which affect the wind loading and wind resistance of stands can be divided into three groups: topographic exposure, soil, and stand factors.
  • These three groups or 'component hazards' form the three sides of the 'Windthrow Triangle.' Relative biophysical hazard for windthrow can be estimated by assessing each side of the triangle independently and then integrating the results.
  • The component hazards are assessed by asking 'diagnostic questions.'
  • While the triangle approach is simplistic, it provides a framework for the systematic observation of damage patterns and associations, which can lead to improved predictions.


Page 16 - Wind Acceleration and Turbulence

For topographic hazard, the diagnostic question is 'are winds speeding up due to terrain obstacle/constriction?'

Wind speeds up over obstacles and produces turbulence in the wake of obstacle; at top of hills will speed up – high hazard; deep behind hill have shelter – low hazard; at base of hill in front have neutral conditions, no speed up or shelter - moderate.

Wind acceleration and turbulence.
Wind acceleration and turbulence.


Page 17 - Topographic Exposure

How would you assess topographic hazard?

Constrictions increase topographic hazard, while shelter reduces topographic hazard. If the area is neither sheltered nor exposed, the topographic hazard is moderate.

In complex terrain, it is necessary to estimate the dominant direction(s) of damaging winds. Storm direction can be estimated on a site-by-site basis by mapping the directions of windthrown trees.

Topographic Exposure


Page 18 - Root and Soil Factors

What impact does soil type have on windthrow hazard?

Strength of anchorage is a function of root/soil mass, root/soil bond or shallow soils and drainage.

The diagnostic question for soil hazard is 'is root anchorage restricted?'

Look at base of root system to see if it is restricted. Unrestricted root systems are typically bowl-like. They are deepest in the centre. Structural roots radiate out in all directions and get smaller and smaller as they reach the perimeter of the root ball.

Moderately restricted root systems extend for some depth into mineral soil but are flat bottomed. Roots at the base are bent or brushed off due to a mechanical barrier or water table.

Severely restricted root systems are superficial and plate-like. Trees attempt to build a platform.

Poor drainage or low strength organic soils decrease the strength of anchorage.

Root and soil factors.
Root and soil factors.


Page 19 - Restricted Plate Root Systems

Severely restricted plate root system on wet shallow soils; trees try to build support platform and move pivot point out away from stem — not as efficient a strategy as deep roots.

Example of moderately restricted rooting. Flat base.
Example of moderately restricted rooting. Flat base.
Moderate soil hazard.

Plate root soil hazard.
Plate root soil hazard.


Page 20 - Are some stands a higher risk than others?

Open Fir Stand

In open uniform stands, trees have grown with full exposure to wind with deep crowns, tapered stems and flat tops.

The diagnostic question for stand hazard is 'are individual trees within the stand adapted to peak wind loads?'

Open grown trees adapt to wind loads. Stand grown trees are sheltered by and damp with their neighbours. As dense stands grow taller, the trees become more slender and less stable.

In very high density, short stands, the individual trees may have low windfirmness, but because they get hung up in the canopy as they fall, the stand edge remains stable during the high wind event. In this case stand hazard is 'low.'

Live crown ratio of codominants is a good indicator of stand density.

Fir in Chilcotin; open uniform stand.
Fir in Chilcotin; open uniform stand.


Page 21 - Dense Uniform Stand

Dense uniform stand; trees have grown with shelter from wind. Note the shallow crowns, slender stems, differentiation into crown classes. Consider what would happen if stand was thinned from above.

Spruce on Moresby; dense uniform stand.
Spruce on Moresby; dense uniform stand.


Page 22 - Integrating Components

The component hazards are integrated in two steps.

Step 1.

  • The first box grid integrates topographic exposure and soil hazards which are intrinsic and constant, to yield 'Site Hazard.'

Step 2.

  • The site hazard is integrated with stand hazard, which changes as stands grow and management practices are applied. When brought together in the second box grid, they yield 'Overall Hazard.'

The integration is conservative. Integrating a moderate and a high yields a high class. The results of the biophysical hazard assessment should be checked in the field during the 'calibration' step and adjusted if necessary.

Biophysical Hazard Assessment Show Larger Image


Biophysical Hazard Assessment - 1 of 7



Biophysical Hazard Assessment - 2 of 7


Biophysical Hazard Assessment - 3 of 7


Biophysical Hazard Assessment - 4 of 7


Biophysical Hazard Assessment - 5 of 7


Biophysical Hazard Assessment - 6 of 7


Biophysical Hazard Assessment - 7 of 7


Page 23 - Examples of Windthrow Damage

Gillard Island centre damaged.
Gillard Island centre damaged.

Centre of island damaged, edges minimal damage; edges have had long-term exposure to winds along strait.

Windthrow at Vancouver Bay, progressive damage.
Windthrow at Vancouver Bay, progressive damage.

Progressive damage in wet bowl with dense tall stand; note the slope changes at far end of bowl, where next picture is taken.

Windthrow at Vancouver Bay
Windthrow at Vancouver Bay, minimal windthrow, defoliation.

Minimal windthrow, soils on slope are well drained, stand is short, exposure the same or higher as the previous photo; trees are defoliated indicating they withstood very high wind loads.