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For a larger glossary of forestry terms please see
http://www.for.gov.bc.ca/hfd/library/documents/glossary/
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.
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