The Rural Voice, 2004-11, Page 41Woodlot Management
Understanding basal area
Steve Bowers
is a forester
and forest
owner, a
member of
the Huron -
Perth
Chapter of
the Ontario
Woodlot
Association
and
Stewardship Co-ordinator with the
Huron Stewardship Council.
One of many factors that need to
be considered by woodlot owners in
order to make appropriate
management decisions is the density
or stocking of trees in their woodlots.
Several features influence density,
including tree spacing and size.
One of the best and most common
approaches to determine density (and
a method commonly used by forestry
professionals) is to measure basal
area. An added incentive to learn
about basal area may be that most
new municipal tree cutting bylaws
require a minimum residual basal
area be left in the woodlot after a
harvest operation.
The basal area of an individual
tree is the cross-sectional area of the
tree measured at breast height (1.3
metres or 4.5 feet above the ground).
For woodlots, basal area is expressed
in square metres per hectare or
square feet per acre. This would
reflect the total cross-sectional area
of all trees on that hectare or acre
(imagine a hectare or acre of forest
with all trees cut off at breast height
— basal area would be the total
surface area of all the stumps). A
higher number indicates a greater
portion of the woodlot's land area is
occupied by tree stems, thus the
woodlot has a higher density.
A woodlot with many smaller
trees may have the same basal area as
another woodlot with fewer but
larger trees. Obviously, larger trees
occupy more space and need more
room to grow than smaller ones. As
most woodlots in southern Ontario
have a mixture of tree size classes,
basal area provides a good way of
measuring stand density in these
mixed forests to help determine
suitable management activities.
Generally, a site has the capacity
to support a certain basal area of tree
stems. If the basal area of a woodlot
exceeds that density by too much,
growth of all trees slows, as it is
spread over a too many stems.
Eventually, density may climb to the
point where growth can only occur if
there is mortality of some stems to
allow space for growth on those
remaining. Alternatively, if basal
area is too low, there are not enough
stems of sufficient size to fully
realize the growth potential for the
site.
A management objective for most
woodlots will be to keep basal area in
the range where it is high enough that
the site is fully occupied by trees, but
below the point where growth of
individual trees is slowed too much.
In hardwood stands such as those we
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Ontario NOG 2W0
TREE MARKING FOR TIMBER
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CALL (519) 335-6768
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NOVEMBER 2004 37