BMP - Tree Risk Assessment - 68
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to support the asymmetric crown. Asymmetric crowns can, however,
contribute to failure when other problems are present, such as compromised root anchorage.
Live crown ratio. The ratio of the vertical extent (height) of the live
crown to the height of the entire tree is the live crown ratio (LCR).
LCR is affected by growing conditions, pruning history, previous
branch failures, or natural branch shedding. Low LCR most commonly develops naturally where trees are growing in dense stands,
but it can also be created by crown raising. Low LCR can be a
condition of concern, especially if the tree originally developed in
forest conditions and was recently exposed to higher wind conditions. Ideally, the LCR should be greater than 65%-that is, the
upper 65% of the tree has live branches. When ratios are less than
50%, there may be reason for concern when other structural defects
are present. In some species, an LCR less than 33% may indicate
a higher likelihood of failure. For some tropical tree genera with
extremely hard wood (e.g., Casuarina spp.), a low LCR may not
necessarily be a cause for concern.
Taper (slenderness). Taper is the change in diameter over the length
of trunks, branches, and roots. Taper is important for even distribution
of mechanical stress. The degree of taper can be calculated by dividing
tree height or branch length by the trunk or branch diameter (H/D).
The same units are used for both height and diameter (feet or meters).
Accurate measurements are essential when determining both height
or length and diameter because minor errors can have a major impact
on the ratio. When H/D is large (that is, the tree is very slender for its
height), the tree may be more prone to failure than when the tree has
moderate or high taper with a small H/D ratio. Reported critical H/D
values vary from 50:1 to 90:1. Because of the size of this range, H/D
ratio is rarely used by itself to classify likelihood of failure.
The amount of stem taper is often related to LCR. Lower LCRs are
associated with less taper. As with LCR, trees that were originally
grown in forest conditions, and recently exposed to more wind because of the removal of adjacent trees, are of greatest concern. Taper
concerns do not apply to palms or to trees in an undisturbed forested
area where individual trees are surrounded by other trees and are
therefore protected from falling and the effects of wind.
68
BMP - Tree Risk Assessment
Table of Contents for the Digital Edition of BMP - Tree Risk Assessment
Table of Contents
BMP - Tree Risk Assessment - Cover1
BMP - Tree Risk Assessment - Cover2
BMP - Tree Risk Assessment - i
BMP - Tree Risk Assessment - ii
BMP - Tree Risk Assessment - Table of Contents
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