Windthrow (module 5, p. 3)
Define the term, windthrow
It is important to recognize that windthrow (blowdown) is a naturally occurring phenomenon. All stands, and therefore retention strategies such as WTPs and riparian reserve zones, are prone to windthrow. Resource managers must realize that some level of windthrow is likely unavoidable no matter what the silvicultural system (e.g., clearcut versus partial cut) or leave strategy, and can be tolerated. However, in order to minimize the risk of high volume blowdown events (in terms of scale or frequency), foresters must carefully consider windthrow risk when scheduling block location, block layout, and any associated retention strategies.
Windthrow has two subheadings:
- Windthrow considerations in patch planning
- Windthrow considerations in riparian areas
Windthrow considerations in patch planning
- Reducing the wind force acting on a tree or group of trees can be achieved by selecting locations for WTPs and green tree retention where the boundaries and orientations favour low wind speed and/or good anchorage. Areas of highest wind velocity tend to be along ridge tops, and in valley bottoms where the prevailing wind direction is parallel to the ridgeline.
- Avoid locating leave patches in areas that have evidence of previous extensive or chronic windthrow.
- Avoid locating leave patches in areas with shallow or poorly drained soils.
- Utilize natural landscape boundaries to create windfirm edges (e.g., rock bluffs, bogs, non-merchantable timber, avalanche tracks).
- If a windward stand boundary proves to be windfirm (most endemic windthrow occurs in the first three years after cutting), utilize it as a windfirm edge.
- From that point, log progressively into the wind.
- In order to disperse the cut more windfirm, boundaries need to be identified.
- Stand edges should be left relatively uniform and smooth. They should not have sharp corners or indentations that are exposed to the wind.
- On high and moderate windthrow hazard sites, edge feathering can be used to reduce the drag force on boundary trees, stabilizing the opening boundary.
- Fallers should perform edge feathering or other windthrow prescriptions concurrently with the layout of the patch.
- Tree topping and spiral pruning are techniques (done by specialists) that can also be used to reduce the wind force on boundary trees or other trees (e.g., seed trees or shelterwood trees) that have a high windthrow hazard.
Are there other considerations? If so, what are they?
Windthrow considerations in riparian areas
In areas of high or moderate windthrow hazard, consider expanding the outer boundaries of riparian management areas (RMAs) to windfirm terrain features.
- Higher retention rates should be considered along the edge of RMAs where windthrow hazard is high/moderate.
- Select the most windthrow resistant trees in the management zone for retention,
- Reduce the force of the wind on the crowns of retained trees by topping or pruning.
For detailed information on management recommendations and practices in riparian areas, consult the Riparian Management Area Guidebook.
Figure 17 (right) shows a cutblock design that incorporates the following factors:
- Wildlife tree patch design
- Riparian management area requirements
- Choice of silvicultural system (clearcut with reserves and seed tree)
- Windthrow hazard
- Worker safety (safe location of standing dead trees in WTP and RMA)