Windfall Mitigation
Wind is the most prevalent natural disturbance agent in the Acadian Forest and plays a very important role in influencing the composition and structure of the forest. Recent studies have indicated that wind will continue to play a major role, as climate change is expected to alter the frequency and severity of strong winds in the region (Taylor et al. 2019). Considering this information, windthrow, “the disturbance where a tree or group of trees is uprooted by wind”, may become more widespread with expected increases in severe windstorms in our region.
While windthrow is a natural process in the forest and contributes to coarse woody debris and wildlife habitat, a large amount of windthrow sustained in a concentrated area may be of concern to landowners for some of the following reasons:
Blown down timber may represent a potential loss of merchantable wood volume and/or increased salvage harvesting costs, which may present an economic loss
Access trails and roads may be blocked or infrastructure may be damaged
Aesthetic values may be impacted
Excessive windfall may cause increased fuel loading on the forest floor, which could increase wildfire hazard
While it is impossible to eliminate a natural process such as windthrow from occurring entirely, it may be possible to mitigate excessive blowdown by assessing hazards prior to implementing harvesting related activities. Windthrow hazard is assessed by forest professionals while conducting a Pre-treatment Assessment (PTA) of a potential working area and all landowners will benefit from understanding some of the factors that influence windthrow hazards on their woodlots.
The main factors influencing windthrow hazard are wind exposure and soil conditions.
Wind exposure. – This is based on slope, slope position, topography, and proximity to coastal areas. Wind exposure ratings have been mapped for all of Nova Scotia by NS Department of Lands and Forests (NSDLF) to consistently assess wind exposure. This map can be viewed on the PTA Reference Viewer, maintained by the NSDNR Forestry Division, found online via this link
Soil conditions – The main soil factor influencing windthrow hazard is potential rooting depth, which is directly related to soil moisture content, soil texture, stoniness and overall soil depth.
Generally, windthrow hazard in relation to soils will increase with:
higher soil moisture content, which prevents stable rooting
presence of finer textured soils (with higher clay content), that may limit rooting depth and lack strength
Very stony or rocky soils, which limit rooting depth
Presence of shallow soils, with bedrock or cementation present, that prevent deep rooting, regardless of species
Other important stand level factors to consider include stand density, rooting patterns, size of tree crowns and overall tree health.
Individual tree species also have different rooting depths that influence windthrow.
Common shallow rooted species include: spruces (red, black, white); balsam fir; and intolerant hardwoods, such as aspen and red maple. (NSDNR 2018)
Common deep rooted species include: pines (red, white, jack) and tolerant hardwoods such as sugar maple, yellow birch and red oak (NSDNR 2018).
Mature forest adjacent to large non-natural openings, such as recent timber harvests, residential developments or large fields, may also have an increased windthrow risk, as they will be more exposed to winds through adjacent openings.
While various partial harvest methods, such as commercial thinning and selection harvesting, may be preferred by some landowners as an alternative to harvest methods such as clearcutting, all of the factors affecting windthrow hazard should be assessed and considered when prescribing and undertaking partial harvest treatments.
Where a partial harvest treatment is chosen, some of the following guidelines should be considered to mitigate excessive windthrow:
Orient harvest trails perpendicular to direction of prevailing winds and follow topographical features where possible and maximize distance between trails
In areas with conditions indicating higher windthrow hazard (rocky, stony terrain; exposed edges; finer, clay-based soils) where a partial harvest is preferred, harvest stand in small patches rather than uniformly thinning between individual trees. Since remaining trees will be left in patches rather than as individual stems, the overall post-harvest windthrow risk may be minimized.
During the first entry in a stand, consider lighter removals during initial harvest and monitor post-harvest conditions in relation to windthrow. Adjust harvest removals during subsequent entries based on stands response.
It must be understood that there is some risk of blowdown when partial harvesting methods are chosen and every stand has unique characteristics and conditions that will influence its suitability. In some cases, a landowner’s objectives may lead them to deciding against any harvesting taking place if the risk of windthrow outweighs the ecological or economic benefits associated with a harvest treatment.
In closing, wind has been a major influence on the forests of our region and will continue to be into the future. While we can do our part to mitigate the hazards associated with windthrow and try our best not to create conditions that have unwanted effects on our woodlots, we must keep in mind that it is as natural for trees to blow down in our forests as it is for trees to grow in them and nature’s processes don’t always coincide with our goals, objectives or desires.
Sources:
Forest Ecosystem Classification for Nova Scotia (2013). NS Department of Natural Resources
NS Forest Management Guide (2018). NS Department of Natural Resources
Forest structure more important than topography in determining windthrow during Hurricane Juan in Canada’s Acadian forest. 2019. Taylor, A.R.; Dracup, E.; MacLean, D.A.; Boulanger, Y.; Endicott, S. Forest Ecology and Management 434: 255-263.
The windthrow triangle: A relative windthrow hazard assessment procedure for forest managers. 1995. Mitchell, S. The Forestry Chronicle. 71. 446-450.