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 Rennell Sound QCI.
Page 4 - Example of Impact in HemlockAmabilis Types
In moist/cool areas, wind has a major influence on stand structure
and the distribution of stands across the landscape.
HA (hemlockamabilis) and CH (cedarhemlock) 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 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 (45% 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.
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.
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.
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.
Streamside Management Areas
- branches in creek, and
- erosion along bank,
by ensuring trees are windfirm.
Streamside management areas
Page 8 - Boston Bar
Windthrow provides bark beetle habitat.
Page 14 - Windthrow Hazard Classification
What different approaches could a forest manager take towards assessing
- 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
- 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.
Critical turning moment.
Critical turning moment is based on winching tests of trees of different
species and sizes on different soil types.
Crown drag is measured in wind tunnels; wind speed over terrain is
measured in the field or modelling in wind tunnels or with numerical
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
- 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.
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.
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
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.
Moderate 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.
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.
Page 22 - Integrating Components
The component hazards are integrated in two steps.
- The first box grid integrates topographic exposure and soil hazards
which are intrinsic and constant, to yield 'Site Hazard.'
- 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
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
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Biophysical Hazard Assessment - 7 of 7
Page 23 - Examples of Windthrow Damage
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.
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, 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.