Riparian Windthrow Northern Vancouver Island

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Riparian Windthrow Northern Vancouver Island

• Terry Rollerson
• Kerry McGourlick

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Study objectives

• Document the extent/character of riparian
  windthrow on Northern Vancouver Island
• Evaluate the factors associated with riparian
  windthrow
• Document the effects on stream channels
• Develop windthrow control strategies
• Develop a monitoring/database system

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Methodology

• Ground surveys of 76 km of pre and post FPC
  riparian edges, S1 to S6 streams
• 447 variable length plots defined by homogeneous
  stand, soil, terrain, stream reach boundary
  characteristics
• Visual estimates of windthrow percentage,
  penetration, spatial pattern, orientation and
  effects on streams

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Visible stream effects

• 95 of the stream reaches surveyed had no
  visible streambank disturbance.
• 4 had 5 streambank disturbance
• 1 of the stream reaches had 10-20 streambank
  disturbance
• 64 of the stream reaches were spanned by
  windthrown trees and 36 were not.

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Stream class vs windthrow
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Windthrow distribution
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Windthrow penetration pattern
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Windthrow severity index (WSI
WTPenetration Distance)
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Single vs double-sided strips
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Treatment plus leave strip effects
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Windthrow vs width (1-sided strips)
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Windthrow vs width (2-sided strips)
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Windthrow vs boundary exposure
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Wind Exposure Index WEI (BE 1 rank) (BE 2
rank)
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Windthrow vs exposure index
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Windthrow vs valley axis
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Windthrow vs boundary geometry
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Windthrow vs rooting depth
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Windthrow vs soil drainage class
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Windthrow vs dominant tree species
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Windthrow vs height class (map)
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Windthrow vs stand age
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Untreated and feathered riparian strips
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Best Predictors

• Dominant species
• Wind exposure
• Rooting depth
• Boundary-slope geometry
• Stand characteristics (age/height/density)
• Leave type (1 vs 2-sided)
• Strip width
• Edge treatment

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Summary

• Average riparian windthrow 21
• Average penetration distance 13 m
• Windthrow and penetration is greatest on windward
  boundaries 16 m
• Deeper rooting and taller trees are associated
  with more windthrow

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Summary continued ...

• 2-sided riparian strips have more windthrow than
  1-sided (external) strips (15 vs 27)
• Feathering reduces windthrow (11 vs 19)
• Wide strips are more windfirm than narrow strips
• Young stands are more vulnerable
• e.g., 1908 windthrow
• second growth

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Summary continued ...

• Cedar dominated stands are more windfirm
• Boundaries setback from the edges of gullies have
  less windthrow
• Windthrown trees span streams, but rarely cause
  significant bank disturbance

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Some options for high risk areas

• Log to streambank on one side
• stable gullies
• small streams
• Substitute more retention on low hazard S4s for
  less retention on high hazard S3s
• Fall, leave logs spanning stream for LWD
• Remove doms and co-doms (fall some across), but
  retain and top intermediates

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Options for high risk areas contd

• Fall tall trees growing on streambanks
• Log to base of escarpments with floodplains
• Log lee edges in young stands. Allow the stand
  time to adapt to increased wind forces before
  logging windward edges, or ..
• log a narrow strip along windward edges so that
  there is some wind protection from upwind stand
  edges for a period of time

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Some Preliminary Observations on Retention
Silviculture
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Observations on retention

• Pre-harvest windthrow assessment is critical
• Manage with local data for local conditions
• treat conventional wisdom with caution
• Manage for risk not hazard
• Carry out post-mortems blocks and basins
• Monitor and document windthrow patterns
• Adapt

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Observations on retention

• The factors controlling windthrow with retention
  silviculture will be similar to those found with
  conventional clearcutting
• Manage the edges of large patches as you would
  external block boundaries
• Manage dispersed retention using block strata and
  individual tree characteristics
• Make use of previously exposed areas/edges

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Observations on retention

• Large patches may survive better than small
• Narrow strips and ribbons are vulnerable
• Multi-storied stands/edges that can be easily
  feathered will be more windfirm
• Young uniform stands - may be vulnerable
• Minimize windward edges
• Setback from windward edges along the tops of
  escarpments and unstable gullies