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Forest Practices Branch Office

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University of British Columbia



For a larger glossary of forestry terms please see


Arctic front: Large cold front or the transition zone originating in the polar arctic region. Comprised of a large cold air mass which moves south replacing the warmer air mass. The air behind an Arctic front is noticeably colder and drier than the air ahead of it. When a cold front passes through, temperatures can drop more than 15 degrees within the first hour. Large frontal systems are associated with strong winds.

Biophysical hazard: The combination of the topographic, soils and stand hazard components. It represents the intrinsic windloading and wind stability of trees on the site prior to treatment.

Catastrophic windthrow: Caused by peak winds which recur infrequently and cause breakage.

Empirical approach to windthrow assessment: Uses regression techniques to predict the probability of damage as a function of environmental and management variables.

Endemic windthrow: Caused by peak winds which recur every 1 to 3 years, causing more uprooting than breaking.

Lee: Refers to the side opposite of windward. Protected from direct exposure to the wind.

Mechanical approach to windthrow assessment: Predicts the critical windspeed for overturning from winching and wind tunnel studies, and the probability of critical wind speed from wind mapping/modelling work.

Observational approach to windthrow assessment: Uses a checklist of indicators.

Pacific front: Dominant frontal systems or transition zones occurring along the coast of British Columbia where a large warm air mass moves in off the Pacific Ocean replacing a cold air mass. Pacific fronts generally move from southwest to northeast and the air behind is warmer and more moist than the air ahead of it. Frontal systems are associated with strong winds.

Parallel openings: Refers to openings with their sides facing the same direction/parallel to the prevailing winds.

Perpendicular openings: Refers to openings with sides facing at right angles to the prevailing winds.

Site hazard: By integrating topographic exposure and soils in the first assessment matrix, an overall site hazard rating is determined. This rating is combined with stand hazard in the second matrix to determine overall biophysical hazard.

Soil hazard: Soil characteristics determine, to a large degree, the form tree roots will take and the strength of the root/soil interface. Unrestricted tree root systems are 'bowl-like,' and deepest in the centre. Moderately restricted root systems may extend for some depth into mineral soil but be flat bottomed. A physical barrier or water table causes roots to bend or stop growth. Severely restricted root systems are plate-like, trees a attempting to build a "root platform." These trees are highly susceptible to windthrow. Poor drainage or low strength organic soils decrease the strength of anchorage.

Stand hazard: Stand characteristics address factors relating to individual trees and stands. Open-grown trees adapt to wind loads by adding taper to their stems, for example. Stand-grown trees are sheltered by other trees but must grow in height to stay above their neighbours. These trees become proportionately more slender and less stable. In very high density, short stands, individual trees may have low wind firmness, however the stand edge often remains intact during high wind events.

Topographic exposure hazard: Considers the location of the stand in relation to local topographic features and prevailing endemic winds. Wind speeds up over obstacles and produces turbulence in the wake of obstacles. In addition, wind speeds up along valleys and through passes. Shoulder areas, where valleys change direction, are high hazard areas.

Treatment risk: The way in which a particular treatment increases or decreases the windloading or wind resistance of trees.

Windthrow impact: Refers to the potential harm windthrow could cause if it occurs. Impact is negative if wind damage results in management objectives not being met. If some level of damage is acceptable, this should be indicated in the original silviculture prescription.

Windthrow risk: The likelihood of damage from endemic winds. It is the combination of Biophysical Hazard and Treatment Risk.

Windward: A side or face exposed directly to prevailing winds. Winds blow directly onto windward faces. Opposite of lee.



DEM: Digital Elevation Model.

GIS: Geographic information system; can refer to any computerized digital mapping system.

SAS: (formerly Statistical Analysis System)

TIN: Triangular Irregular Network.

TOPEX-to-distance: TOPEX is a measure of topographic exposure at a given location. Classical topex is the sum of the angle to the skyline in degrees for the eight cardiinal directions. TOPEX-to-distance is classical TOPEX modified to facilitate map-based calculations.

TOPEX2000: TOPEX is a measure of topographic exposure at a given location. For TOPEX2000, the maximum distance in each direction is limited.

TRIM: Terrain Resource Information Management, TRIM mapping consists of 7 027 mapsheets covering the province of British Columbia at a scale of 1:20 000. The cartographic framework for this mapping is the Univeral Transverse Mercator coordinate system, based on NAD83 (1983 North American Datum). Each mapsheet is precisely 12 minutes of longitude wide by 6 minutes of latitude high.

UTM: Universal Transverse Mercator, a widely used grid for dividing up the surface of the globe for mapping purposes. For more information on UTM visit:

Reference Material


BC Ministry of Forests. 1995. Silvicultural Systems Guidebook. For. Prac. Br., Victoria, BC.

BC Ministry of Forests. 1995. Biodiversity Guidebook. For. Prac. Br., Victoria, BC.

BC Ministry of Forests. 1995. Riparian Management Area Guidebook. For. Prac. Br., Victoria, BC.

BC Ministry of Forests. 1999. Mapping and Assessing Terrain Stability Guidebook. Second edition. For. Prac. Br., Victoria, BC.

BC Ministry of Forests. 2001. Gully Assessment Procedure Guidebook. Fourth edition, Version 4.1. For. Prac. Br., Victoria, BC.

FS 712 Windthrow Assessment Field Cards. BC Min. For., Victoria BC.

Mitchell, S.J. 1998. A diagnostic framework for windthrow risk estimation. For. Chron. 74:100–105.

Mitchell, S.J. 2000. Forest health: preliminary interpretations for wind damage. BC Min. For., For. Prac. Br., Victoria BC. Stand Density Management Diagrams.

Mitchell, S.J., T.Hailemariam and Y.Kulis. 2001. Empirical modeling of cutblock edge windthrow risk on Vancouver Island, Canada, using stand level information. For. Ecol. Manage. [In press].

Stathers, R., T. Rollerson and S. Mitchell. Windthrow Handbook for BC Forests. BC Min. For., Resear. Br., Victoria, BC. Working Pap. 9401.

Forest Renewal BC / Forest Service BC
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