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Riparian Windthrow Northern Vancouver Island

Explore riparian windthrow impact on stream channels and develop control strategies. Ground surveys and visible effects revealed key findings, predictors, and management suggestions.

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Riparian Windthrow Northern Vancouver Island

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  1. Riparian Windthrow Northern Vancouver Island Terry Rollerson Kerry McGourlick

  2. 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

  3. 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

  4. 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.

  5. Stream class vs % windthrow

  6. % Windthrow distribution

  7. Windthrow penetration pattern

  8. Windthrow severity index: (WSI = %WT*Penetration Distance)

  9. Single vs double-sided strips

  10. Treatment plus leave strip effects

  11. Windthrow vs width (1-sided strips)

  12. Windthrow vs width (2-sided strips)

  13. Windthrow vs boundary exposure

  14. Wind Exposure Index WEI = (BE 1 rank) + (BE 2 rank) BE=boundary exposure

  15. Windthrow vs exposure index

  16. Windthrow vs valley axis

  17. Windthrow vs boundary geometry

  18. Windthrow vs rooting depth

  19. Windthrow vs soil drainage class

  20. Windthrow vs dominant tree species

  21. Windthrow vs height class (map)

  22. Windthrow vs stand age

  23. Untreated and feathered riparian strips

  24. 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

  25. 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

  26. 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

  27. 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

  28. Some options for high risk areas • Log to streambank on one side: • stable gullies • small streams • Substitute more retention on low hazard S4’s for less retention on high hazard S3’s • Fall, leave logs spanning stream for LWD • Remove doms and co-doms (fall some across), but retain and top intermediates

  29. Options for high risk areas cont’d • 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

  30. Some Preliminary Observations on Retention Silviculture

  31. 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

  32. 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

  33. 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

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